#5737: Template Haskell can't derive Read, Show, Eq instances
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Template Haskell
Version: 7.0.4 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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Given the following code:
{{{
import Language.Haskell.TH
makeAlpha' n = [d| data Alpha = Alpha $(conT n) deriving (Show, Read) |]
}}}
The compiler complains with the following error:
{{{
Can't derive instances where the instance context mentions
type variables that are not data type parameters
Offending constraint: Show t_d
}}}
But manually constructing the AST works without issues:
{{{
makeAlpha n = do
let alpha = mkName "Alpha"
dec <- dataD (return []) alpha []
[normalC alpha [fmap ((,) NotStrict) $ conT n]]
[''Show, ''Read]
return [dec]
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5737>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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