Chad Scherrer wrote:
But why should this...
sumArrays [] = error "Can't apply sumArrays to an empty list" sumArrays (x:xs) = runSTArray (result x) where result x = do x0 <- thaw x mapM_ (x0 +=) xs return x0
work differently than this...
sumArrays' [] = error "Can't apply sumArrays to an empty list" sumArrays' (x:xs) = runSTArray result' where result' = do x0 <- thaw x mapM_ (x0 +=) xs return x0
Are the types of (result x) and result' not exactly the same?
It's the monmorphism restriction, again. Because result' doesn't look like a function, a monomorphic type is inferred for it. runST[U]Array of course doesn't want a monomorphic type. It's got nothing to do with boxed vs. unboxed arrays (I think, I can't be bothered to test it right now). There are at least four ways out: - make result' a function, either as in the first example above or by supplying a dummy () argument - declare the correct polymorphic type for result' - inline result' - (GHC only) compile with -fno-monomorphism-restriction Yes, it's a bit cumbersome. Imperative code is supposed to be cumbersome, after all. :) Udo. -- They seem to have learned the habit of cowering before authority even when not actually threatened. How very nice for authority. I decided not to learn this particular lesson. -- Richard Stallman