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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hmm, difficult to find. The title was "Suboptimal pattern specification", but I just knew
this problem was discussed here or on SO within the last ?? days.

/Str.

On 20.05.2016 12:04, Soegtrop, Michael wrote:
>
> Thanks a lot for pointing me to this answer! Obviously I need to work on my search
> skills …
>
>  
>
> Best regards,
>
>  
>
> Michael
>
>  
>
> *From:*caml-list-request@inria.fr [mailto:caml-list-request@inria.fr] *On Behalf Of
> *Mr. Herr
> *Sent:* Friday, May 20, 2016 10:38 AM
> *To:* caml-list@inria.fr
> *Subject:* Re: [Caml-list] Syntax for several matches, each with a "when" clause,
> but only one result
>
>  
>
>  
>
> 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> <mailto: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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