Re: Instead of pcase

[Ihor Radchenko]

Richard Stallman <rms@gnu.org> writes:

> Constrained variable: (PRED VARIABLE)  or, more generally,
>                       (PRED VARIABLE OTHER-ARGS...)
> ...
>      (symbolp sym)   ; Match any symbol, bind `sym' to it.

This one is clear, especially as ,(symbolp sym) - comma indicates that
the form is special.

>      (> num-foos min-foos)  ; Match any number greater than MIN-FOOS,
>                             ;  bind `num-foos' to it.

But this is confusing - which symbol to bind and which one to eval is
based purely on position in the ,(PRED ...) pattern. In such scenario,
it is very easy to confuse that num-foos will be let-bound. It does not
help that the whole thing looks like an ordinary function call.

I'd suggest to provide a more clear indication about which symbol will
be bound, so that it stands out compared to ordinary variables like
min-foos.

>   Constrained variable constructs can be nested
>
>   For example,
>
>     (< (integerp num-foos) 1)  ; Match any integer number > 1,
>                                ;  bind `num-foos' to it.

This is also confusing. Looking at the above expression, I expect that
we compare the return value of `integerp' with 1, but it is not the case
- integerp returns nil or t. Again, it looks deceivingly similar to
normal function call.

> General constrained variable: (constrain VAR EXPRESSION)
>
>   This general constrained variable pattern binds VAR to the
>   value being matched against, tentatively, then evaluates EXPRESSION.
>   If the result is true, the match succeeds and leaves VAR
>   bound to that value.
>
>   For instance,
>
>     (constrain x (and (> x 0) (< x 100)))
>
>   succeeds if the value being matched aainst is in the open interval (0, 100),
>   and in that case it binds x to that value.

This looks much better, as `constrain' is a special keyword that can be
easily recognized and not confused with a function call.

-- 
Ihor Radchenko // yantar92,
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