2008/10/9 Reiner Pope
The syntax is for the implicit parameter extension[1]. I think you would write your example as
foo (undefined :: Bar x) ?z :: Bar y
Then querying the type of that whole expression with :t will list ?z's type in the expression's constraints. (Of course, you should turn off the monomorphism restriction so that ghc doesn't complain if constraints aren't resolved).
Oh yeah, I forgot about that. ghci> :set -fno-monomorphism-restriction Although I don't know if that affects the results of the :t command.
[1]: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extension...
Reiner
There's also a haskell98 way to do the same thing, it's just a bit more wordy at the ghci prompt, and a bit more work to decode the result: ghci> :t \z -> (foo (undefined :: Bar x) z :: Bar y) Now, the type of this expression is clearly type of z -> Bar y So just read the value before the arrow to get your answer. -- ryan