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

["Soegtrop, Michael" <michael.soegtrop@intel.com>]

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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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