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Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

28ce1f8a

by Andreas Klebinger at 2026-04-03T19:58:44-04:00

Give the Data instance for ModuleName a non-bottom toConstr implementation.

I've also taken the liberty to add Note [Data.Data instances for GHC AST Types]
describing some of the uses of Data.Data I could find.

Fixes #27129

2 changed files:

compiler/GHC/Hs/Instances.hs
compiler/Language/Haskell/Syntax/Module/Name.hs

Changes:

compiler/GHC/Hs/Instances.hs

@@ -17,6 +17,21 @@ module GHC.Hs.Instances where
  -- UndecidableInstances ?
 +{- Note [Data.Data instances for GHC AST Types]
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +We give all of the frontend types and their instantiations (HsSyn) and
 +some other types Data.Data instances. There are two main motivations to
 +do so:
++
 +* For users of the GHC API it allows to write Generic code over the GHC AST.
 +* GHC itself has a few uses of these as well:
 +    * In the showAstData, showAstDataFull helpers to print a representation of
 +      the actual AST using it's constructors rather than just user facing pretty printing.
 +    * It's used to some degree for HIE file generation in the ToHIE instances.
 +    * TH serialization uses it for serialization of Annotations (GHC.Serialized)
 +    * Some of the dump flags use showAstData to produce the actual dump output.
 +-}
++
  import Data.Data hiding ( Fixity )
  import GHC.Prelude

compiler/Language/Haskell/Syntax/Module/Name.hs

@@ -17,7 +17,8 @@ instance Data ModuleName where
    -- don't traverse?
    toConstr x   = constr
      where
 -      constr = mkConstr (dataTypeOf x) "{abstract:ModuleName}" [] Prefix
 +      -- Fake a Algebraic constr. See #27129
 +      constr = mkConstrTag (dataTypeOf x) "{abstract:ModuleName}" 1 [] Prefix
    gunfold _ _  = error "gunfold"
    dataTypeOf _ = mkNoRepType "ModuleName"