[project @ 1997-07-26 22:48:58 by sof]
[ghc.git] / ghc / tests / typecheck / should_compile / tc091.hs
1 --!!! Test polymorphic recursion
2
3
4 -- With polymorphic recursion this one becomes legal
5 -- SLPJ June 97.
6
7 {-
8 To: Lennart Augustsson <augustss@cs.chalmers.se>
9 Cc: partain@dcs.gla.ac.uk, John Peterson (Yale) <peterson-john@cs.yale.edu>,
10 simonpj@dcs.gla.ac.uk
11 Subject: Type checking matter
12 Date: Fri, 23 Oct 92 15:28:38 +0100
13 From: Simon L Peyton Jones <simonpj@dcs.gla.ac.uk>
14
15
16 I've looked at the enclosed again. It seems to me that
17 since "s" includes a recursive call to "sort", inside the body
18 of "sort", then "sort" is monomorphic, and hence so is "s";
19 hence the type signature (which claims full polymorphism) is
20 wrong.
21
22 [Lennart says he can't see any free variables inside "s", but there
23 is one, namely "sort"!]
24
25 Will: one for the should-fail suite?
26
27 Simon
28
29
30 ------- Forwarded Message
31
32
33 From: Lennart Augustsson <augustss@cs.chalmers.se>
34 To: partain
35 Subject: Re: just to show you I'm a nice guy...
36 Date: Tue, 26 May 92 17:30:12 +0200
37
38 > Here's a fairly simple module from our compiler, which includes what
39 > we claim is an illegal type signature (grep ILLEGAL ...).
40 > Last time I checked, hbc accepted this module.
41
42 Not that I don't believe you, but why is this illegal?
43 As far as I can see there are no free variables in the function s,
44 which makes me believe that it can typechecked like a top level
45 definition. And for a top level defn the signature should be
46 all right.
47
48 -- Lennart
49 - ------- End of forwarded message -------
50 -}
51 module ShouldFail where
52
53 sort :: Ord a => [a] -> [a]
54 sort xs = s xs (length xs)
55 where
56 s :: Ord b => [b] -> Int -> [b] -- This signature is WRONG
57 s xs k = if k <= 1 then xs
58 else merge (sort ys) (sort zs)
59 where (ys,zs) = init_last xs (k `div` (2::Int))
60
61 -- Defns of merge and init_last are just dummies with the correct types
62 merge :: Ord a => [a] -> [a] -> [a]
63 merge xs ys = xs
64
65 init_last :: [a] -> Int -> ([a],[a])
66 init_last a b = (a,a)
67