Oh, I disagree with this point of view. Circle is certainly a value, i.e. a full-fledged function, as Brian Beckman correctly surmised. The Haskell designers did not decide "for convenience" that Circle is the same as \x -> Circle x. Rather, that's a fundamental law (the eta law, to be exact) of the lambda calculus, on which Haskell is based. The real reason that the Haskell designers chose to have constructors begin with a capital letter is to make pattern-matching clearer. For example, if one writes (\Foo -> 42) it is clear that Foo is a constructor, and this function will be an error when applied to anything but Foo, whereas (\foo -> 42) will match anything. If the namespaces were not separated in this way, then you wouldn't know whether "foo" was a constructor or a formal parameter, without looking at your whole program -- including imports -- to track down all "data" declarations. I hope this helps, -Paul Ben Rudiak-Gould wrote:
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
-- Professor Paul Hudak Chair, Dept of Computer Science Office: (203) 432-1235 Yale University FAX: (203) 432-0593 P.O. Box 208285 email: paul.hudak@yale.edu New Haven, CT 06520-8285 WWW: www.cs.yale.edu/~hudak