Hi,

I am having trouble getting a small program to compile. The helpful folks at #haskell created a version of the program that does compile - http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28406#a28408 but it is not very clear to them (and to me) why the original program wouldn't type compile in the first place.

Here's the program that refuses to compile -

module Delme () where

data DecisionState = A | B | C | D

d_test :: Eq b => b -> b -> DecisionState -> DecisionState -> ()
d_test test testVal trueState falseState =
    if (test == testVal)
     then d trueState
     else d falseState

d :: DecisionState -> ()
d A = d_test True True B C
d B = d_test 1 2 C D
d C = d_test True False A B
d D = ()

I get an error like -

Delme.hs:13:0:

Contexts differ in length

(Use -XRelaxedPolyRec to allow this)

When matching the contexts of the signatures for

d_test :: forall b.

(Eq b) =>

b -> b -> DecisionState -> DecisionState -> ()

d :: DecisionState -> ()

The signature contexts in a mutually recursive group should all be identical

When generalising the type(s) for d_test, d

Putting in the extension does get the program to type check but the original program should have type compiled in the first place.

The ironic thing we discovered is that if we remove the type declaration for 'd', the program type checks, and GHC then derives the exact same type which we removed!

Can some of the smarter people in the room please shed more light on this?

-- Anupam