RE: [Caml-list] Warning: this match case is unused.

["Krishnaswami, Neel" <neelk@cswcasa.com>]

Rolf Wester [mailto:rolf.wester@ilt.fhg.de] wrote:
> 
> this is probably a trivial question but can someone tell me why:
>  
> let f l n0 =
>   let n = List.length l in
>   match n with 
> 	  n0 -> 1;
> 	| _  ->  0;;
> 
> Results in:
> Warning: this match case is unused.

Yes. Your function will always return 1, because variables on the
left-hand side of a pattern-match are bound like in a let expression. 
So the first branch of the match will always succeed, binding n0 to n, 
and then return 1.

So your code is equivalent to:

let f l n0 = 
  let n = List.length l in
  let n0 = n
  in 1

which is probably not what you want. :) Try writing this as:

let f l n0 = 
  let n = List.length l in 
  if n = n0 then
    1
  else
    0

--
Neel Krishnaswami
neelk@cswcasa.com
 
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