Sandbox::https://github.com/haskell/cabal/pull/11703

sheaf · 2026-04-03T15:27:43Z

This is a rebase of #9871. I had to create a new PR as Matthew is no longer active (and that PR was made from his personal fork).

Template B: This PR does not modify behaviour or interface.

This is a major refactoring in how cabal-install works, with a few knock-on internal changes to some Cabal library functionality.
It should not cause any breakage for users (I tested clc-stackage and it all builds unchanged with this branch).

This PR modifies cabal-install to directly go through the Cabal library when building packages, instead of the Setup interface (except of course for build-type: Custom packages).

In particular, to build a package with build-type: Hooks, cabal-install will compile the package's SetupHooks file into an external hooks executable using the new hooks-exe package. All hooked operations are then performed by communicating with this external executable, instead of going through an opaque Setup executable. The hooks-exe package provides both functionality for compiling an external hooks executable and for interfacing with it. This makes use of the CommunicationHandle API that I added to process.

The main change is to SetupWrapper: the old InternalMethod becomes LibraryMethod, which builds the package by calling Cabal library functions. This is used to build all packages unless build-type: Custom or there is a Cabal library version mismatch which requires us to fall back to compiling Setup.hs and running that. The SelfExec method as well as forceExternalSetupMethod are removed: they no longer have any purpose, as builds can now be carried out concurrently thanks to 7b90583 and edb808a.

The new Distribution.Client.InLibrary module contains all the necessary framework for taking information that cabal-install has and invoking Cabal library functions to perform the appropriate action (configure, build, test, bench, ...). Most of these are pretty simple; the main difficulty is with configure as we need to jump to the part of the Cabal configure code that continues after figuring out information about the system (e.g. compiler information), to avoid wasting work with Cabal rediscovering things that cabal-install already knows. This required a bit of refactoring in Distribution.Simple.Configure.

TODO:

Update bootstrap plans.
Rebase over Implement recompilation checking for pre-build rules #11731.
Rebase over Mark output in LogProgress #11753.
Validate this version of the patch against clc-stackage and update the PR description accordingly.

sheaf · 2026-04-17T12:28:03Z

I now consider this patch ready for review. Significant changes since #9871:

I further refactored the Cabal library configure function so that all the logic that pertains to adjusting BuildOptions based on compiler and toolchain capabilities (e.g. support for bytecode libraries, profiling dynamic, etc) can be shared. This avoids duplicating logic inside cabal-install and prevents the codepaths from going out of sync.
I continued refactoring SetupWrapper, taking into account suggestions by @andreabedini on the original PR (e.g. moving more functions to the top level, see e.g. cabalLibFromOptions and cabalLibVersionToUse). I also took the opportunity to more clearly demarcate v1-only logic. On top of that, I was able to get rid of the self-exec setup method, now that Cabal library: allow setting the logging handle #11077 has landed. So there is only "external" (via Setup executable) and "in library" now.
The test I recently added for custom-setup builds fail when constraining dependencies in the Cabal-hooks package stack #11331 revealed that the way I had set up the hooks-exe implicit dependency in the previous iteration locked out users from using a different Cabal version in setup-depends entirely, as opposed to falling back to the external setup method. So I moved the necessary logic for compiling an external hooks executable into the Cabal library, which I now realise is the only place where it makes sense for it to live. The logic for invoking the external hooks executable remains separate.

I still need to check stackage compilation. With the changes in (1) implemented, I am now significantly more confident that we are not changing behaviour.

geekosaur

I think that, despite not intending to change behavior, this probably warrants a changelog entry marked significant.

sheaf · 2026-04-20T09:49:51Z

I think that, despite not intending to change behavior, this probably warrants a changelog entry marked significant.

I've written a changelog entry now, please let me know what you think!

sheaf · 2026-04-20T16:50:25Z

I have successfully built all of clc-stackage with this patch (with the exception of the minisat-solver package which fails with a C compiler error that I'm almost certain is unrelated).

I will instrument this version of cabal-install to compare LocalBuildInfo to further validate these changes.

sheaf · 2026-04-22T11:18:12Z

I have spent this morning diffing the output of cabal-install before and after this patch. I have about 2GB of dumped LocalBuildInfo files, and the differences I am seeing are:

ProgramDb
a. ghc and ghc-pkg are now FoundOnSystem instead of UserSpecified. I think this is expected because we no longer have to rely on --with-ghc flags to communicate the GHC from cabal-install to the Cabal library.
b. We consistently add data_dir environment variables to the environment of other programs. This is to ensure that build tools invoked at compile time (e.g. a call to alex in a Template Haskell splice) get the proper data directory.
Cosmetic differences in values for configTests, configBenchmarks, configAllowDependingOnPrivateLibs, configCoverageFor. This is just an artifact about where in the code the default values are applied (e.g. whether we see NoFlag or Flag False); I believe it has no incidence on the actual builds.
configInstallDirs: we now pass correct values for flibdir, includedir and mandir, which was not possible via the CLI interface because there are no ConfigFlags for those fields.

I also found a harmless minor inconsistency in targetBuildDepends where we weren't passing "ThisVersion" constraints corresponding to versions found by the solver. I've rectified that, but I don't expect it to make any difference given that we already pin everything by passing exact unit ids for all dependencies.

I've attached dumps of the LocalBuildInfo of Agda for reference, where the only changes are those described above (and some noise from build directories and ABI hashes changing).

Agda-HEAD.txt
Agda-InLib.txt

philderbeast · 2026-04-22T12:47:45Z

I also found a harmless minor inconsistency in targetBuildDepends where we weren't passing "ThisVersion" constraints corresponding to versions found by the solver. I've rectified that, but I don't expect it to make any difference given that we already pin everything by passing exact unit ids for all dependencies.

Are these some of those differences?

It looks like Agda-InLib.txt accepts any version for these.

sheaf · 2026-04-22T13:00:32Z

I also found a harmless minor inconsistency in targetBuildDepends where we weren't passing "ThisVersion" constraints corresponding to versions found by the solver. I've rectified that, but I don't expect it to make any difference given that we already pin everything by passing exact unit ids for all dependencies.

Are these some of those differences?

It looks like Agda-InLib.txt accepts any version for these.

Yes, that's what I was talking about. I don't think it makes a difference because we pass exact unit IDs for all dependencies (those determined by the solver). I updated this PR to includes the "ThisVersion" constraints determined by the solver just for the sake of consistency.

sheaf · 2026-04-23T11:10:36Z

I believe I've addressed everything. I would appreciate a review on #11731 so that I can land that first; that's a self-contained change to recompilation checking for SetupHooks pre-build rules.

mergify · 2026-04-26T11:43:30Z

dcoutts · 2026-04-30T08:30:13Z

🎉

andreabedini · 2026-05-02T03:54:08Z

Good work @sheaf and @mpickering!

MicroCabal rewrites the dependencies of every package it builds with mhs: it adds ghc-compat, which holds modules that are part of base in GHC but not in mhs, such as GHC.Stack and Language.Haskell.TH.Syntax, and it uses the forks array-mhs and random-mhs instead of array and random, which mhs cannot compile. Packages written for GHC rely on both. Distribution.Simple.MHS.rewritePackageDescription does the same, and cabal-install applies it to all the packages it gives to the solver (rewriteSourcePackages, next to the rewriting for allow-newer), so that the solver chooses ghc-compat and the forks, and cabal-install builds them from Hackage like any other dependency. Nothing has to be installed for mhs beforehand, and offline, where they cannot be had, the packages are left as they are. See Note [MicroHs rewrites of packages]. Rewriting the packages before solving is enough thanks to a recent change of cabal-install, which calls Cabal in-library (haskell#11703): it configures a package with the description of its plan, which is rewritten already, rather than having a Setup script read the .cabal file of the package again. Before that change, configure had to rewrite the package once more. Only a Custom setup script still reads the .cabal file, and mhs cannot compile one. As with MicroCabal, the version macros of a fork are also defined under the name of the package it forks: QuickCheck, which depends on random, tests MIN_VERSION_random(1,3,0) to define its RandomGen instance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0176vAefrw8jPKXdh9h1wKuj

PaleRoses · 2026-10-15T04:45:26Z

hey, heads up, this one made our no-op builds noticeably worse.

elabGPkgDescription is the whole package's GenericPackageDescription, and it now sits in every component's ElaboratedConfiguredPackage. the plan cache serializes both the elaborated and the improved plan, so a package with N components writes its full GPD 2N times, on top of the PackageDescription copies that were already there. that's quadratic in components per package, for a field that's "just used to report errors/warnings".

minimal repro: one package, N public sublibraries, one module each, a 16-flag common stanza. cabal build all once, then a no-op cabal build all under +RTS -s. ghc 9.14.1, macOS arm64, sizes are exact, times are single runs so take them loosely:

sublibs	improved-plan, 3.16.1.0	improved-plan, 3.18.1.0	no-op, 3.16.1.0	no-op, 3.18.1.0
10	0.39 MB	0.79 MB	1.19 s	1.55 s
40	3.6 MB	7.5 MB	1.46 s	3.58 s
80	13.3 MB	27.3 MB	1.79 s	2.83 s

on our real project (538 components across 44 packages, the biggest has 60) improved-plan is 56 MB on 3.16.1.0 and 127 MB on 3.18.1.0, same sources. a no-op cabal build on 3.18 allocates 10.9 GB, peaks at 720 MB live and spends more than half its time in GC, about 3 s per invocation, just to tell me there's nothing to build. a bigger -A makes it worse. one of our common warning flags is in the 3.16 cache 21,777 times and in the 3.18 cache 43,137 times.

separately, and i'm not sure it's from this PR: the per-unit configured programs seem to carry more copies of PATH now. the ghcup bin dir shows up 5,149 times in the 3.16 cache and 16,732 times in 3.18's.

we can't just stay on 3.16 either: ghc 9.14.2-rc2 wants jsem protocol v2, and with 3.16's semaphore it says "protocol version mismatch (got v1, supported v2)" and compiles sequentially.

could the GPD be stored once per package, or left out of the cached plan and re-read when a diagnostic actually needs it? happy to test a patch.

MicroCabal rewrites the dependencies of every package it builds with mhs: it adds ghc-compat, which holds modules that are part of base in GHC but not in mhs, such as GHC.Stack and Language.Haskell.TH.Syntax, and it uses the forks array-mhs and random-mhs instead of array and random, which mhs cannot compile. Packages written for GHC rely on both. Distribution.Simple.MHS.rewritePackageDescription does the same, and cabal-install applies it to all the packages it gives to the solver (rewriteSourcePackages, next to the rewriting for allow-newer), so that the solver chooses ghc-compat and the forks, and cabal-install builds them from Hackage like any other dependency. Nothing has to be installed for mhs beforehand, and offline, where they cannot be had, the packages are left as they are. See Note [MicroHs rewrites of packages]. Rewriting the packages before solving is enough thanks to a recent change of cabal-install, which calls Cabal in-library (haskell#11703): it configures a package with the description of its plan, which is rewritten already, rather than having a Setup script read the .cabal file of the package again. Before that change, configure had to rewrite the package once more. Only a Custom setup script still reads the .cabal file, and mhs cannot compile one. As with MicroCabal, the version macros of a fork are also defined under the name of the package it forks: QuickCheck, which depends on random, tests MIN_VERSION_random(1,3,0) to define its RandomGen instance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0176vAefrw8jPKXdh9h1wKuj

MicroCabal rewrites the dependencies of every package it builds with mhs: it adds ghc-compat, which holds modules that are part of base in GHC but not in mhs, such as GHC.Stack and Language.Haskell.TH.Syntax, and it uses the forks array-mhs and random-mhs instead of array and random, which mhs cannot compile. Packages written for GHC rely on both. Distribution.Simple.MHS.rewritePackageDescription does the same, and cabal-install applies it to all the packages it gives to the solver (rewriteSourcePackages, next to the rewriting for allow-newer), so that the solver chooses ghc-compat and the forks, and cabal-install builds them from Hackage like any other dependency. Nothing has to be installed for mhs beforehand, and offline, where they cannot be had, the packages are left as they are. See Note [MicroHs rewrites of packages]. Rewriting the packages before solving is enough thanks to a recent change of cabal-install, which calls Cabal in-library (#11703): it configures a package with the description of its plan, which is rewritten already, rather than having a Setup script read the .cabal file of the package again. Before that change, configure had to rewrite the package once more. Only a Custom setup script still reads the .cabal file, and mhs cannot compile one. As with MicroCabal, the version macros of a fork are also defined under the name of the package it forks: QuickCheck, which depends on random, tests MIN_VERSION_random(1,3,0) to define its RandomGen instance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0176vAefrw8jPKXdh9h1wKuj

MicroCabal rewrites the dependencies of every package it builds with mhs: it adds ghc-compat, which holds modules that are part of base in GHC but not in mhs, such as GHC.Stack and Language.Haskell.TH.Syntax, and it uses the forks array-mhs and random-mhs instead of array and random, which mhs cannot compile. Packages written for GHC rely on both. Distribution.Simple.MHS.rewritePackageDescription does the same, and cabal-install applies it to all the packages it gives to the solver (rewriteSourcePackages, next to the rewriting for allow-newer), so that the solver chooses ghc-compat and the forks, and cabal-install builds them from Hackage like any other dependency. Nothing has to be installed for mhs beforehand, and offline, where they cannot be had, the packages are left as they are. ghc-compat is added at the root of every component, unless the component depends on it there already, with bounds of its own; a dependency under a condition, or in another component, is no reason to leave it out. See Note [MicroHs rewrites of packages]. Rewriting the packages before solving is enough thanks to a recent change of cabal-install, which calls Cabal in-library (#11703): it configures a package with the description of its plan, which is rewritten already, rather than having a Setup script read the .cabal file of the package again. Before that change, configure had to rewrite the package once more. Only a Custom setup script still reads the .cabal file, and mhs cannot compile one. As with MicroCabal, the version macros of a fork are also defined under the name of the package it forks: QuickCheck, which depends on random, tests MIN_VERSION_random(1,3,0) to define its RandomGen instance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0176vAefrw8jPKXdh9h1wKuj

MicroCabal rewrites the dependencies of every package it builds with mhs: it adds ghc-compat, which holds modules that are part of base in GHC but not in mhs, such as GHC.Stack and Language.Haskell.TH.Syntax, and it uses the forks array-mhs and random-mhs instead of array and random, which mhs cannot compile. Packages written for GHC rely on both. Distribution.Simple.MHS.rewritePackageDescription does the same, and cabal-install applies it to all the packages it gives to the solver (rewriteSourcePackages, next to the rewriting for allow-newer), so that the solver chooses ghc-compat and the forks, and cabal-install builds them from Hackage like any other dependency. Nothing has to be installed for mhs beforehand, and offline, where they cannot be had, the packages are left as they are. ghc-compat is added at the root of every component, unless the component depends on it there already, with bounds of its own; a dependency under a condition, or in another component, is no reason to leave it out. A re-export that names the package of a module, array:Data.Array say, names the fork too, as Cabal resolves it by that name. See Note [MicroHs rewrites of packages]. Rewriting the packages before solving is enough thanks to a recent change of cabal-install, which calls Cabal in-library (#11703): it configures a package with the description of its plan, which is rewritten already, rather than having a Setup script read the .cabal file of the package again. Before that change, configure had to rewrite the package once more. Only a Custom setup script still reads the .cabal file, and mhs cannot compile one. As with MicroCabal, the version macros of a fork are also defined under the name of the package it forks: QuickCheck, which depends on random, tests MIN_VERSION_random(1,3,0) to define its RandomGen instance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0176vAefrw8jPKXdh9h1wKuj

sheaf mentioned this pull request Apr 3, 2026

Directly call in-library functions to build packages #9871

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Mark output in LogProgress #11753

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mergify Bot added ready and waiting Mergify is waiting out the cooldown period merge delay passed Applied (usually by Mergify) when PR approved and received no updates for 2 days queued labels Apr 24, 2026

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Merge of #11703

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Implement recompilation checking for pre-build rules #11731

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merge queue: embarking master (ff9daa2) and [#11731 + #11703] together #11765

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Revert "Merge pull request #10534 #11820

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Mikolaj mentioned this pull request Jul 9, 2026

Wrong relative paths in error messages when building a multi-package project #11206

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Build packages with MicroHs ulysses4ever/cabal#41

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Plan caches hold a copy of the package descriptions for every component #12463

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MicroHS support #11392

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