On Sun, Jul 10, 2011 at 06:49, Patrick Browne
My main question is in understanding the relationship between the arguments of the functions getX and getY in the class and in the instance. It seems to me that the constructor Pt 1 2 produces one element of type Point which has two components. How does this square with the class definition of getX which has two arguments?
One argument. But your main confusion is between the data constructor which takes two arguments, and the type constructor which takes one. Point a -- type constructor, takes a type a and produces a new type Point a Pt a a -- data constructor, takes two values of some type a and produces a new value of type Point a Type constructors are used in type signatures; data constructors are used in code. You can't use Pt a a as a type; it's a (polymorphic) value. Conversely, you can't use Point a as a value (that is, "foo (Point a)" is an error, because foo requires a value, not a type). -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms