Hi
Try doing a Hoogle search for "c1 (c2 x) -> c2 (c1 x)". Hoogle correctly states that Data.Traversable.sequence will do it for you.
Now try doing "c1 k (c2 x) -> c2 (c1 k x)". The 'sequence' function will also do this, but now Hoogle returns 0 results.
This is puzzling, since AFAIK, the above two type signatures are "equvilent" in some sense. (Specifically, replace every type X with type Y and you get from one to the other.) To me, this looks like a Hoogle bug. (It goes without saying that Hoogle also failed to find anything for the more specific type signature I was searching for, despite the fact that 'sequence' unifies with it.)
Hoogle is not a "unification engine" - since that is very rarely what people want out of a type search engine. What it is is an approximate matcher. Let's compare the two types: Your search :: c1 k (c2 x) -> c2 (c1 k x) sequence :: Monad m => [m a] -> m [a] Are you expecting c1 (:: * -> * -> *) to unify with [] (:: * -> *)? That seems kind incorrect at the very last. Additionally, those types don't look all that close. But, let's briefly consider unification (and why Hoogle doesn't used it). Consider the search: Eq a => [(a,b)] -> a -> b What the user wants is lookup, which sadly doesn't unify. However, undefined unifies perfectly. Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================