id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4128,Can't capture classes inside a template haskell type quotation,lilac,,"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.",feature request,new,low,7.6.2,Template Haskell,6.12.1,,,,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,4230,,