[mercury-users] functional dependencies (again)

[Michael Day]

About a year ago I posted this:

   :- typeclass foo(A, B) given A >> B.

   :- instance foo(list(T), T). % an error in the second parameter

   :- pred bar(C) <= foo(C, int). % an error in the constraint

The last example is now supported, which has made using multi-parameter
type classes slightly more practical. However the simple functional
dependency from the first example still can't be used, which is
unfortunate as this simple case is enough to solve a great variety of
problems that arise with multi-parameter type classes.

How much work would be required to add support for just this case?

Michael

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