Hello Simon, You are requested us to give you examples of bad messages about program errors. This message is due to use of "all" instead of "and". in this case it would be great to hear that "all" has type of "(a -> Bool) -> [a] -> Bool" instead of needed "[Bool] -> Bool". May be in this case GHC tries to be smarter than needed :-) data PackedFilePath = PackedFilePath { fpPackedDirectory :: !String , fpPackedBasename :: !String , fpHash :: Int } map2eq f x y = (f x) == (f y) instance Eq PackedFilePath where a==b = all [ map2eq fpHash a b , map2eq fpPackedBasename a b , map2eq fpPackedDirectory a b ] c.hs:10:11: Couldn't match `Bool' against `t -> t1' Expected type: Bool Inferred type: t -> t1 Expected type: Bool Inferred type: [a] -> Bool Probable cause: `all' is applied to too few arguments in the call (all [map2eq fpHash a b, map2eq fpPackedBasename a b, map2eq fpPackedDirectory a b]) The HUGS message again a little more informative: ERROR "C:\temp\c.hs":10 - Type error in application *** Expression : all [map2eq fpHash a b,map2eq fpPackedBasename a b,map2eq fpPackedDirectory a b] *** Term : [map2eq fpHash a b,map2eq fpPackedBasename a b,map2eq fpPackedDirectory a b] *** Type : [Bool] *** Does not match : a -> Bool btw, i share the viewpoint that GHC messages are close to unusable. The main information i extract from them is an exact error location. may be (may be!) it is better to render error messages in terms of terms ;) with KNOWN TYPES, such as "all" in this case. while the Haskell officially is functional language and any term can be a fixed function or some data, in real programs there is some base things which have a fixed meaning and organize the whole computation process. concentrating on such things (i.e. functions with known types) instead of concentrating on types of isolated terms may approach you to not-so-smart programmers using GHC like me :) may be it is a viewpoint of old imperative programmer which accustomed to having two different beasts - procedures with well-known types and data which can or cannot be processed by these procedures, but both GHC and HUGS in this example says nothing about that cause of error message was that "all" wants arguments of this concrete types -- Best regards, Bulat mailto:bulatz@HotPOP.com