I was one of the people on #haskell discussing this with Anupam. Note that that when you remove the signature of d, the result complies and ghci will state the inferred type of d is exactly the signature that you are not allowed to write. In my opinion, this is a bug in the Haskell 98 report where it says ``If the programmer supplies explicit type signatures for more than one variable in a declaration group, the contexts of these signatures must be identical up to renaming of the type variables. The problem is that we cannot give a type signature to d with exactly the constraints of d_test because d doesn't have any type variable in its type signature. At the very least the Haskell report should allow type checking to proceed if everything in a declaration group has a signature even if the signatures don't have identical constraints. A trac ticket is needed for Haskell 2011, if one doesn't already exist. On Sat, 31 Jul 2010, Anupam Jain wrote:
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
-- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''