On Wed, Mar 18, 2009 at 11:49 AM, Tom.Amundsen
Hi All,
I am new to Haskell. I just started reading "Real World Haskell" a few days ago, so I apologize for being such a noob.
But, I am curious why I see a lot of code where people do pattern matching via multiple function declarations instead of using the case ... of ... construct? For example:
[code] foldl' _ zero [] = zero foldl' step zero (x:xs) = let new = step zero x in new `seq` foldl' step new xs [/code]
Well, in this particular case, note that you have two equations written. These equations are true of foldl', and in fact foldl' is the *least defined * function satisfying these equations (see http://en.wikibooks.org/wiki/Haskell/Denotational_semantics for the underlying theory of "definedness"). It a very pretty idea. However, this happy view of equations breaks down once you start using the "fall through" semantics, as in: foo (x:y:xs) = x + y foo xs = 0 In which the second equation does not always hold (foo [1,2,3] = 0 is false). Because of this, I am beginning to prefer not writing functions using "fall through", although occasionally it is too damned convenient to pass up. Luke