On Fri, 27 Jun 2003 12:22:46 +0400 "Serge D. Mechveliani" <mechvel@botik.ru> wrote:

Dear Haskellers,

Once I wrote a function

f' :: Int -> Int -> Int -> (Int, Int)

f' n m l = let (d,k) = (gcd n m, quot n d) in (k, l*k)

(the simplified version of real program), placing erroneousely d to both parts of the pattern matching (d,k) = ...

The intended program was

f n m l = let {d = gcd n m; k = quot n d} in (k, l*k)

But f' gives the intended results, at least in the GHC implementation. So that I did not notice the `error' for a long time.

Is really the Haskell pattern matching semantic so that f and f' are equivalent ?

Haskell is a lazy language. You can define values in terms of themselves (with emphasis on -values-). Here's a (relatively) simple example of doing this for more and (some) explanation see http://haskell.org/hawiki/TyingTheKnot. -- list_rep_min: replace list with it's minimum value in one pass list_rep_min l = result where (result,mini) = lrm l mini lrm [x] m = ([m],x) lrm (x:xs) m = (m:ms,x `min` m') where (ms,m') = lrm xs m

Is it an occasional implementation feature that hide the `error'?

No. It's a language feature, though it does cause problems. My advice is simply to avoid shadowing (local) variables. I am usually pretty good about following it, but one particular time I didn't, I ended up with a function that seemed 'mysteriously' broken. I felt pretty stupid when I finally noticed what it was. Oddly, enough I was actually thinking that I was shadowing too much as I wrote the code.