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Remove quantified constraints in FieldGrammar#11729

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leana8959:remove-fg-quantified-constraints
Apr 13, 2026
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Remove quantified constraints in FieldGrammar#11729
mergify[bot] merged 1 commit intohaskell:masterfrom
leana8959:remove-fg-quantified-constraints

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@leana8959 leana8959 commented Apr 10, 2026 •
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Previously field grammar was written with quantified constraints, this PR refactors them to concrete types (there's actually not a lot of them!). This would help the exact print project and in general make it easier to scrutinize which term is requiring which constraint.

EDIT: at the request of Jappie, here's an error caused by an application of type family on a quantified constraint when implementing exact print. Removing the quantification helps the compiler resolve this constraint.

error message 
src/Distribution/PackageDescription/FieldGrammar.hs:718:6: error: [GHC-73138]
    • Illegal type synonym family application ‘Annotate
                                                 mod (RelativePath from to)’ in instance:
        Newtype
          [Annotate mod (RelativePath from to)]
          (ListWith mod FSep (RelativePathNT from to) (RelativePath from to))
    • In the quantified constraint ‘forall from (to :: FileOrDir).
                                    Newtype
                                      [Annotate mod (RelativePath from to)]
                                      (ListWith
                                         mod FSep (RelativePathNT from to) (RelativePath from to))’
      In the type signature:
        buildInfoFieldGrammar' :: forall mod
                                         c
                                         g. (FieldGrammarWith mod c g,
                                             Applicative (g mod (BuildInfoWith mod)),
                                             L.HasBuildInfoWith mod (BuildInfoWith mod),
                                             Newtype [Annotate mod LegacyExeDependency] (ListWith mod CommaFSep (Identity LegacyExeDependency) LegacyExeDependency),
                                             c (ListWith mod CommaFSep (Identity LegacyExeDependency) LegacyExeDependency),
                                             Newtype [Annotate mod ExeDependency] (ListWith mod CommaFSep (Identity ExeDependency) ExeDependency),
                                             c (ListWith mod CommaFSep (Identity ExeDependency) ExeDependency),
                                             Newtype [Annotate mod String] (ListWith mod NoCommaFSep Token' String),
                                             c (ListWith mod NoCommaFSep Token' String),
                                             Newtype [Annotate mod PkgconfigDependency] (ListWith mod CommaFSep (Identity PkgconfigDependency) PkgconfigDependency),
                                             c (ListWith mod CommaFSep (Identity PkgconfigDependency) PkgconfigDependency),
                                             forall from to.
                                             Newtype [Annotate mod (RelativePath from to)] (ListWith mod FSep (RelativePathNT from to) (RelativePath from to)),
                                             c (ListWith mod FSep (RelativePathNT Framework File) (RelativePath Framework File)),
                                             Newtype [Annotate mod (DependencyWith mod)] (ListWith mod CommaVCat (Identity (DependencyWith mod)) (DependencyWith mod)),
                                             c (ListWith mod CommaVCat (Identity (DependencyWith mod)) (DependencyWith mod))) =>
                                            g mod (BuildInfoWith mod) (BuildInfoWith mod)
    |
718 |   :: forall mod c g
    |      ^^^^^^^^^^^^^^...

This change doesn't change behaviour or interface.

  • Patches conform to the coding conventions.
  • Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

@leana8959 leana8959 changed the title Remove fg quantified constraints Remove quantified constraints in FieldGrammar Apr 10, 2026
@leana8959 leana8959 force-pushed the remove-fg-quantified-constraints branch from 4a6f208 to 2cdb355 Compare April 10, 2026 11:56
@leana8959 leana8959 marked this pull request as ready for review April 10, 2026 11:59
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Seems reasonable!

@leana8959 leana8959 marked this pull request as draft April 10, 2026 12:25
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leana8959 commented Apr 10, 2026 •
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Drafting because I forgot to remove the language extension "QuantifiedConstraints".

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Yes, this looks fine (modulo actually removing the LANGUAGE pragma of course). I felt slightly bad for adding the quantified constraints in the first place; I agree it's not so bad to spell them out. Thanks!

@leana8959 leana8959 force-pushed the remove-fg-quantified-constraints branch from 2cdb355 to c0dd136 Compare April 10, 2026 12:38
@leana8959 leana8959 marked this pull request as ready for review April 10, 2026 21:39
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