This type error comes up such a lot that I'm copying this message
to the Haskell mailing list.
| The following program does not typecheck under ghc-2.01 unless you
| uncomment the type signature for test. (ghc-0.29 seems to propagate
| the equality attribute correctly, and doesn't require the annotation.)
|
| module Test (test) where
| -- test :: Eq a => [[a]] -> [a] -> [[a]]
| test l xs = [ys | ys <- l, ys == map id xs]
The problem here is that there isn't a most general type for test; this is a
shortcoming in the language, not the implementation. Consider: map has type
map :: Monad m => (a->b) -> m a -> m b
Now, you are taking equality over those ys's, that equality over things
of type (m b). So the "right" type for test is:
test :: (Monad m, Eq (m a)) => [m a] -> m a -> [m a]
But alas, Haskell doesn't like the Eq (m a) constraint, so it complains
of a type error.
The solution is presumably to generalise the type system a bit, so that we
recover the principal-type property.
But we want to do the *right* generalisation. I know Mark Jones is thinking
about this; others are welcome to do so too!
Simon
