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Re: [Emacs-diffs] map 988d721: Add a pcase pattern for maps and `map-let
From: |
Stefan Monnier |
Subject: |
Re: [Emacs-diffs] map 988d721: Add a pcase pattern for maps and `map-let' based on it |
Date: |
Tue, 02 Jun 2015 21:06:44 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux) |
> + "pcase pattern matching map elements.
> +Matches if the object is a map (list, hash-table or array), and
> +binds values from ARGS to the corresponding element of the map.
> +
> +ARGS can be an alist of key/binding pairs of a list of keys."
I think this would benefit from a bit more detail. E.g. you could say
"ARGS is a list of elements of the form (KEY PAT)" or something
like that.
> + (seq-map (lambda (elt)
> + (if (consp elt)
> + `(app (pcase--flip map-elt ',(car elt)) ,(cdr elt))
> + `(app (pcase--flip map-elt ',elt) ,elt)))
> + args))
Hmm... It looks like it's actually elements of the form (KEY . PAT) or
elements of the form SYMBOL which stands for (SYMBOL . SYMBOL).
Some alternatives to consider:
- Use (KEY PAT) instead of (KEY . PAT). This makes the source code
a bit more concise, so I'm in favor of the (KEY PAT) form.
- Use a plist rather than an alist. I like alists better in general, so
I'm fine with this choice.
- let KEY be evaluated rather than having it be quoted. I.e. require
the programmer to write 'a when looking up the key `a'. It costs an
extra quote in some/many cases, but does give you extra power.
Of course, the downside is that it begs the question of what to do
with the SYMBOL case.
FWIW, in dash.el, the `-let' includes a similar destructuring
facility, with patterns of the form:
(&plist key0 a0 ... keyN aN) - bind value mapped by keyK in the
`source` plist to aK. If the
value is not found, aK is nil.
(&alist key0 a0 ... keyN aN) - bind value mapped by keyK in the
`source` alist to aK. If the
value is not found, aK is nil.
(&hash key0 a0 ... keyN aN) - bind value mapped by keyK in the
`source` hash table to aK. If the
value is not found, aK is nil.
where, AFAICT the `key's are expressions which are evaluated before
being passed to the respective lookup function. IOW when you want
a constant key you need to quote it, unless that constant is
self-quoting (e.g. a keyword).
Stefan
- Re: [Emacs-diffs] map 988d721: Add a pcase pattern for maps and `map-let' based on it,
Stefan Monnier <=