Implement --{enable,disable}-executables

[hvr]

This may have been subsumed by other tickets (e.g. #4087) but I figured it may be worth sketching a more modest enhancement which "completes" cabal's UI (scroll down for rationale):

What?

We already have the ability to enable/disable tests and benchmark, via

    --enable-tests                   Enable dependency checking and compilation
                                     for test suites listed in the package
                                     description file.
    --disable-tests                  Disable dependency checking and
                                     compilation for test suites listed in the
                                     package description file.

    --enable-benchmarks              Enable dependency checking and compilation
                                     for benchmarks listed in the package
                                     description file.
    --disable-benchmarks             Disable dependency checking and
                                     compilation for benchmarks listed in the
                                     package description file.

which translates to cabal.project files as

tests: True
benchmarks: False

package bar
  tests: False
  benchmarks: True

as well as the --constraint syntax to force the solver to enable tests/benchmarks

  --constraint="bar bench"
  --constraint="bar test"

(but I'm not sure this works properly in new-build btw; also I'm missing a no-bench/no-test token for symmetry)

What I'm suggesting now is to extend this logic to executable components, i.e.

    --enable-executables             Enable dependency checking and compilation
                                     for executables listed in the package
                                     description file.
    --disable-executables            Disable dependency checking and
                                     compilation for executables listed in the
                                     package description file.

and

  --constraint="bar exe"
  --constraint="bar no-exe"

as well as the respective executables: <bool> property for cabal.project files

The default settings would be:

• executables are enabled by default for local packages (i.e. those specified via packages: ...)
• executables are obviously enabled by default where they're needed to satisfy build-tool-depends goals
• else, executables are disabled by default (e.g. when only the library of a package is requested via build-depends

Why?

Finally, the rationale for this feature request (beyond improving symmetry of executables w/ tests & benchmark) is the situation that you have packages which bundle libraries + executables to form the unit of distribution. This has become more attractive with new-build's per-component building, where you can depend on the library part of a package w/o cabal building all the executables (& their deps) as well.

However, cabal currently still has to solve for all executables even if all you ever need is the library component.

The library may often have a smaller dependency footprint and would therefore puts less pressure on the constraint solver.

However, the executables often have additional library dependencies, like e.g. optparse-applicative or even heavier dependencies (http libraries, GUI toolkits, terminal UI toolkits, etc).

So having to satisfy the executable's constraints provides no benefit for build-depends dependencies, and can instead result in the cabal solver not finding a solution, even though there would be if executables' constraints would have been ignored.

[grayjay]

@hvr #4087 should also allow cabal to skip building executables for non-local packages when they aren't needed as build tools. I was wondering how the two features should interact. Would it make sense for --enable-executables to force executables to be built but for --disable-executables to still allow executables to be built if they are needed as build tools? I'll probably start working on #4087 at the Haskell eXchange hackathon.

[runeksvendsen]

I'd like to add another reason to the Why section:

Building and linking executables is slow. If I just want to run the test suite of my local library, there should be no need to wait until an otherwise unused executable is done building and linking.

[phadej]

@runeksvendsen currently cabal solves for all executables, but doesn't build them. So your motivation point doesn't apply.

[runeksvendsen]

@phadej that's not the behaviour I'm seeing for cabal-install v3.6.2.0:

cabal file:

$ cat *.cabal
cabal-version: 1.12

-- This file has been generated from package.yaml by hpack version 0.34.4.
--
-- see: https://github.com/sol/hpack

name:           test-cabal-build-execs
version:        0.1.0.0
description:    Please see the README on GitHub at <https://github.com/githubuser/test-cabal-build-execs#readme>
homepage:       https://github.com/githubuser/test-cabal-build-execs#readme
bug-reports:    https://github.com/githubuser/test-cabal-build-execs/issues
author:         Author name here
maintainer:     [email protected]
copyright:      2022 Author name here
license:        BSD3
license-file:   LICENSE
build-type:     Simple
extra-source-files:
    README.md
    ChangeLog.md

source-repository head
  type: git
  location: https://github.com/githubuser/test-cabal-build-execs

library
  exposed-modules:
      Lib
  other-modules:
      Paths_test_cabal_build_execs
  hs-source-dirs:
      src
  build-depends:
      base >=4.7 && <5
  default-language: Haskell2010

executable test-cabal-build-execs-exe
  main-is: Main.hs
  other-modules:
      Paths_test_cabal_build_execs
  hs-source-dirs:
      app
  ghc-options: -threaded -rtsopts -with-rtsopts=-N
  build-depends:
      base >=4.7 && <5
    , test-cabal-build-execs
  default-language: Haskell2010

test-suite test-cabal-build-execs-test
  type: exitcode-stdio-1.0
  main-is: Spec.hs
  other-modules:
      Paths_test_cabal_build_execs
  hs-source-dirs:
      test
  ghc-options: -threaded -rtsopts -with-rtsopts=-N
  build-depends:
      base >=4.7 && <5
    , test-cabal-build-execs
  default-language: Haskell2010

Building:

$ cabal build --enable-tests
Resolving dependencies...
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - test-cabal-build-execs-0.1.0.0 (lib) (first run)
 - test-cabal-build-execs-0.1.0.0 (test:test-cabal-build-execs-test) (first run)
 - test-cabal-build-execs-0.1.0.0 (exe:test-cabal-build-execs-exe) (first run)
Configuring library for test-cabal-build-execs-0.1.0.0..
Preprocessing library for test-cabal-build-execs-0.1.0.0..
Building library for test-cabal-build-execs-0.1.0.0..
[1 of 2] Compiling Lib              ( src/Lib.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/build/Lib.o, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/build/Lib.dyn_o )
[2 of 2] Compiling Paths_test_cabal_build_execs ( /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/build/autogen/Paths_test_cabal_build_execs.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/build/Paths_test_cabal_build_execs.o, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/build/Paths_test_cabal_build_execs.dyn_o )
Configuring test suite 'test-cabal-build-execs-test' for test-cabal-build-execs-0.1.0.0..
Configuring executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
Preprocessing test suite 'test-cabal-build-execs-test' for test-cabal-build-execs-0.1.0.0..
Building test suite 'test-cabal-build-execs-test' for test-cabal-build-execs-0.1.0.0..
Preprocessing executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
Building executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
[1 of 2] Compiling Main             ( app/Main.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe-tmp/Main.o )
[1 of 2] Compiling Main             ( test/Spec.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/t/test-cabal-build-execs-test/build/test-cabal-build-execs-test/test-cabal-build-execs-test-tmp/Main.o )
[2 of 2] Compiling Paths_test_cabal_build_execs ( /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/autogen/Paths_test_cabal_build_execs.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe-tmp/Paths_test_cabal_build_execs.o )
[2 of 2] Compiling Paths_test_cabal_build_execs ( /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/t/test-cabal-build-execs-test/build/test-cabal-build-execs-test/autogen/Paths_test_cabal_build_execs.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/t/test-cabal-build-execs-test/build/test-cabal-build-execs-test/test-cabal-build-execs-test-tmp/Paths_test_cabal_build_execs.o )
Linking /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe ...
Linking /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/t/test-cabal-build-execs-test/build/test-cabal-build-execs-test/test-cabal-build-execs-test ...

Here's the above log with only lines relevant to the test-cabal-build-execs-exe executable:

Configuring executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
Preprocessing executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
Building executable 'test-cabal-build-execs-exe' for test-cabal-build-execs-0.1.0.0..
[1 of 2] Compiling Main             ( app/Main.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe-tmp/Main.o )
[2 of 2] Compiling Paths_test_cabal_build_execs ( /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/autogen/Paths_test_cabal_build_execs.hs, /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe-tmp/Paths_test_cabal_build_execs.o )
Linking /Users/rune/tmp/test-cabal-build-execs/dist-newstyle/build/x86_64-osx/ghc-8.10.7/test-cabal-build-execs-0.1.0.0/x/test-cabal-build-execs-exe/build/test-cabal-build-execs-exe/test-cabal-build-execs-exe ...

[Mikolaj]

I thought @phadej meant cabal test or cabal bench?

[phadej]

I meant dependencies, i.e. libraries on Hackage.

For local packages use cabal build libs etc target selectors to build only what you want.