nix-local-build.rst

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 :doc:`How to package Haskell code <how-to-package-haskell-code>` for instructions). You can configure and build it using Nix-style local builds with this command (configuring is not necessary):

$ cabal build

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

$ cabal repl

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

$ cabal run <executable name> [executable args]

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 build all

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

$ cd cabal-install
$ cabal build

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

$ cabal 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 build package-tests

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

Cookbook

How can I profile my library/application?

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

profiling: True

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

package p
    profiling-detail: toplevel-functions

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

How can I have a reproducible set of versions for my dependencies?

You can use cabal freeze to save the solver results to a file.

Since Cabal 3.8, an alternative approach is to use a :ref:`remote project configuration file<conditionals and imports>`: to specify a set of versions for packages.

One provider of such package sets is Stackage, and its package sets are called snapshots. The Stackage snapshots contain a set of packages from Hackage that have all been verified to build with a given version of GHC.

For example, the snapshot named lts-19.2 contains versioned packages which all compile on GHC 9.0.2. You can conveniently review the versions of packages in lts-19.2. Using the following cabal.project file, Cabal will use the versions of packages that the this snapshot specifies:

packages: .
import: https://www.stackage.org/lts-19.2/cabal.config

Please note that project files do not get bundled in Cabal package tarballs, made using e.g. cabal sdist. Project files are intended for use in local development environments.

How it works

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 fields of a project. Packages in the former two fields will usually have their source code stored in a folder in your project, while extra-packages lists packages residing on Hackage that are treated as being local anyway.

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.

2. An external package is any package which is not listed in the packages, optional-packages and extra-packages fields. 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 based on all of the inputs which 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.

Use cabal path --store-dir to show where your global package store is located. This is configurable via the 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 (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 build are only applied to local packages, so that adding a flag to cabal 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.

Where are my build products?

A major deficiency in the current implementation of cabal 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 cabal 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.

Caching

Nix-style local builds support a robust caching system which helps to 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 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 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 is determined using cabal sdist --list-only. Thus if you do not list all your source files in a Cabal file, Cabal may fail to recompile when you edit them.

config (binary)

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.