Re: Instead of pcase

[Richard Stallman]

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  > 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.

What a strange example.  This is connterintuitive and I could not see
at first what it would mean.  But I worked it out by following the
rules.

(match* PATTERN :key1) matches PATTERN against the data object :key1.

(memq (alist-get key alist-val) '(:key1 :key2 :key3))
as a pattern means to match (alist-get key alist-val) aainst it,
and if that matches, then call memq to determine whether this matches,

It would be matched against :key1.

Matching (alist-get key alist-val) against :key1 would succeed,
so `key' would be bound to `key1'.

So the memq call would return t.  And the pattern would match,
binding `key' to :key1.

It isn't "wrong", but I can't see the point using that strange 
pattern.  What purpose did you have in mind in writing it?
Is there a simpler way to do that?


-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)