Improve error message for kind mismatches

[guest]

ext :: Ev st Request res -> ServerPart (EvPar res st) Request Result
ext x = Handle $ \req -> do f <- ask
                            liftIO $ f x req

That's from HAppS/Protocols/HTTP.hs

While trying to bend my head around the involved types, feeling understanding already creeping somewhere, I, in a desperate attempt, copied the code into my source and changed it to

ext' :: Ev st Request res -> ServerPart (EvPar res st) Request Result
ext' x = Handle $ \req -> req

hoping to get a clean, expressive and enlightening type error,

I got this:

    Kind mis-match
    Expected kind `* -> *', but `Result' has kind `*'
    In the type `ServerPart (EvPar res st) Request Result'
    In the type `Ev st Request res
                 -> ServerPart (EvPar res st) Request Result'
    In the type signature for `ext'':
      ext' :: Ev st Request res
              -> ServerPart (EvPar res st) Request Result

Which made me wonder, google and then groan, as I couldn't even figure out what the heck a kind-mismatch is or means, not to mention why ghc points me to the type instead of the code.

My request is for the documentation section "Error messages and their meaning" aka "RTF this if you got that". The terminology of some ghc errors is just begging for references to papers.

[chevalier@…]

A kind is to a type what a type is to a value. The error message is saying that the type ServerPart (EvPar res st) Request Result is ill-formed. Since the error message only concerns the type, not the code, that's why the error message doesn't mention the code. In this case, the error is saying that you're using a type (Result) in a context where a type constructor is expected (analogous to applying the constant 1 to an argument). Looking at the type definition for ServerPart should be enlightening.

[simonpj]

I think we ought at least to be able to improve the error message a bit -- something like "Request is not applied to enough arguments".

Let's try to do this for 6.10.

Simon

[simonmar]

Adding a more descriptive description

[simonpj]

Fixed by

Wed May 13 16:11:30 BST 2009  simonpj@microsoft.com
  * Improve error reporting for kind errors (fix Trac #1633)
  
  A long-standing improvement to the error message for kinds. Now instead of
      Expected kind `* -> *', but `Int' has kind `*'
  we get
      The first argument of `T' should have kind `* -> *',
      but `Int' has kind `*'

Simon