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
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