Brian Beckman wrote:
data Shape = Circle Float | Square Float
I read this something along the lines of "'Shape' is a type constructor, for use in other type-defining expressions, and 'Circle' and 'Sqare' are its two data constructors, which should be used like functions of type 'Float -> Shape'". Indeed, typing "Circle" at the Hugs prompt reveals that Haskell has a "function" named "Circle" with type "Float -> Shape."
However, I don't know of other circumstances where (1) I can declare functions with capitalized names (Hugs groans about syntax errors if I attempt the following:
Circle2 :: Float -> Shape Circle2 = Circle
And (2) where the argument-types of a function can be declared on the function's right-hand side.
I remember being confused in a similar way by data constructors when I learned Haskell. You might find it easier to think of "Circle" and "Square" as part of the name of a value. "Circle 1.2" is one of the values in the type Shape, for example; it's not a function call which returns the value, it just *is* the value. Circle by itself doesn't really mean anything -- it's not a value of any type -- and Haskell could have been designed to make it a syntax error. But for convenience Haskell's designers decided to treat it as though it meant (\x -> Circle x). -- Ben