Template Haskell: reify returns incorrect types when ommiting type signatures
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 [] )
Trac metadata
Trac field | Value |
---|---|
Version | 6.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Template Haskell |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | alfonso.acosta@gmail.com |
Operating system | Multiple |
Architecture | Unknown |