On Monday 18 April 2011 04:38:17, Perry James wrote:
Hi all,
I'm working through Brown & Cook's "Monadic Memoization Mixins" and have come across some code that I can't get to compile with GHC.
The error I get for
arrayDict :: (*MArray *arr (*Maybe *b) m, *Ix *a, *Ord *a) => a -> arr a (*Maybe *b) -> *Dict *a b m
is Non type-variable argument in the constraint: MArray arr (Maybe b) m (Use -XFlexibleContexts to permit this) In the type signature for `arrayDict': arrayDict :: (MArray arr (Maybe b) m, Ix a, Ord a) => a -> arr a (Maybe b) -> Dict a b m
That's clear enough, I think, you have to enable FlexibleContexts. (Btw, also NPlusKPatterns for newer GHCs.)
Enabling FlexibleContexts causes other errors. What could be going wrong? In case this isn't enough context, I've posted the full source code for this section of the paper at smipple<http://www.smipple.net/snippet/Perry%20James/Monadic%20Memoizati on%20Mixins> .
Simple, lines 80-81: arrayDict size arr = (check, store) where check a = if a > size then return Nothing else return readArray arr a readArray array index :: monad (Maybe b) so the return in the else branch shouldn't be. Remove it and it compiles.
Thanks in advance for any help, Perry
Cheers, Daniel