Re: Instead of pcase

[Ihor Radchenko <yantar92@posteo.net>]

Richard Stallman <rms@gnu.org> writes:

> For tests which don't fit that form, you can use `constrain'.
> Here's a valid example:
>
> `(,(integerp smaller)
>   ,(constrain bigger (> (= (mod (- (integerp bigger) dec) 5) 0) smaller)))
>
> Does the text I sent fail to make this clear?  Would you please take
> a look, and if it is not clear, tell me why?

Your description is clear. However, it is somewhat similar to the
existing `pcase' description in a way that it is also clear, but goes in
odds with the usual Elisp structures - I see nested "constrained
variable" construct yet as another DSL. I see myself coming to back the
documentation of `cond*' again and again, just as I do with `pcase' now.

Let me try to illustrate the problem with another example:

    ;; ALIST-VAL = '((:key1 . 1) (:key4 . 2))
    ;; Match alist key where the key is one of a know set of keys.
    (match* (memq (alist-get key alist-val) '(:key1 :key2 :key3)) :key1)

From the description of "constrained variable" construct, the above will
(1) test if ALIST-VAL contains a record with KEY=:key1 key; (2) test if
KEY=:key1 is one of :key1 :key2 :key3.

However, the above is also deceivingly similar to Elisp expression that
tests the return value of `alist-get': (memq 1 '(:key1 :key2 :key3))

-- 
Ihor Radchenko // yantar92,
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