5.1. Quickstart

Suppose that you are in a directory containing a single Cabal package which you wish to build (if you haven’t set up a package yet check out developing packages for instructions). You can configure and build it using Nix-style local builds with this command (configuring is not necessary):

$ cabal v2-build

To open a GHCi shell with this package, use this command:

$ cabal v2-repl

To run an executable defined in this package, use this command:

$ cabal v2-run <executable name> [executable args]

5.1.1. Developing multiple packages

Many Cabal projects involve multiple packages which need to be built together. To build multiple Cabal packages, you need to first create a cabal.project file which declares where all the local package directories live. For example, in the Cabal repository, there is a root directory with a folder per package, e.g., the folders Cabal and cabal-install. The cabal.project file specifies each folder as part of the project:

packages: Cabal/
          cabal-install/

The expectation is that a cabal.project is checked into your source control, to be used by all developers of a project. If you need to make local changes, they can be placed in cabal.project.local (which should not be checked in.)

Then, to build every component of every package, from the top-level directory, run the command: (using cabal-install-2.0 or greater.)

$ cabal v2-build

To build a specific package, you can either run v2-build from the directory of the package in question:

$ cd cabal-install
$ cabal v2-build

or you can pass the name of the package as an argument to cabal v2-build (this works in any subdirectory of the project):

$ cabal v2-build cabal-install

You can also specify a specific component of the package to build. For example, to build a test suite named package-tests, use the command:

$ cabal v2-build package-tests

Targets can be qualified with package names. So to request package-tests from the Cabal package, use Cabal:package-tests.

Unlike sandboxes, there is no need to setup a sandbox or add-source projects; just check in cabal.project to your repository and v2-build will just work.

5.2. Cookbook

5.2.1. How can I profile my library/application?

Create or edit your cabal.project.local, adding the following line:

profiling: True

Now, cabal v2-build will automatically build all libraries and executables with profiling. You can fine-tune the profiling settings for each package using profiling-detail:

package p
    profiling-detail: toplevel-functions

Alternately, you can call cabal v2-build --enable-profiling to temporarily build with profiling.

5.3. How it works

5.3.1. Local versus external packages

One of the primary innovations of Nix-style local builds is the distinction between local packages, which users edit and recompile and must be built per-project, versus external packages, which can be cached across projects. To be more precise:

  1. A local package is one that is listed explicitly in the packages, optional-packages or extra-packages field of a project. Usually, these refer to packages whose source code lives directly in a folder in your project (although, you can list an arbitrary Hackage package in extra-packages to force it to be treated as local).

Local packages, as well as the external packages (below) which depend on them, are built inplace, meaning that they are always built specifically for the project and are not installed globally. Inplace packages are not cached and not given unique hashes, which makes them suitable for packages which you want to edit and recompile.

  1. An external package is any package which is not listed in the packages field. The source code for external packages is usually retrieved from Hackage.

When an external package does not depend on an inplace package, it can be built and installed to a global store, which can be shared across projects. These build products are identified by a hash that over all of the inputs which would influence the compilation of a package (flags, dependency selection, etc.). Just as in Nix, these hashes uniquely identify the result of a build; if we compute this identifier and we find that we already have this ID built, we can just use the already built version.

The global package store is ~/.cabal/store (configurable via global store-dir option); if you need to clear your store for whatever reason (e.g., to reclaim disk space or because the global store is corrupted), deleting this directory is safe (v2-build will just rebuild everything it needs on its next invocation).

This split motivates some of the UI choices for Nix-style local build commands. For example, flags passed to cabal v2-build are only applied to local packages, so that adding a flag to cabal v2-build doesn’t necessitate a rebuild of every transitive dependency in the global package store.

In cabal-install 2.0 and above, Nix-style local builds also take advantage of a new Cabal library feature, per-component builds, where each component of a package is configured and built separately. This can massively speed up rebuilds of packages with lots of components (e.g., a package that defines multiple executables), as only one executable needs to be rebuilt. Packages that use Custom setup scripts are not currently built on a per-component basis.

5.3.2. Where are my build products?

A major deficiency in the current implementation of v2-build is that there is no programmatic way to access the location of build products. The location of the build products is intended to be an internal implementation detail of v2-build, but we also understand that many unimplemented features can only be reasonably worked around by accessing build products directly.

The location where build products can be found varies depending on the version of cabal-install:

  • In cabal-install-1.24, the dist directory for a package p-0.1 is stored in dist-newstyle/build/p-0.1. For example, if you built an executable or test suite named pexe, it would be located at dist-newstyle/build/p-0.1/build/pexe/pexe.
  • In cabal-install-2.0, the dist directory for a package p-0.1 defining a library built with GHC 8.0.1 on 64-bit Linux is dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1. When per-component builds are enabled (any non-Custom package), a subcomponent like an executable or test suite named pexe will be stored at dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/c/pexe; thus, the full path of the executable is dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/c/pexe/build/pexe/pexe (you can see why we want this to be an implementation detail!)
  • In cabal-install-2.2 and above, the /c/ part of the above path
    is replaced with one of /l/, /x/, /f/, /t/, or /b/, depending on the type of component (sublibrary, executable, foreign library, test suite, or benchmark respectively). So the full path to an executable named pexe compiled with GHC 8.0.1 on a 64-bit Linux is now dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/x/pexe/build/pexe/pexe; for a benchmark named pbench it now is dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/b/pbench/build/pbench/pbench;

The paths are a bit longer in 2.0 and above but the benefit is that you can transparently have multiple builds with different versions of GHC. We plan to add the ability to create aliases for certain build configurations, and more convenient paths to access particularly useful build products like executables.

5.3.3. Caching

Nix-style local builds sport a robust caching system which help reduce the time it takes to execute a rebuild cycle. While the details of how cabal-install does caching are an implementation detail and may change in the future, knowing what gets cached is helpful for understanding the performance characteristics of invocations to v2-build. The cached intermediate results are stored in dist-newstyle/cache; this folder can be safely deleted to clear the cache.

The following intermediate results are cached in the following files in this folder (the most important two are first):

solver-plan (binary)
The result of calling the dependency solver, assuming that the Hackage index, local cabal.project file, and local cabal files are unmodified. (Notably, we do NOT have to dependency solve again if new build products are stored in the global store; the invocation of the dependency solver is independent of what is already available in the store.)
source-hashes (binary)
The hashes of all local source files. When all local source files of a local package are unchanged, cabal v2-build will skip invoking setup build entirely (saving us from a possibly expensive call to ghc --make). The full list of source files participating in compilation are determined using setup sdist --list-sources (thus, if you do not list all your source files in a Cabal file, you may fail to recompile when you edit them.)
config (same format as cabal.project)
The full project configuration, merged from cabal.project (and friends) as well as the command line arguments.
compiler (binary)
The configuration of the compiler being used to build the project.
improved-plan (binary)
Like solver-plan, but with all non-inplace packages improved into pre-existing copies from the store.
plan.json (JSON)

A JSON serialization of the computed install plan intended for integrating cabal with external tooling. The cabal-plan package provides a library for parsing plan.json files into a Haskell data structure as well as an example tool showing possible applications.

Todo

Document JSON schema (including version history of schema)

Note that every package also has a local cache managed by the Cabal build system, e.g., in $distdir/cache.

There is another useful file in dist-newstyle/cache, plan.json, which is a JSON serialization of the computed install plan and is intended for integrating with external tooling.

5.4. Commands

We now give an in-depth description of all the commands, describing the arguments and flags they accept.

5.4.1. cabal v2-configure

cabal v2-configure takes a set of arguments and writes a cabal.project.local file based on the flags passed to this command. cabal v2-configure FLAGS; cabal new-build is roughly equivalent to cabal v2-build FLAGS, except that with new-configure the flags are persisted to all subsequent calls to v2-build.

cabal v2-configure is intended to be a convenient way to write out a cabal.project.local for simple configurations; e.g., cabal v2-configure -w ghc-7.8 would ensure that all subsequent builds with cabal v2-build are performed with the compiler ghc-7.8. For more complex configuration, we recommend writing the cabal.project.local file directly (or placing it in cabal.project!)

cabal v2-configure inherits options from Cabal. semantics:

  • Any flag accepted by ./Setup configure.
  • Any flag accepted by cabal configure beyond ./Setup configure, namely --cabal-lib-version, --constraint, --preference and --solver.
  • Any flag accepted by cabal install beyond ./Setup configure.
  • Any flag accepted by ./Setup haddock.

The options of all of these flags apply only to local packages in a project; this behavior is different than that of cabal install, which applies flags to every package that would be built. The motivation for this is to avoid an innocuous addition to the flags of a package resulting in a rebuild of every package in the store (which might need to happen if a flag actually applied to every transitive dependency). To apply options to an external package, use a package stanza in a cabal.project file.

5.4.2. cabal v2-update

cabal v2-update updates the state of the package index. If the project contains multiple remote package repositories it will update the index of all of them (e.g. when using overlays).

Some examples:

$ cabal v2-update                  # update all remote repos
$ cabal v2-update head.hackage     # update only head.hackage

5.4.3. cabal v2-build

cabal v2-build takes a set of targets and builds them. It automatically handles building and installing any dependencies of these targets.

A target can take any of the following forms:

  • A package target: package, which specifies that all enabled components of a package to be built. By default, test suites and benchmarks are not enabled, unless they are explicitly requested (e.g., via --enable-tests.)

  • A component target: [package:][ctype:]component, which specifies a specific component (e.g., a library, executable, test suite or benchmark) to be built.

  • All packages: all, which specifies all packages within the project.

  • Components of a particular type: package:ctypes, all:ctypes: which specifies all components of the given type. Where valid ctypes are:

    • libs, libraries,
    • flibs, foreign-libraries,
    • exes, executables,
    • tests,
    • benches, benchmarks.

In component targets, package: and ctype: (valid component types are lib, flib, exe, test and bench) can be used to disambiguate when multiple packages define the same component, or the same component name is used in a package (e.g., a package foo defines both an executable and library named foo). We always prefer interpreting a target as a package name rather than as a component name.

Some example targets:

$ cabal v2-build lib:foo-pkg       # build the library named foo-pkg
$ cabal v2-build foo-pkg:foo-tests # build foo-tests in foo-pkg

(There is also syntax for specifying module and file targets, but it doesn’t currently do anything.)

Beyond a list of targets, cabal v2-build accepts all the flags that cabal v2-configure takes. Most of these flags are only taken into consideration when building local packages; however, some flags may cause extra store packages to be built (for example, --enable-profiling will automatically make sure profiling libraries for all transitive dependencies are built and installed.)

In addition cabal v2-build accepts these flags:

  • --only-configure: When given we will forgoe performing a full build and abort after running the configure phase of each target package.

5.4.4. cabal v2-repl

cabal v2-repl TARGET loads all of the modules of the target into GHCi as interpreted bytecode. In addition to cabal v2-build’s flags, it takes an additional --repl-options flag.

To avoid ghci specific flags from triggering unneeded global rebuilds these flags are now stripped from the internal configuration. As a result --ghc-options will no longer (reliably) work to pass flags to ghci (or other repls). Instead, you should use the new --repl-options flag to specify these options to the invoked repl. (This flag also works on cabal repl and Setup repl on sufficiently new versions of Cabal.)

Currently, it is not supported to pass multiple targets to v2-repl (v2-repl will just successively open a separate GHCi session for each target.)

It also provides a way to experiment with libraries without needing to download them manually or to install them globally.

This command opens a REPL with the current default target loaded, and a version of the vector package matching that specification exposed.

$ cabal v2-repl --build-depends "vector >= 0.12 && < 0.13"

Both of these commands do the same thing as the above, but only exposes base, vector, and the vector package’s transitive dependencies even if the user is in a project context.

$ cabal v2-repl --ignore-project --build-depends "vector >= 0.12 && < 0.13"
$ cabal v2-repl --project='' --build-depends "vector >= 0.12 && < 0.13"

This command would add vector, but not (for example) primitive, because it only includes the packages specified on the command line (and base, which cannot be excluded for technical reasons).

$ cabal v2-repl --build-depends vector --no-transitive-deps

5.4.5. cabal v2-run

cabal v2-run [TARGET [ARGS]] runs the executable specified by the target, which can be a component, a package or can be left blank, as long as it can uniquely identify an executable within the project. Tests and benchmarks are also treated as executables.

See the v2-build section for the target syntax.

Except in the case of the empty target, the strings after it will be passed to the executable as arguments.

If one of the arguments starts with - it will be interpreted as a cabal flag, so if you need to pass flags to the executable you have to separate them with --.

$ cabal v2-run target -- -a -bcd --argument

‘v2-run’ also supports running script files that use a certain format. With a script that looks like:

#!/usr/bin/env cabal
{- cabal:
build-depends: base ^>= 4.11
            , shelly ^>= 1.8.1
-}

main :: IO ()
main = do
    ...

It can either be executed like any other script, using cabal as an interpreter, or through this command:

$ cabal v2-run script.hs
$ cabal v2-run script.hs -- --arg1 # args are passed like this

5.4.6. cabal v2-freeze

cabal v2-freeze writes out a freeze file which records all of the versions and flags which that are picked by the solver under the current index and flags. Default name of this file is cabal.project.freeze but in combination with a --project-file=my.project flag (see project-file) the name will be my.project.freeze. A freeze file has the same syntax as cabal.project and looks something like this:

constraints: HTTP ==4000.3.3,
             HTTP +warp-tests -warn-as-error -network23 +network-uri -mtl1 -conduit10,
             QuickCheck ==2.9.1,
             QuickCheck +templatehaskell,
             -- etc...

For end-user executables, it is recommended that you distribute the cabal.project.freeze file in your source repository so that all users see a consistent set of dependencies. For libraries, this is not recommended: users often need to build against different versions of libraries than what you developed against.

5.4.7. cabal v2-bench

cabal v2-bench [TARGETS] [OPTIONS] runs the specified benchmarks (all the benchmarks in the current package by default), first ensuring they are up to date.

5.4.8. cabal v2-test

cabal v2-test [TARGETS] [OPTIONS] runs the specified test suites (all the test suites in the current package by default), first ensuring they are up to date.

5.4.9. cabal v2-haddock

cabal v2-haddock [FLAGS] [TARGET] builds Haddock documentation for the specified packages within the project.

If a target is not a library haddock-benchmarks, haddock-executables, haddock-internal, haddock-tests will be implied as necessary.

5.4.10. cabal v2-exec

cabal v2-exec [FLAGS] [--] COMMAND [--] [ARGS] runs the specified command using the project’s environment. That is, passing the right flags to compiler invocations and bringing the project’s executables into scope.

5.4.11. cabal v2-install

cabal v2-install [FLAGS] PACKAGES builds the specified packages and symlinks/copies their executables in installdir (usually ~/.cabal/bin).

For example this command will build the latest cabal-install and symlink its cabal executable:

$ cabal v2-install cabal-install

In addition, it’s possible to use cabal v2-install to install components of a local project. For example, with an up-to-date Git clone of the Cabal repository, this command will build cabal-install HEAD and symlink the cabal executable:

$ cabal v2-install exe:cabal

Where symlinking is not possible (eg. on Windows), --install-method=copy can be used:

$ cabal v2-install exe:cabal --install-method=copy --installdir=~/bin

Note that copied executables are not self-contained, since they might use data-files from the store.

It is also possible to “install” libraries using the --lib flag. For example, this command will build the latest Cabal library and install it:

$ cabal v2-install --lib Cabal

This works by managing GHC environments. By default, it is writing to the global environment in ~/.ghc/$ARCH-$OS-$GHCVER/environments/default. v2-install provides the --package-env flag to control which of these environments is modified.

This command will modify the environment file in the current directory:

$ cabal v2-install --lib Cabal --package-env .

This command will modify the environment file in the ~/foo directory:

$ cabal v2-install --lib Cabal --package-env foo/

Do note that the results of the previous two commands will be overwritten by the use of other v2-style commands, so it is not recommended to use them inside a project directory.

This command will modify the environment in the “local.env” file in the current directory:

$ cabal v2-install --lib Cabal --package-env local.env

This command will modify the myenv named global environment:

$ cabal v2-install --lib Cabal --package-env myenv

If you wish to create a named environment file in the current directory where the name does not contain an extension, you must reference it as ./myenv.

You can learn more about how to use these environments in this section of the GHC manual.

5.4.12. cabal v2-clean

cabal v2-clean [FLAGS] cleans up the temporary files and build artifacts stored in the dist-newstyle folder.

By default, it removes the entire folder, but it can also spare the configuration and caches if the --save-config option is given, in which case it only removes the build artefacts (.hi, .o along with any other temporary files generated by the compiler, along with the build output).

5.4.13. cabal v2-sdist

cabal v2-sdist [FLAGS] [TARGETS] takes the crucial files needed to build TARGETS and puts them into an archive format ready for upload to Hackage. These archives are stable and two archives of the same format built from the same source will hash to the same value.

cabal v2-sdist takes the following flags:

  • -l, --list-only: Rather than creating an archive, lists files that would be included. Output is to stdout by default. The file paths are relative to the project’s root directory.
  • -o, --output-dir: Sets the output dir, if a non-default one is desired. The default is dist-newstyle/sdist/. --output-dir - will send output to stdout unless multiple archives are being created.
  • -z, --null: Only used with --list-only. Separates filenames with a NUL byte instead of newlines.

v2-sdist is inherently incompatible with sdist hooks, not due to implementation but due to fundamental core invariants (same source code should result in the same tarball, byte for byte) that must be satisfied for it to function correctly in the larger v2-build ecosystem. autogen-modules is able to replace uses of the hooks to add generated modules, along with the custom publishing of Haddock documentation to Hackage.

5.5. Configuring builds with cabal.project

cabal.project files support a variety of options which configure the details of your build. The general syntax of a cabal.project file is similar to that of a Cabal file: there are a number of fields, some of which live inside stanzas:

packages: */*.cabal
with-compiler: /opt/ghc/8.0.1/bin/ghc

package cryptohash
  optimization: False

In general, the accepted field names coincide with the accepted command line flags that cabal install and other commands take. For example, cabal v2-configure --enable-profiling will write out a project file with profiling: True.

The full configuration of a project is determined by combining the following sources (later entries override earlier ones):

  1. ~/.cabal/config (the user-wide global configuration)
  2. cabal.project (the project configuration)
  3. cabal.project.freeze (the output of cabal v2-freeze)
  4. cabal.project.local (the output of cabal v2-configure)

5.5.1. Specifying the local packages

The following top-level options specify what the local packages of a project are:

packages: package location list (space or comma separated)
Default value:./*.cabal

Specifies the list of package locations which contain the local packages to be built by this project. Package locations can take the following forms:

  1. They can specify a Cabal file, or a directory containing a Cabal file, e.g., packages: Cabal cabal-install/cabal-install.cabal.
  2. They can specify a glob-style wildcards, which must match one or more (a) directories containing a (single) Cabal file, (b) Cabal files (extension .cabal), or (c) tarballs which contain Cabal packages (extension .tar.gz). For example, to match all Cabal files in all subdirectories, as well as the Cabal projects in the parent directories foo and bar, use packages: */*.cabal ../{foo,bar}/
  3. They can specify an http, https or file URL, representing the path to a remote tarball to be downloaded and built.

There is no command line variant of this field; see #3585.

optional-packages: package location list (space or comma-separated)
Default value:./*/*.cabal

Like packages, specifies a list of package locations containing local packages to be built. Unlike packages, if we glob for a package, it is permissible for the glob to match against zero packages. The intended use-case for optional-packages is to make it so that vendored packages can be automatically picked up if they are placed in a subdirectory, but not error if there aren’t any.

There is no command line variant of this field.

extra-packages: package list with version bounds (comma separated)

[STRIKEOUT:Specifies a list of external packages from Hackage which should be considered local packages.] (Not implemented)

There is no command line variant of this field.

All local packages are vendored, in the sense that if other packages (including external ones from Hackage) depend on a package with the name of a local package, the local package is preferentially used. This motivates the default settings:

packages: ./*.cabal
optional-packages: ./*/*.cabal

…any package can be vendored simply by making a checkout in the top-level project directory, as might be seen in this hypothetical directory layout:

foo.cabal
foo-helper/     # local package
unix/           # vendored external package

All of these options support globs. cabal v2-build has its own glob format:

  • Anywhere in a path, as many times as you like, you can specify an asterisk * wildcard. E.g., */*.cabal matches all .cabal files in all immediate subdirectories. Like in glob(7), asterisks do not match hidden files unless there is an explicit period, e.g., .*/foo.cabal will match .private/foo.cabal (but */foo.cabal will not).
  • You can use braces to specify specific directories; e.g., {vendor,pkgs}/*.cabal matches all Cabal files in the vendor and pkgs subdirectories.

Formally, the format described by the following BNF:

Todo

convert globbing grammar to proper ABNF syntax

FilePathGlob    ::= FilePathRoot FilePathGlobRel
FilePathRoot    ::= {- empty -}        # relative to cabal.project
                  | "/"                # Unix root
                  | [a-zA-Z] ":" [/\\] # Windows root
                  | "~"                # home directory
FilePathGlobRel ::= Glob "/"  FilePathGlobRel # Unix directory
                  | Glob "\\" FilePathGlobRel # Windows directory
                  | Glob         # file
                  | {- empty -}  # trailing slash
Glob      ::= GlobPiece *
GlobPiece ::= "*"            # wildcard
            | [^*{},/\\] *   # literal string
            | "\\" [*{},]    # escaped reserved character
            | "{" Glob "," ... "," Glob "}" # union (match any of these)

5.5.1.1. Specifying Packages from Remote Version Control Locations

Starting with Cabal 2.4, there is now a stanza source-repository-package for specifying packages from an external version control which supports the following fields:

A simple example is shown below:

packages: .

source-repository-package
    type: git
    location: https://github.com/hvr/HsYAML.git
    tag: e70cf0c171c9a586b62b3f75d72f1591e4e6aaa1

source-repository-package
    type: git
    location: https://github.com/well-typed/cborg
    tag: 3d274c14ca3077c3a081ba7ad57c5182da65c8c1
    subdir: cborg

5.5.2. Global configuration options

The following top-level configuration options are not specific to any package, and thus apply globally:

verbose: nat
--verbose=n, -vn
Default value:1

Control the verbosity of cabal commands, valid values are from 0 to 3.

The command line variant of this field is --verbose=2; a short form -v2 is also supported.

jobs: nat or $ncpus
--jobs=n, -jn , --jobs=$ncpus
Default value:1

Run nat jobs simultaneously when building. If $ncpus is specified, run the number of jobs equal to the number of CPUs. Package building is often quite parallel, so turning on parallelism can speed up build times quite a bit!

The command line variant of this field is --jobs=2; a short form -j2 is also supported; a bare --jobs or -j is equivalent to --jobs=$ncpus.

keep-going: boolean
--keep-going
Default value:False

If true, after a build failure, continue to build other unaffected packages.

The command line variant of this field is --keep-going.

--builddir=DIR

Specifies the name of the directory where build products for build will be stored; defaults to dist-newstyle. If a relative name is specified, this directory is resolved relative to the root of the project (i.e., where the cabal.project file lives.)

This option cannot be specified via a cabal.project file.

--project-file=FILE

Specifies the name of the project file used to specify the rest of the top-level configuration; defaults to cabal.project. This name not only specifies the name of the main project file, but also the auxiliary project files cabal.project.freeze and cabal.project.local; for example, if you specify --project-file=my.project, then the other files that will be probed are my.project.freeze and my.project.local.

If the specified project file is a relative path, we will look for the file relative to the current working directory, and then for the parent directory, until the project file is found or we have hit the top of the user’s home directory.

This option cannot be specified via a cabal.project file.

--store-dir=DIR

Specifies the name of the directory of the global package store.

5.5.3. Solver configuration options

The following settings control the behavior of the dependency solver:

constraints: constraints list (comma separated)
--constraint="pkg > 2.0"

Add extra constraints to the version bounds, flag settings, and other properties a solver can pick for a package. For example:

constraints: bar == 2.1

A package can be specified multiple times in constraints, in which case the specified constraints are intersected. This is useful, since the syntax does not allow you to specify multiple constraints at once. For example, to specify both version bounds and flag assignments, you would write:

constraints: bar == 2.1,
             bar +foo -baz

Valid constraints take the same form as for the constraint command line option.

preferences: preference (comma separated)
--preference="pkg > 2.0"

Like constraints, but the solver will attempt to satisfy these preferences on a best-effort basis. The resulting install is locally optimal with respect to preferences; specifically, no single package could be replaced with a more preferred version that still satisfies the hard constraints.

Operationally, preferences can cause the solver to attempt certain version choices of a package before others, which can improve dependency solver runtime.

One way to use preferences is to take a known working set of constraints (e.g., via cabal v2-freeze) and record them as preferences. In this case, the solver will first attempt to use this configuration, and if this violates hard constraints, it will try to find the minimal number of upgrades to satisfy the hard constraints again.

The command line variant of this field is --preference="pkg >= 2.0"; to specify multiple preferences, pass the flag multiple times.

allow-newer: none, all or list of scoped package names (space or comma separated)
--allow-newer , --allow-newer=[none, all , [scope:][^]pkg]
Default value:none

Allow the solver to pick an newer version of some packages than would normally be permitted by than the build-depends bounds of packages in the install plan. This option may be useful if the dependency solver cannot otherwise find a valid install plan.

For example, to relax pkgs build-depends upper bound on dep-pkg, write a scoped package name of the form:

allow-newer: pkg:dep-pkg

If the scope shall be limited to specific releases of pkg, the extended form as in

allow-newer: pkg-1.2.3:dep-pkg, pkg-1.1.2:dep-pkg

can be used to limit the relaxation of dependencies on dep-pkg by the pkg-1.2.3 and pkg-1.1.2 releases only.

The scoped syntax is recommended, as it is often only a single package whose upper bound is misbehaving. In this case, the upper bounds of other packages should still be respected; indeed, relaxing the bound can break some packages which test the selected version of packages.

The syntax also allows to prefix the dependee package with a modifier symbol to modify the scope/semantic of the relaxation transformation in a additional ways. Currently only one modifier symbol is defined, i.e. ^ (i.e. caret) which causes the relaxation to be applied only to ^>= operators and leave all other version operators untouched.

However, in some situations (e.g., when attempting to build packages on a new version of GHC), it is useful to disregard all upper-bounds, with respect to a package or all packages. This can be done by specifying just a package name, or using the keyword all to specify all packages:

-- Disregard upper bounds involving the dependencies on
-- packages bar, baz. For quux only, relax
-- 'quux ^>= ...'-style constraints only.
allow-newer: bar, baz, ^quux

-- Disregard all upper bounds when dependency solving
allow-newer: all

-- Disregard all `^>=`-style upper bounds when dependency solving
allow-newer: ^all

For consistency, there is also the explicit wildcard scope syntax * (or its alphabetic synonym all). Consequently, the examples above are equivalent to the explicitly scoped variants:

allow-newer: all:bar, *:baz, *:^quux

allow-newer: *:*
allow-newer: all:all

allow-newer: *:^*
allow-newer: all:^all

In order to ignore all bounds specified by a package pkg-1.2.3 you can combine scoping with a right-hand-side wildcard like so

-- Disregard any upper bounds specified by pkg-1.2.3
allow-newer: pkg-1.2.3:*

-- Disregard only `^>=`-style upper bounds in pkg-1.2.3
allow-newer: pkg-1.2.3:^*

allow-newer is often used in conjunction with a constraint (in the cfg-field:constraints field) forcing the usage of a specific, newer version of a package.

The command line variant of this field is e.g. --allow-newer=bar. A bare --allow-newer is equivalent to --allow-newer=all.

allow-older: none, all, list of scoped package names (space or comma separated)
--allow-older , --allow-older=[none, all , [scope:][^]pkg]
Since:Cabal 2.0
Default value:none

Like allow-newer, but applied to lower bounds rather than upper bounds.

The command line variant of this field is --allow-older=all. A bare --allow-older is equivalent to --allow-older=all.

index-state: HEAD, unix-timestamp, ISO8601 UTC timestamp.
Since:Cabal 2.0
Default value:HEAD

This allows to change the source package index state the solver uses to compute install-plans. This is particularly useful in combination with freeze-files in order to also freeze the state the package index was in at the time the install-plan was frozen.

-- UNIX timestamp format example
index-state: @1474739268

-- ISO8601 UTC timestamp format example
-- This format is used by 'cabal v2-configure'
-- for storing `--index-state` values.
index-state: 2016-09-24T17:47:48Z
reject-unconstrained-dependencies: all, none
--reject-unconstrained-dependencies=[all|none]
Default value:none
Since:2.6

By default, the dependency solver can include any package that it’s aware of in a build plan. If you wish to restrict the build plan to a closed set of packages (e.g., from a freeze file), use this flag.

When set to all, all non-local packages that aren’t goals must be explicitly constrained. When set to none, the solver will consider all packages.

5.5.4. Package configuration options

Package options affect the building of specific packages. There are three ways a package option can be specified:

  • They can be specified at the top-level, in which case they apply only to local package, or
  • They can be specified inside a package stanza, in which case they apply to the build of the package, whether or not it is local or external.
  • They can be specified inside an package * stanza, in which case they apply to all packages, local ones from the project and also external dependencies.

For example, the following options specify that optimization should be turned off for all local packages, and that bytestring (possibly an external dependency) should be built with -fno-state-hack:

optimization: False

package bytestring
    ghc-options: -fno-state-hack

ghc-options is not specifically described in this documentation, but is one of many fields for configuring programs. They take the form progname-options and progname-location, and can only be set inside package stanzas. (TODO: They are not supported at top-level, see #3579.)

At the moment, there is no way to specify an option to apply to all external packages or all inplace packages. Additionally, it is only possible to specify these options on the command line for all local packages (there is no per-package command line interface.)

Some flags were added by more recent versions of the Cabal library. This means that they are NOT supported by packages which use Custom setup scripts that require a version of the Cabal library older than when the feature was added.

flags: list of +flagname or -flagname (space separated)
--flags="+foo -bar", -ffoo , -f-bar

Force all flags specified as +flagname to be true, and all flags specified as -flagname to be false. For example, to enable the flag foo and disable bar, set:

flags: +foo -bar

If there is no leading punctuation, it is assumed that the flag should be enabled; e.g., this is equivalent:

flags: foo -bar

Flags are per-package, so it doesn’t make much sense to specify flags at the top-level, unless you happen to know that all of your local packages support the same named flags. If a flag is not supported by a package, it is ignored.

See also the solver configuration field constraints.

The command line variant of this flag is --flags. There is also a shortened form -ffoo -f-bar.

A common mistake is to say cabal v2-build -fhans, where hans is a flag for a transitive dependency that is not in the local package; in this case, the flag will be silently ignored. If haskell-tor is the package you want this flag to apply to, try --constraint="haskell-tor +hans" instead.

with-compiler: executable
--with-compiler=executable

Specify the path to a particular compiler to be used. If not an absolute path, it will be resolved according to the PATH environment. The type of the compiler (GHC, GHCJS, etc) must be consistent with the setting of the compiler field.

The most common use of this option is to specify a different version of your compiler to be used; e.g., if you have ghc-7.8 in your path, you can specify with-compiler: ghc-7.8 to use it.

This flag also sets the default value of with-hc-pkg, using the heuristic that it is named ghc-pkg-7.8 (if your executable name is suffixed with a version number), or is the executable named ghc-pkg in the same directory as the ghc directory. If this heuristic does not work, set with-hc-pkg explicitly.

For inplace packages, cabal v2-build maintains a separate build directory for each version of GHC, so you can maintain multiple build trees for different versions of GHC without clobbering each other.

At the moment, it’s not possible to set with-compiler on a per-package basis, but eventually we plan on relaxing this restriction. If this is something you need, give us a shout.

The command line variant of this flag is --with-compiler=ghc-7.8; there is also a short version -w ghc-7.8.

with-hc-pkg: executable
--with-hc-pkg=executable

Specify the path to the package tool, e.g., ghc-pkg. This package tool must be compatible with the compiler specified by with-compiler (generally speaking, it should be precisely the tool that was distributed with the compiler). If this option is omitted, the default value is determined from with-compiler.

The command line variant of this flag is --with-hc-pkg=ghc-pkg-7.8.

optimization: nat
--enable-optimization
--disable-optimization
Default value:1

Build with optimization. This is appropriate for production use, taking more time to build faster libraries and programs.

The optional nat value is the optimisation level. Some compilers support multiple optimisation levels. The range is 0 to 2. Level 0 disables optimization, level 1 is the default. Level 2 is higher optimisation if the compiler supports it. Level 2 is likely to lead to longer compile times and bigger generated code. If you are not planning to run code, turning off optimization will lead to better build times and less code to be rebuilt when a module changes.

When optimizations are enabled, Cabal passes -O2 to the C compiler.

We also accept True (equivalent to 1) and False (equivalent to 0).

Note that as of GHC 8.0, GHC does not recompile when optimization levels change (see GHC #10923), so if you change the optimization level for a local package you may need to blow away your old build products in order to rebuild with the new optimization level.

The command line variant of this flag is -O2 (with -O1 equivalent to -O). There are also long-form variants --enable-optimization and --disable-optimization.

configure-options: args (space separated)
--configure-option=arg

A list of extra arguments to pass to the external ./configure script, if one is used. This is only useful for packages which have the Configure build type. See also the section on system-dependent parameters.

The command line variant of this flag is --configure-option=arg, which can be specified multiple times to pass multiple options.

compiler: ghc, ghcjs, jhc, lhc, uhc or haskell-suite
--compiler=compiler
Default value:ghc

Specify which compiler toolchain to be used. This is independent of with-compiler, because the choice of toolchain affects Cabal’s build logic.

The command line variant of this flag is --compiler=ghc.

tests: boolean
--enable-tests
--disable-tests
Default value:False

Force test suites to be enabled. For most users this should not be needed, as we always attempt to solve for test suite dependencies, even when this value is False; furthermore, test suites are automatically enabled if they are requested as a built target.

The command line variant of this flag is --enable-tests and --disable-tests.

benchmarks: boolean
--enable-benchmarks
--disable-benchmarks
Default value:False

Force benchmarks to be enabled. For most users this should not be needed, as we always attempt to solve for benchmark dependencies, even when this value is False; furthermore, benchmarks are automatically enabled if they are requested as a built target.

The command line variant of this flag is --enable-benchmarks and --disable-benchmarks.

extra-prog-path: paths (newline or comma separated)
--extra-prog-path=PATH
Since:Cabal 1.18

A list of directories to search for extra required programs. Most users should not need this, as programs like happy and alex will automatically be installed and added to the path. This can be useful if a Custom setup script relies on an exotic extra program.

The command line variant of this flag is --extra-prog-path=PATH, which can be specified multiple times.

run-tests: boolean
--run-tests
Default value:False

Run the package test suite upon installation. This is useful for saying “When this package is installed, check that the test suite passes, terminating the rest of the build if it is broken.”

Warning

One deficiency: the run-tests setting of a package is NOT recorded as part of the hash, so if you install something without run-tests and then turn on run-tests, we won’t subsequently test the package. If this is causing you problems, give us a shout.

The command line variant of this flag is --run-tests.

debug-info: integer
--enable-debug-info=⟨n⟩
--disable-debug-info
Since:Cabal 1.22
Default value:False

If the compiler (e.g., GHC 7.10 and later) supports outputing OS native debug info (e.g., DWARF), setting debug-info: True will instruct it to do so. See the GHC wiki page on DWARF for more information about this feature.

(This field also accepts numeric syntax, but until GHC 8.2 this didn’t do anything.)

The command line variant of this flag is --enable-debug-info and --disable-debug-info.

split-sections: boolean
--enable-split-sections
--disable-split-sections
Since:Cabal 2.2
Default value:False

Use the GHC -split-sections feature when building the library. This reduces the final size of the executables that use the library by allowing them to link with only the bits that they use rather than the entire library. The downside is that building the library takes longer and uses a bit more memory.

This feature is supported by GHC 8.0 and later.

The command line variant of this flag is --enable-split-sections and --disable-split-sections.

split-objs: boolean
--enable-split-objs
--disable-split-objs
Default value:False

Use the GHC -split-objs feature when building the library. This reduces the final size of the executables that use the library by allowing them to link with only the bits that they use rather than the entire library. The downside is that building the library takes longer and uses considerably more memory.

It is generally recommend that you use split-sections instead of split-objs where possible.

The command line variant of this flag is --enable-split-objs and --disable-split-objs.

executable-stripping: boolean
--enable-executable-stripping
--disable-executable-stripping
Default value:True

When installing binary executable programs, run the strip program on the binary. This can considerably reduce the size of the executable binary file. It does this by removing debugging information and symbols.

Not all Haskell implementations generate native binaries. For such implementations this option has no effect.

(TODO: Check what happens if you combine this with debug-info.)

The command line variant of this flag is --enable-executable-stripping and --disable-executable-stripping.

library-stripping: boolean
--enable-library-stripping
--disable-library-stripping
Since:Cabal 1.20

When installing binary libraries, run the strip program on the binary, saving space on the file system. See also executable-stripping.

The command line variant of this flag is --enable-library-stripping and --disable-library-stripping.

program-prefix: prefix
--program-prefix=prefix

[STRIKEOUT:Prepend prefix to installed program names.] (Currently implemented in a silly and not useful way. If you need this to work give us a shout.)

prefix may contain the following path variables: $pkgid, $pkg, $version, $compiler, $os, $arch, $abi, $abitag

The command line variant of this flag is --program-prefix=foo-.

program-suffix: suffix
--program-suffix=suffix

[STRIKEOUT:Append suffix to installed program names.] (Currently implemented in a silly and not useful way. If you need this to work give us a shout.)

The most obvious use for this is to append the program’s version number to make it possible to install several versions of a program at once: program-suffix: $version.

suffix may contain the following path variables: $pkgid, $pkg, $version, $compiler, $os, $arch, $abi, $abitag

The command line variant of this flag is --program-suffix='$version'.

shared: boolean
--enable-shared
--disable-shared
Default value:False

Build shared library. This implies a separate compiler run to generate position independent code as required on most platforms.

The command line variant of this flag is --enable-shared and --disable-shared.

executable-dynamic: boolean
--enable-executable-dynamic
--disable-executable-dynamic
Default value:False

Link executables dynamically. The executable’s library dependencies should be built as shared objects. This implies shared: True unless shared: False is explicitly specified.

The command line variant of this flag is --enable-executable-dynamic and --disable-executable-dynamic.

library-for-ghci: boolean
--enable-library-for-ghci
--disable-library-for-ghci
Default value:True

Build libraries suitable for use with GHCi. This involves an extra linking step after the build.

Not all platforms support GHCi and indeed on some platforms, trying to build GHCi libs fails. In such cases, consider setting library-for-ghci: False.

The command line variant of this flag is --enable-library-for-ghci and --disable-library-for-ghci.

relocatable:
--relocatable
Since:Cabal 1.22
Default value:False

[STRIKEOUT:Build a package which is relocatable.] (TODO: It is not clear what this actually does, or if it works at all.)

The command line variant of this flag is --relocatable.

static: boolean
--enable-static
--disable-static
Default value:False

Roll this and all dependent libraries into a combined .a archive. This uses GHCs -staticlib flag, which is available for iOS and with GHC 8.4 and later for other platforms as well.

executable-static: boolean
--enable-executable-static
--disable-executable-static
Default value:False

Build fully static executables. This link all dependent libraries into executables statically, including libc. This passes -static and -optl=-static to GHC.

extra-include-dirs: directories (comma or newline separated list)
--extra-include-dirs=DIR

An extra directory to search for C header files. You can use this flag multiple times to get a list of directories.

You might need to use this flag if you have standard system header files in a non-standard location that is not mentioned in the package’s .cabal file. Using this option has the same affect as appending the directory dir to the include-dirs field in each library and executable in the package’s .cabal file. The advantage of course is that you do not have to modify the package at all. These extra directories will be used while building the package and for libraries it is also saved in the package registration information and used when compiling modules that use the library.

The command line variant of this flag is --extra-include-dirs=DIR, which can be specified multiple times.

extra-lib-dirs: directories (comma or newline separated list)
--extra-lib-dirs=DIR

An extra directory to search for system libraries files.

The command line variant of this flag is --extra-lib-dirs=DIR, which can be specified multiple times.

extra-framework-dirs: directories (comma or newline separated list)
--extra-framework-dirs=DIR

An extra directory to search for frameworks (OS X only).

You might need to use this flag if you have standard system libraries in a non-standard location that is not mentioned in the package’s .cabal file. Using this option has the same affect as appending the directory dir to the extra-lib-dirs field in each library and executable in the package’s .cabal file. The advantage of course is that you do not have to modify the package at all. These extra directories will be used while building the package and for libraries it is also saved in the package registration information and used when compiling modules that use the library.

The command line variant of this flag is --extra-framework-dirs=DIR, which can be specified multiple times.

profiling: boolean
--enable-profiling
--disable-profiling
Since:Cabal 1.22
Default value:False

Build libraries and executables with profiling enabled (for compilers that support profiling as a separate mode). It is only necessary to specify profiling for the specific package you want to profile; cabal v2-build will ensure that all of its transitive dependencies are built with profiling enabled.

To enable profiling for only libraries or executables, see library-profiling and executable-profiling.

For useful profiling, it can be important to control precisely what cost centers are allocated; see profiling-detail.

The command line variant of this flag is --enable-profiling and --disable-profiling.

profiling-detail: level
--profiling-detail=level
Since:Cabal 1.24

Some compilers that support profiling, notably GHC, can allocate costs to different parts of the program and there are different levels of granularity or detail with which this can be done. In particular for GHC this concept is called “cost centers”, and GHC can automatically add cost centers, and can do so in different ways.

This flag covers both libraries and executables, but can be overridden by the library-profiling-detail field.

Currently this setting is ignored for compilers other than GHC. The levels that cabal currently supports are:

default
For GHC this uses exported-functions for libraries and toplevel-functions for executables.
none
No costs will be assigned to any code within this component.
exported-functions
Costs will be assigned at the granularity of all top level functions exported from each module. In GHC, this is for non-inline functions. Corresponds to -fprof-auto-exported.
toplevel-functions
Costs will be assigned at the granularity of all top level functions in each module, whether they are exported from the module or not. In GHC specifically, this is for non-inline functions. Corresponds to -fprof-auto-top.
all-functions
Costs will be assigned at the granularity of all functions in each module, whether top level or local. In GHC specifically, this is for non-inline toplevel or where-bound functions or values. Corresponds to -fprof-auto.

The command line variant of this flag is --profiling-detail=none.

library-profiling-detail: level
--library-profiling-detail=level
Since:Cabal 1.24

Like profiling-detail, but applied only to libraries

The command line variant of this flag is --library-profiling-detail=none.

library-vanilla: boolean
--enable-library-vanilla
--disable-library-vanilla
Default value:True

Build ordinary libraries (as opposed to profiling libraries). Mostly, you can set this to False to avoid building ordinary libraries when you are profiling.

The command line variant of this flag is --enable-library-vanilla and --disable-library-vanilla.

library-profiling: boolean
--enable-library-profiling
--disable-library-profiling
Since:Cabal 1.22
Default value:False

Build libraries with profiling enabled. You probably want to use profiling instead.

The command line variant of this flag is --enable-library-profiling and --disable-library-profiling.

executable-profiling: boolean
--enable-executable-profiling
--disable-executable-profiling
Since:Cabal 1.22
Default value:False

Build executables with profiling enabled. You probably want to use profiling instead.

The command line variant of this flag is --enable-executable-profiling and --disable-executable-profiling.

coverage: boolean
--enable-coverage
--disable-coverage
Since:Cabal 1.22
Default value:False

Build libraries and executables (including test suites) with Haskell Program Coverage enabled. Running the test suites will automatically generate coverage reports with HPC.

The command line variant of this flag is --enable-coverage and --disable-coverage.

library-coverage: boolean
--enable-library-coverage
--disable-library-coverage
Deprecated:
Since:Cabal 1.22
Default value:False

Deprecated, use coverage.

The command line variant of this flag is --enable-library-coverage and --disable-library-coverage.

documentation: boolean
--enable-documentation
--disable-documentation
Default value:False

Enables building of Haddock documentation

The command line variant of this flag is --enable-documentation and --disable-documentation.

documentation: true does not imply haddock-benchmarks, haddock-executables, haddock-internal or haddock-tests. These need to be enabled separately if desired.

doc-index-file: templated path
--doc-index-file=TEMPLATE

A central index of Haddock API documentation (template cannot use $pkgid), which should be updated as documentation is built.

The command line variant of this flag is --doc-index-file=TEMPLATE

The following commands are equivalent to ones that would be passed when running setup haddock. (TODO: Where does the documentation get put.)

haddock-hoogle: boolean
Default value:False

Generate a text file which can be converted by Hoogle into a database for searching. This is equivalent to running haddock with the --hoogle flag.

The command line variant of this flag is --hoogle (for the haddock command).

haddock-html: boolean
Default value:True

Build HTML documentation.

The command line variant of this flag is --html (for the haddock command).

haddock-html-location: templated path

Specify a template for the location of HTML documentation for prerequisite packages. The substitutions are applied to the template to obtain a location for each package, which will be used by hyperlinks in the generated documentation. For example, the following command generates links pointing at [Hackage] pages:

html-location: 'http://hackage.haskell.org/packages/archive/$pkg/latest/doc/html'

Here the argument is quoted to prevent substitution by the shell. If this option is omitted, the location for each package is obtained using the package tool (e.g. ghc-pkg).

The command line variant of this flag is --html-location (for the haddock subcommand).

haddock-executables: boolean
Default value:False

Run haddock on all executable programs.

The command line variant of this flag is --executables (for the haddock subcommand).

haddock-tests: boolean
Default value:False

Run haddock on all test suites.

The command line variant of this flag is --tests (for the haddock subcommand).

haddock-benchmarks: boolean
Default value:False

Run haddock on all benchmarks.

The command line variant of this flag is --benchmarks (for the haddock subcommand).

haddock-all: boolean
Default value:False

Run haddock on all components.

The command line variant of this flag is --all (for the haddock subcommand).

haddock-internal: boolean
Default value:False

Build haddock documentation which includes unexposed modules and symbols.

The command line variant of this flag is --internal (for the haddock subcommand).

haddock-css: path

The CSS file that should be used to style the generated documentation (overriding haddock’s default.)

The command line variant of this flag is --css (for the haddock subcommand).

Default value:False

Generated hyperlinked source code using HsColour, and have Haddock documentation link to it.

The command line variant of this flag is --hyperlink-source (for the haddock subcommand).

haddock-hscolour-css: path

The CSS file that should be used to style the generated hyperlinked source code (from HsColour).

The command line variant of this flag is --hscolour-css (for the haddock subcommand).

haddock-contents-location: URL

A baked-in URL to be used as the location for the contents page.

The command line variant of this flag is --contents-location (for the haddock subcommand).

haddock-keep-temp-files: boolean

Keep temporary files.

The command line variant of this flag is --keep-temp-files (for the haddock subcommand).

5.5.5. Advanced global configuration options

write-ghc-environment-files: always, never, or ghc8.4.4+
--write-ghc-environment-files=policy
Default value:never

Whether a GHC package environment file should be created after a successful build.

Since Cabal 3.0, defaults to never. Before that, defaulted to creating them only when compiling with GHC 8.4.4 and older (GHC 8.4.4 is the first version that supports the -package-env - option that allows ignoring the package environment files).

http-transport: curl, wget, powershell, or plain-http
--http-transport=transport
Default value:curl

Set a transport to be used when making http(s) requests.

The command line variant of this field is --http-transport=curl.

ignore-expiry: boolean
--ignore-expiry
Default value:False

If True, we will ignore expiry dates on metadata from Hackage.

In general, you should not set this to True as it will leave you vulnerable to stale cache attacks. However, it may be temporarily useful if the main Hackage server is down, and we need to rely on mirrors which have not been updated for longer than the expiry period on the timestamp.

The command line variant of this field is --ignore-expiry.

remote-repo-cache: directory
--remote-repo-cache=DIR
Default value:~/.cabal/packages

[STRIKEOUT:The location where packages downloaded from remote repositories will be cached.] Not implemented yet.

The command line variant of this flag is --remote-repo-cache=DIR.

logs-dir: directory
--logs-dir=DIR
Default value:~/.cabal/logs

[STRIKEOUT:The location where build logs for packages are stored.] Not implemented yet.

The command line variant of this flag is --logs-dir=DIR.

build-summary: template filepath
--build-summary=TEMPLATE
Default value:~/.cabal/logs/build.log

[STRIKEOUT:The file to save build summaries. Valid variables which can be used in the path are $pkgid, $compiler, $os and $arch.] Not implemented yet.

The command line variant of this flag is --build-summary=TEMPLATE.

local-repo: directory
--local-repo=DIR
Deprecated:

[STRIKEOUT:The location of a local repository.] Deprecated. See “Legacy repositories.”

The command line variant of this flag is --local-repo=DIR.

world-file: path
--world-file=FILE
Deprecated:

[STRIKEOUT:The location of the world file.] Deprecated.

The command line variant of this flag is --world-file=FILE.

Undocumented fields: root-cmd, symlink-bindir, build-log, remote-build-reporting, report-planned-failure, one-shot, offline.

Most users generally won’t need these.

solver: modular
--solver=modular

This field is reserved to allow the specification of alternative dependency solvers. At the moment, the only accepted option is modular.

The command line variant of this field is --solver=modular.

max-backjumps: nat
--max-backjumps=N
Default value:4000

Maximum number of backjumps (backtracking multiple steps) allowed while solving. Set -1 to allow unlimited backtracking, and 0 to disable backtracking completely.

The command line variant of this field is --max-backjumps=4000.

reorder-goals: boolean
--reorder-goals
--no-reorder-goals
Default value:False

When enabled, the solver will reorder goals according to certain heuristics. Slows things down on average, but may make backtracking faster for some packages. It’s unlikely to help for small projects, but for big install plans it may help you find a plan when otherwise this is not possible. See #1780 for more commentary.

The command line variant of this field is --(no-)reorder-goals.

count-conflicts: boolean
--count-conflicts
--no-count-conflicts
Default value:True

Try to speed up solving by preferring goals that are involved in a lot of conflicts.

The command line variant of this field is --(no-)count-conflicts.

minimize-conflict-set: boolean
--minimize-conflict-set
--no-minimize-conflict-set
Default value:False

When there is no solution, try to improve the solver error message by finding a minimal conflict set. This option may increase run time significantly, so it is off by default.

The command line variant of this field is --(no-)minimize-conflict-set.

strong-flags: boolean
--strong-flags
--no-strong-flags
Default value:False

Do not defer flag choices. (TODO: Better documentation.)

The command line variant of this field is --(no-)strong-flags.

allow-boot-library-installs: boolean
--allow-boot-library-installs
--no-allow-boot-library-installs
Default value:False

By default, the dependency solver doesn’t allow base, ghc-prim, integer-simple, integer-gmp, and template-haskell to be installed or upgraded. This flag removes the restriction.

The command line variant of this field is --(no-)allow-boot-library-installs.

cabal-lib-version: version
--cabal-lib-version=version

This field selects the version of the Cabal library which should be used to build packages. This option is intended primarily for internal development use (e.g., forcing a package to build with a newer version of Cabal, to test a new version of Cabal.) (TODO: Specify its semantics more clearly.)

The command line variant of this field is --cabal-lib-version=1.24.0.1.