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

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

[Rolf Wester]

Thanks to all who replied. Your answers where very instructive
for me and I hopefully now got how pattern matching works.

Thanks again

Rolf Wester 
-------------------------------------
Rolf Wester
rolf.wester@ilt.fraunhofer.de
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RE: [Caml-list] Warning: this match case is unused.

[Krishnaswami, Neel]

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

[Yann Coscoy]

Rolf Wester wrote:

> Hi,
> 
> 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.


The second n0 in pattern « n0 -> 1 » is the introduction of a fresh 
variable. It is not binded to n0 previously introduced in « f l n0 = ».

I guess the function you had in mind is:


let f l n0 =
  let n = List.length l in
  match n with 
	  n1 when n1=n0 -> 1;
	| _  -> 0;;





-- 
Yann Coscoy - Trusted Logic
5 rue du Bailliage 78000 Versailles
+33 (0) 1 30 97 74 76
http://www.trusted-logic.fr


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

[Pixel]

"Rolf Wester" <rolf.wester@ilt.fhg.de> writes:

>   match n with 
> 	  n0 -> 1;
> 	| _  ->  0;;

make it

   match n with 
 	  n0' when n0 = n0' -> 1;
 	| _  ->  0;;

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

[Rolf Wester]

Hi,

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.

Thanks

Rolf Wester

-------------------------------------
Rolf Wester
rolf.wester@ilt.fraunhofer.de
-------------------
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