oleg@pobox.com wrote:
Is there a canonical example example that exhibits this behavior? Yes, it was discussed back in 2003. Here's the canonical form:
g::(Show a,Show b) => a->b g = undefined
--h :: Show a => b -> a h x = g(h x)
Both Hugs and GHC (in the pure Haskell98 mode!) are happy with h. Both compilers infer the type for h, shown in the comment. If you give h the type signature -- the one that is inferred by the compilers themselves -- both compilers complain.
Strangely, ghci accepts it on the command line: $ ghci [...] GHC Interactive, version 6.4.2, for Haskell 98. [...] Prelude> let g :: (Show a, Show b) => a -> b; g = undefined; h :: Show a => b -> a; h x = g (h x) in h 1 *** Exception: Prelude.undefined but not from a file to be loaded. Hugs +98 does not accept it on command line as expected. What's going on? Regards, apfelmus