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diff --git a/third_party/bazel/rules_haskell/docs/haskell-use-cases.rst b/third_party/bazel/rules_haskell/docs/haskell-use-cases.rst deleted file mode 100644 index a8c4340cf7..0000000000 --- a/third_party/bazel/rules_haskell/docs/haskell-use-cases.rst +++ /dev/null @@ -1,283 +0,0 @@ -.. _use-cases: - -Common Haskell Build Use Cases -============================== - -Picking a compiler ------------------- - -Unlike Bazel's native C++ rules, rules_haskell does not auto-detect -a Haskell compiler toolchain from the environment. This is by design. -We require that you declare a compiler to use in your ``WORKSPACE`` -file. - -There are two common sources for a compiler. One is to use the -official binary distributions from `haskell.org`_. This is done using -the `ghc_bindist`_ rule. - -The compiler can also be pulled from Nixpkgs_, a set of package -definitions for the `Nix package manager`_. Pulling the compiler from -Nixpkgs makes the build more hermetic, because the transitive closure -of the compiler and all its dependencies is precisely defined in the -``WORKSPACE`` file. Use `rules_nixpkgs`_ to do so (where ``X.Y.Z`` -stands for any recent release):: - - load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") - - http_archive( - name = "io_tweag_rules_nixpkgs", - strip_prefix = "rules_nixpkgs-X.Y.Z", - urls = ["https://github.com/tweag/rules_nixpkgs/archive/vX.Y.Z.tar.gz"], - ) - - load( - "@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", - "nixpkgs_git_repository", - "nixpkgs_package" - ) - - nixpkgs_git_repository( - name = "nixpkgs", - revision = "18.09", # Any tag or commit hash - ) - - nixpkgs_package( - name = "ghc", - repositories = { "nixpkgs": "@nixpkgs//:default.nix" } - attribute_path = "haskell.compiler.ghc843", # Any compiler version - build_file = "@io_tweag_rules_haskell//haskell:ghc.BUILD", - ) - - register_toolchains("//:ghc") - -This workspace description specifies which Nixpkgs version to use, -then exposes a Nixpkgs package containing the GHC compiler. The -description assumes that there exists a ``BUILD`` file at the root of -the repository that includes the following:: - - haskell_toolchain( - name = "ghc", - # Versions here and in WORKSPACE must match. - version = "8.4.3", - # Use binaries from @ghc//:bin to define //:ghc toolchain. - tools = ["@ghc//:bin"], - ) - -.. _Bazel+Nix blog post: https://www.tweag.io/posts/2018-03-15-bazel-nix.html -.. _Nix package manager: https://nixos.org/nix -.. _Nixpkgs: https://nixos.org/nixpkgs/manual/ -.. _ghc_bindist: http://api.haskell.build/haskell/ghc_bindist.html#ghc_bindist -.. _haskell.org: https://haskell.org -.. _haskell_binary: http://api.haskell.build/haskell/haskell.html#haskell_binary -.. _haskell_library: http://api.haskell.build/haskell/haskell.html#haskell_library -.. _rules_nixpkgs: https://github.com/tweag/rules_nixpkgs - -Loading targets in a REPL -------------------------- - -Rebuilds are currently not incremental *within* a binary or library -target (rebuilds are incremental across targets of course). Any change -in any source file will trigger a rebuild of all source files listed -in a target. In Bazel, it is conventional to decompose libraries into -small units. In this way, libraries require less work to rebuild. -Still, for interactive development full incrementality and fast -recompilation times are crucial for a good developer experience. We -recommend making all development REPL-driven for fast feedback when -source files change. - -Every `haskell_binary`_ and every `haskell_library`_ target has an -optional executable output that can be run to drop you into an -interactive session. If the target's name is ``foo``, then the REPL -output is called ``foo@repl``. - -Consider the following binary target:: - - haskell_binary( - name = "hello", - srcs = ["Main.hs", "Other.hs"], - deps = ["//lib:some_lib"], - ) - -The target above also implicitly defines ``hello@repl``. You can call -the REPL like this (requires Bazel 0.15 or later):: - - $ bazel run //:hello@repl - -This works for any ``haskell_binary`` or ``haskell_library`` target. -Modules of all libraries will be loaded in interpreted mode and can be -reloaded using the ``:r`` GHCi command when source files change. - -Building code with Hackage dependencies (using Nix) ---------------------------------------------------- - -Each Haskell library or binary needs a simple build description to -tell Bazel what source files to use and what the dependencies are, if -any. Packages on Hackage don't usually ship with `BUILD.bazel` files. -So if your code depends on them, you either need to write a build -description for each package, generate one (see next section), or -decide not to use Bazel to build packages published on Hackage. This -section documents one way to do the latter. - -Nix is a package manager. The set of package definitions is called -Nixpkgs. This repository contains definitions for most actively -maintained Cabal packages published on Hackage. Where these packages -depend on system libraries like zlib, ncurses or libpng, Nixpkgs also -contains package descriptions for those, and declares those as -dependencies of the Cabal packages. Since these definitions already -exist, we can reuse them instead of rewriting these definitions as -build definitions in Bazel. See the `Bazel+Nix blog post`_ for a more -detailed rationale. - -To use Nixpkgs in Bazel, we need `rules_nixpkgs`_. See `Picking -a compiler`_ for how to import Nixpkgs rules into your workspace and -how to use a compiler from Nixpkgs. To use Cabal packages from -Nixpkgs, replace the compiler definition with the following:: - - nixpkgs_package( - name = "ghc", - repositories = { "nixpkgs": "@nixpkgs//:default.nix" }, - nix_file = "//:ghc.nix", - build_file = "@io_tweag_rules_haskell//haskell:ghc.BUILD", - ) - -This definition assumes a ``ghc.nix`` file at the root of the -repository. In this file, you can use the Nix expression language to -construct a compiler with all the packages you depend on in scope:: - - with (import <nixpkgs> {}); - - haskellPackages.ghcWithPackages (p: with p; [ - containers - lens - text - ]) - -Each package mentioned in ``ghc.nix`` can then be imported using -`haskell_toolchain_library`_ in ``BUILD`` files. - -.. _haskell_toolchain_library: http://api.haskell.build/haskell/haskell.html#haskell_toolchain_library - -Building code with Hackage dependencies (using Hazel) ------------------------------------------------------ - -.. todo:: - - Explain how to use Hazel instead of Nix - -Generating API documentation ----------------------------- - -The `haskell_doc`_ rule can be used to build API documentation for -a given library (using Haddock). Building a target called -``//my/pkg:mylib_docs`` would make the documentation available at -``bazel-bin/my/pkg/mylib_docs/index/index.html``. - -Alternatively, you can use the -``@io_tweag_rules_haskell//haskell:haskell.bzl%haskell_doc_aspect`` -aspect to ask Bazel from the command-line to build documentation for -any given target (or indeed all targets), like in the following: - -.. code-block:: console - - $ bazel build //my/pkg:mylib \ - --aspects @io_tweag_rules_haskell//haskell:haskell.bzl%haskell_doc_aspect - -.. _haskell_doc: http://api.haskell.build/haskell/haddock.html#haskell_doc - -Linting your code ------------------ - -The `haskell_lint`_ rule does not build code but runs the GHC -typechecker on all listed dependencies. Warnings are treated as -errors. - -Alternatively, you can directly check a target using - -.. code-block:: console - - $ bazel build //my/haskell:target \ - --aspects @io_tweag_rules_haskell//haskell:haskell.bzl%haskell_lint_aspect - -.. _haskell_lint: http://api.haskell.build/haskell/lint.html#haskell_lint - -Checking code coverage ----------------------- - -"Code coverage" is the name given to metrics that describe how much source -code is covered by a given test suite. A specific code coverage metric -implemented here is expression coverage, or the number of expressions in -the source code that are explored when the tests are run. - -Haskell's ``ghc`` compiler has built-in support for code coverage analysis, -through the hpc_ tool. The Haskell rules allow the use of this tool to analyse -``haskell_library`` coverage by ``haskell_test`` rules. To do so, you have a -few options. You can add -``expected_covered_expressions_percentage=<some integer between 0 and 100>`` to -the attributes of a ``haskell_test``, and if the expression coverage percentage -is lower than this amount, the test will fail. Alternatively, you can add -``expected_uncovered_expression_count=<some integer greater or equal to 0>`` to -the attributes of a ``haskell_test``, and instead the test will fail if the -number of uncovered expressions is greater than this amount. Finally, you could -do both at once, and have both of these checks analyzed by the coverage runner. -To see the coverage details of the test suite regardless of if the test passes -or fails, add ``--test_output=all`` as a flag when invoking the test, and there -will be a report in the test output. You will only see the report if you -required a certain level of expression coverage in the rule attributes. - -For example, your BUILD file might look like this: :: - - haskell_library( - name = "lib", - srcs = ["Lib.hs"], - deps = [ - "//tests/hackage:base", - ], - ) - - haskell_test( - name = "test", - srcs = ["Main.hs"], - deps = [ - ":lib", - "//tests/hackage:base", - ], - expected_covered_expressions_percentage = 80, - expected_uncovered_expression_count = 10, - ) - -And if you ran ``bazel coverage //somepackage:test --test_output=all``, you -might see a result like this: :: - - INFO: From Testing //somepackage:test: - ==================== Test output for //somepackage:test: - Overall report - 100% expressions used (9/9) - 100% boolean coverage (0/0) - 100% guards (0/0) - 100% 'if' conditions (0/0) - 100% qualifiers (0/0) - 100% alternatives used (0/0) - 100% local declarations used (0/0) - 100% top-level declarations used (3/3) - ============================================================================= - -Here, the test passes because it actually has 100% expression coverage and 0 -uncovered expressions, which is even better than we expected on both counts. - -There is an optional ``haskell_test`` attribute called -``strict_coverage_analysis``, which is a boolean that changes the coverage -analysis such that even having better coverage than expected fails the test. -This can be used to enforce that developers must upgrade the expected test -coverage when they improve it. On the other hand, it requires changing the -expected coverage for almost any change. - -There a couple of notes regarding the coverage analysis functionality: - -- Coverage analysis currently is scoped to all source files and all - locally-built Haskell dependencies (both direct and transitive) for a given - test rule. -- Coverage-enabled build and execution for ``haskell_test`` targets may take - longer than regular. However, this has not effected regular ``run`` / - ``build`` / ``test`` performance. - -.. _hpc: <http://hackage.haskell.org/package/hpc> |