Reto Kramer wrote:
The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter.
class (C a) data (C foo) => XY foo = X foo | Y foo
bar :: a -> XY a bar aFoo = X aFoo
As suggested, this works:
bar :: (C a) => a -> XY a
Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)?
Hi Reto - If you'd not given any signature at all the compiler would have inferred the correct type for bar, but since you gave an explicit signature, the compiler had no option but to complain that you missed out the C a constraint. (ie if you decide to provide a signature you must give the full signature including constraints since the compiler won't add anything to them - partial signatures are not (yet) supported) Regards, Brian. -- http://www.metamilk.com