At 3:07 PM -0500 7/24/01, Cagdas Ozgenc wrote:
Hi, I am extremely new to Haskell. This will be my first question, so go easy. I have just read Chapter 1 on Simon Thompson's book. for example a function declaration is given as follows
scale : : Picture -> Int -> Picture
If the first two types are input variables why does the syntax require me to use arrows twice? I mean isn't the following syntax more readable (hypothetically)?
scale : : Picture , Int -> Picture
Is there a specific reason not to be able to distinguish the input parameters from the output parameter? Thanks
Welcome to Haskell! May you find it as rewarding as I have. As you read further, you'll find that Haskell allows functions to be applied to less than their full complement of arguments. For example, if pict :: Picture, then scale pict is a function which, applied to an Int, produces a Picture. That is, scale pict :: Int -> Picture This feature, known as "Currying", is very useful for making rather general functions which can be specialized by supplying some of their arguments. The type notation has been designed to accommodate Currying in two ways: 0. each parameter type is followed by an arrow 1. the arrow has been designed to be right-associative. The arrow's right-associativity means that these two types are identical: scale : : Picture -> Int -> Picture scale : : Picture -> (Int -> Picture) That is, you are free to think of scale as either of a function of two arguments which returns a Picture a function of one argument which returns (a function of one argument which returns a Picture) That equivalence would be a bit less clear using the comma syntax you suggest. Cheers, --Ham ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer Mail Code C0500 512-471-9525 The University of Texas at Austin Taylor Hall 5.138 Austin, Texas 78712-1188 ham@cs.utexas.edu hrichrds@swbell.net ------------------------------------------------------------------