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Re: [Emacs-diffs] master 9dd95bf: * lisp/emacs-lisp/pcase.el (pcase--u1)

From: Stefan Monnier
Subject: Re: [Emacs-diffs] master 9dd95bf: * lisp/emacs-lisp/pcase.el (pcase--u1): Fix bignums
Date: Fri, 26 Oct 2018 15:37:20 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

>> >> Would you feel the same if `pcase` always used `equal` and the
>> >> optimization to `eql` were performed in the byte-compiler instead?
>> > I don't know, and it's not really relevant, is it?
>> It is: the use of `eq` or `eql` here is an optimization which pcase
>> performs only because the compiler doesn't do it.
> I disagree.  We describe our code, not what the compiler produces out
> of it.

Here I disagree: the pcase doc does not describe the generated code, it
only describes the resulting behavior.

>> >   ‘KEYWORD’
>> >   ‘INTEGER’
>> >   ‘STRING’
>> >        Matches if EXPVAL is equal to the literal object.  The equality
>> >        predicate depends on the type of the object; e.g., symbols are
>> >        compared using 'eq', and strings using 'equal'.
>> I think we should say here that the semantics is that of `equal` and not
>> that of `eq` (or `=` or whatever else).
>> The above would allow `pcase` to use `eq` for integers.
> Where does it say something that has that effect?

`eq` is a notion of equality, so "is equal to the literal object" is
a valid description even if we use `eq`.  Futhermore the subsequent text
makes it clear that `pcase` would be free to use whichever notion of
equality it deems appropriate since "The equality predicate depends on
the type of the object".

>> IOW if pcase uses `eq` on integers (as it did until yesterday) and
>> some code uses pcase to match a bignum, the above would let us say
>> that the bug is in the pcase use rather than in the pcase
>> implementation.
>> Compared to the current doc, it also leaves it unclear whether 1.0 would
>> match the '1 pattern.
> So you now agree to describe in detail what predicate is used for each
> data type?

Yes, that's what I've done: `equal` is the predicate used.  ;-)


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