Re: [Haskell-beginners] The case expression

23 Jan 2009

      On Thu, Jan 22, 2009 at 10:01 PM, Erik de Castro Lopo
 wrote:
...
Hi all,
Ocaml's match .. with expression (very similar to Haskell's case)
allows multiple matches for a single result (naive example):
let f x =
       match x with
       | 0 -> "zero"
       | 1 | 3 | 5 | 7 -> "odd"
       | 2 | 4 | 6 | 8 -> "even"
       _ -> "bad number"
Is there a similar thing in Haskell? At the moment I have to do
something like :
f x =
       case x of
         0 -> "zero"
         1 -> "odd"
         3 -> "odd"
         5 -> "odd"
         7 -> "odd"
         2 -> "even"
         4 -> "even"
         6 -> "even"
         8 -> "even"
         _ -> "bad number"
Well you can guard each clause:

case x of
  0 -> "zero"
  n | even n -> "even"
     | odd n -> "odd"
     | otherwise -> "bad number"

Still not quite exactly what you've got.

So you could really hammer it with blunt force:

case x of
  0 -> "zero"
  n | n `elem` [1,3,5,7] -> "odd"
     | n `elem` [2, 4, 6, 8] -> "even"
     | otherwise -> "bad number"

By the way, zero is even. http://en.wikipedia.org/wiki/Zero_is_even

Re: [Haskell-beginners] The case expression

David Morse