At 9:24 AM -0400 2005/5/7, Daniel Carrera wrote:
Hello,
Right now I'm using type declarations like:
f :: Int -> [Int]
So f returns a list of Ints.
Is there a way to tell Haskell that a list or array must have exactly (say) 256 elements? I'd love to have Haskell make sure that the array I build is the correct size.
Well, for starters, lists and arrays are two entirely different topics. I've noticed that Haskell newbies sometimes confuse them --possibly the use of [] in list types and enumerations triggers an unconscious association with [] used in conventional languages for array indexing. It's easy to define a function that constructs a list of a given size, but that size is not part of the list's type. As far as I know, the last programming language that included arrays' sizes in their types was Standard Pascal, and it turned out to be an unmitigated disaster. Because array parameters were typed with their sizes, a procedure for searching arrays of size 100 could not be used for arrays of any other size. Every useful (non-Standard) dialect of Pascal provided a way around that restriction, as did Pascal's successor, Modula-2, and as far as I know the mistake has not been repeated. As for Haskell arrays, I've been programming in Haskell since the early 1990's, and I've never really needed them. Most Haskell programmers find lists much more useful, and you'd probably be better off concentrating on lists until you encounter a problem in which arrays are really needed. When you do, you'll discover that in Haskell as in most other languages, an array's size is not part of its type. It's nice to see you taking up Haskell with such enthusiasm. If you agree with many of us that Haskell's one of today's best programming programming languages, I hope you'll help spread the word. With best wishes, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-0233 ham@cs.utexas.edu hrichrds@swbell.net http://www.cs.utexas.edu/users/ham/richards ------------------------------------------------------------------