Ticket #2222: ReifyPlusTypeInferenceBugs.hs

File ReifyPlusTypeInferenceBugs.hs, 2.0 KB (added by morabbin, 15 months ago)

Exhibits problems described in #2222

Line 
1{-# LANGUAGE TemplateHaskell #-}
2module ReifyPlusTypeInferenceBugs where
3
4import Language.Haskell.TH
5
6-- First problem:
7--  * reify doesn't return the expected type of names binded to
8--    polymorphic expressions
9
10-- a :: Num a => a
11-- uncommenting the line above fixes the problem
12a = 1
13
14-- The following splice should print
15-- "inside b: forall a_0 . GHC.Num.Num a_0 => a_0"
16-- but instead, it merely prints a type variable "inside b: t_0"
17b = $(do VarI _ t _ _ <- reify 'a
18         runIO $ putStrLn ("inside b: " ++ pprint t)
19         [| undefined |])
20
21
22-- Second problem:
23--  * reify doesn't return the expected type of names binded to
24--    TH-spliced expressions if no explicit type signature
25--    declaration is provided.
26
27
28-- c :: Bool
29-- uncommenting the line above fixes the problem
30c = $([| True |])
31
32
33-- this splice should print "inside d: GHC.Base.Bool"
34-- but, again, it prints just a type variable: "inside d: t_0"
35d = $(do VarI _ t _ _ <- reify 'c
36         runIO $ putStrLn ("inside d: " ++ pprint t)
37         [| undefined |] )
38
39-- Strangely enough, reify works differently if called inside a declaration
40-- splice. This time, the type returned is closer to be right
41-- but unnecesary type variables are included:
42-- "type of c: forall a_0 a_1 . GHC.Base.Bool"
43$(do VarI _ t _ _ <- reify 'c
44     runIO $ putStrLn ("type of c: " ++ pprint t)
45     return [] )
46
47
48-- Even more strange is the fact that the order of declaration of
49-- splices seems to matter. Declaring the exact example again ....
50
51-- e :: Bool
52-- uncommenting the line above solves the problem
53e = $([| True |])
54
55-- this splice works as expected!!! ???
56-- "inside f: GHC.Base.Bool"
57f = $(do VarI _ t _ _ <- reify 'e
58         runIO $ putStrLn ("inside f: " ++ pprint t)
59         [| undefined |] )
60
61-- Here, we still get unnecesary variables, but, for some reason,
62-- _just_ one in this case:
63-- "type of e: forall a_0 . GHC.Base.Bool"
64$(do VarI _ t _ _ <- reify 'e
65     runIO $ putStrLn ("type of e: " ++ pprint t)
66     return [] )