Can't capture classes inside a template haskell type quotation
GHC rejects the following:
{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
class C a where
data X = X
fmap return $ instanceD (cxt []) [t| C $(conT ''X) |] []
with the error:
test.hs:5:23:
Class `C' used as a type
In the Template Haskell quotation [t| C $(conT 'X) |]
In the second argument of `instanceD', namely `[t| C $(conT 'X) |]'
In the expression: instanceD (cxt []) [t| C $(conT 'X) |] []
This is frustrating, since that quotation would have resulted in exactly the TH AST fragment I wanted, had the error not occurred. Instead I must resort to:
instanceD (cxt []) (appT (conT ''Storable) (conT ''X)) []
Incidentally, the error message produced is wrong: The error says 'X whereas I wrote ''X.
Trac metadata
Trac field | Value |
---|---|
Version | 6.12.1 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Template Haskell |
Test case | |
Differential revisions | |
BlockedBy | |
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Blocking | |
CC | |
Operating system | |
Architecture |