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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #9262
Closed
Open
Issue created Jul 03, 2014 by Richard Eisenberg@raeDeveloper

Allow free variables in reifyInstances

When I say

{-# LANGUAGE TemplateHaskell #-}

module Bug where

import Language.Haskell.TH

$( do insts <- reifyInstances ''Eq [ListT `AppT` VarT (mkName "a")]
      runIO $ putStrLn $ pprint insts
      return [] )

I get

Bug.hs:7:4:
    Not in scope: type variable ‘a’
    In the argument of reifyInstances: GHC.Classes.Eq [a]

But, I wanted the declaration for instance Eq a => Eq [a]. The error message isn't wrong, exactly -- a really isn't in scope -- but I think we can do better by just treating variables as fresh.

Trac metadata
Trac field Value
Version 7.8.2
Type FeatureRequest
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Template Haskell
Test case
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