#2222: Template Haskell: reify returns incorrect types when ommiting type
signatures
------------------------+---------------------------------------------------
Reporter: fons | Owner:
Type: bug | Status: new
Priority: normal | Component: Template Haskell
Version: 6.8.2 | Severity: major
Keywords: | Testcase:
Architecture: Unknown | Os: Multiple
------------------------+---------------------------------------------------
Replicable with GHC versions 6.8.2 and 6.9 (20080219 snapshot).
Workaround: supply type signatures.
See the following examples:
`ReifyPlusTypeInferenceBugs.hs`
{{{
{-# LANGUAGE TemplateHaskell #-}
module ReifyPlusTypeInferenceBugs where
import Language.Haskell.TH
-- First problem:
-- * reify doesn't return the expected type of names binded to
-- polymorphic expressions
-- a :: Num a => a
-- uncommenting the line above fixes the problem
a = 1
-- The following splice should print
-- "inside b: forall a_0 . GHC.Num.Num a_0 => a_0"
-- but instead, it merely prints a type variable "inside b: t_0"
b = $(do VarI _ t _ _ <- reify 'a
runIO $ putStrLn ("inside b: " ++ pprint t)
[| undefined |])
-- Second problem:
-- * reify doesn't return the expected type of names binded to
-- TH-spliced expressions if no explicit type signature
-- declaration is provided.
-- c :: Bool
-- uncommenting the line above fixes the problem
c = $([| True |])
-- this splice should print "inside d: GHC.Base.Bool"
-- but, again, it prints just a type variable: "inside d: t_0"
d = $(do VarI _ t _ _ <- reify 'c
runIO $ putStrLn ("inside d: " ++ pprint t)
[| undefined |] )
-- Strangely enough, reify works differently if called inside a
declaration
-- splice. This time, the type returned is closer to be right
-- but unnecesary type variables are included:
-- "type of c: forall a_0 a_1 . GHC.Base.Bool"
$(do VarI _ t _ _ <- reify 'c
runIO $ putStrLn ("type of c: " ++ pprint t)
return [] )
-- Even more strange is the fact that the order of declaration of
-- splices seems to matter. Declaring the exact example again ....
-- e :: Bool
-- uncommenting the line above solves the problem
e = $([| True |])
-- this splice works as expected!!! ???
-- "inside f: GHC.Base.Bool"
f = $(do VarI _ t _ _ <- reify 'e
runIO $ putStrLn ("inside f: " ++ pprint t)
[| undefined |] )
-- Here, we still get unnecesary variables, but, for some reason,
-- _just_ one in this case:
-- "type of e: forall a_0 . GHC.Base.Bool"
$(do VarI _ t _ _ <- reify 'e
runIO $ putStrLn ("type of e: " ++ pprint t)
return [] )
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2222>
GHC <http://www.haskell.org/ghc/>
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