[Caml-list] Suboptimal pattern matching specification

[Caml-list] Suboptimal pattern matching specification

[Daniel Bünzli]

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

Re: [Caml-list] Suboptimal pattern matching specification

[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
>
>
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs