Claus made a suggestion about type error messages: | Apart from bracketing fully applied type synonyms, the error message | could be improved by providing the missing bit of information about | 'Memo': | | D:\home\Haskell\tmp\desktop\types.hs:11:11: | Couldn't match expected type `Memo d1' | against inferred type `Memo d' | (type Memo d :: * -> *) | In the second argument of `(.)', namely `appl' | In the expression: abst . appl | In the definition of `f': f = abst . appl I've implemented this idea; note the "NB" line below: NoMatchErr.hs:20:11: Couldn't match expected type `Memo d' against inferred type `Memo d1' NB: `Memo' is a (non-injective) type function In the second argument of `(.)', namely `appl' In the expression: abst . appl In the definition of `f': f = abst . appl (Rather than give its kind, I thought it was better to focus on the reason for the mis-match, namely the non-injectivity.) Simon