[...]
>  I was expecting "when" to be right distributive over "|". I find
> OCaml's behaviour not very intuitive in such a situation.
[...]

The when construct introduces a guard to a pattern matching
clause. This means that a when construct is global to the entire
pattern of the clause it appears in; it means in particular that no
when construct can be nested into a pattern and the question of its
distributivity wrt any other construct is pointless.

>  Is there a good reason for this?

Yes: simplicity, complexity invariants preservation for runtime
pattern analysis decisions even in the presence of when clauses,
easier understanding of the compiler warnings, simplicity and
well-definedness semantics (in particular the desired invariance wrt
the order of evaluation).

>  Cheers,
> 
>  Sébastien.

Best regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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