Reiner Pope wrote:
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).
[1]:
http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#implicit-parameters
Woah... that's some pretty crazy stuff, right there. Just wow.
It would take me months to comprehend how that works. Heh. Oh well, it's
useful for debugging the type system though. ;-)
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