Re: [Caml-list] Syntax for several matches, each with a "when" clause, but only one result

["Mr. Herr" <misterherr@freenet.de>]

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On 20.05.2016 09:57, Soegtrop, Michael wrote:
>
> Dear OCaml Users,
>
>  
>
> sometimes I want to do something like
>
>  
>
> match expr with
>
> | case1 when cond1
>
> | case2 when cond2
>
> | case3 when cond3 -> result
>
>  
>
> but this doesn’t work. I have to write
>
>  
>
> match expr with
>
> | case1 when cond1 -> result
>
> | case2 when cond2 -> result
>
> | case3 when cond3 -> result
>
>  
>
> Usually only some of the matches have a when clause. Is there a way to avoid
> copying the result term (other than writing a function) ?
>
>  
>
>
I could not see a working web link to this old list message from 2016-04-07, so I
just give you a copy of the answer by
Gabriel Scherer:

No, indeed you have to use a local definition to avoid code
duplication in this case.

My understanding of the design stance of pattern-matching in OCaml is
as follows: the syntax of patterns is bounded by what can be matched
efficiently. This explains why "when" has a second-class status
(first-class when cannot be matched efficiently); sometimes the user
has to pay for this rigidity. But, on the positive side, it is a
simple and clear stance, and it correlates with the availability of
good tooling, namely exhaustivity and useless-clause warnings.

On Thu, Apr 7, 2016 at 9:36 AM, Daniel Bünzli
<daniel.buenzli@erratique.ch> wrote:

> Hello,
>
> Something I run quite often is the following pattern matching
>
> match v with
> | None | Some c when sat c -> expr
> | Some …
>
> which doesn't compile and forces me to write
>
> match v with
> | None -> expr
> | Some c when sat c -> expr
> | Some …
>
> and leads to code duplication or the introduction of a definition to avoid it.
>
> Am I missing a syntax bit ?
>
> Best,
>
> Daniel


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