bug#49407: Request: Specify default values in `map-let` in Map.el

[Okam]

On 2023-07-22 01:48 UTC, Michael Heerdegen wrote:
> 
> | diff --git a/lisp/emacs-lisp/map.el b/lisp/emacs-lisp/map.el
> | index 7a48ba47434..7f127f312de 100644
> | --- a/lisp/emacs-lisp/map.el
> | +++ b/lisp/emacs-lisp/map.el
> | @@ -50,18 +50,23 @@ map
> |
> |  ARGS is a list of elements to be matched in the map.
> |
> | -Each element of ARGS can be of the form (KEY PAT), in which case KEY is
> | -evaluated and searched for in the map.  The match fails if for any KEY
> | -found in the map, the corresponding PAT doesn't match the value
> | -associated with the KEY.
> | +Each element of ARGS can be of the form (KEY PAT) or (KEY PAT DEFAULT),
> | +
> | +in which case KEY is evaluated and searched for in the map and
> 
> That line break in the last sentence is somehow... broken.
> 
> | +DEFAULT is the evaluated value used if KEY is not found.  The
> | +match fails if for any KEY found in the map, the corresponding
> | +PAT doesn't match the value associated with the KEY.  The match
> | +fails if for any KEY not found in the map, the corresponding PAT
> | +doesn't match the DEFAULT value, if given.
> 
> Here is something grammatically not right in the last sentence.  And I
> think we should say that that last case (PAT doesn't match DEFAULT) only
> lets matching fail when the KEY is not found.
> 
> 
> | -Keys in ARGS not found in the map are ignored, and the match doesn't
> | -fail."
> | +When no DEFAULT value is given, the default value for keys in
> | +ARGS not found in the map is nil, and the match doesn't fail so
> | +long at PAT, if given, matches nil."
> |    `(and (pred mapp)
> |          ,@(map--make-pcase-bindings args)))
> 
> Typo in doc ("at").  And can't we simply say that omitting DEFAULT is
> equivalent to a DEFAULT of nil?
> 
> | @@ -599,7 +606,11 @@ map--make-pcase-bindings
> |    "Return a list of pcase bindings from ARGS to the elements of a map."
> |    (mapcar (lambda (elt)
> |              (cond ((consp elt)
> | -                   `(app (pcase--flip map-elt ,(car elt)) ,(cadr elt)))
> | +                   (if (cddr elt)
> | +                       `(app (lambda (arg)
> | +                               (map-elt arg ,(car elt) ,(caddr elt)))
> | +                             ,(cadr elt))
> | +                     `(app (pcase--flip map-elt ,(car elt)) ,(cadr elt))))
> 
> If we don't extend `pcase--flip' to support this case, I think your
> lambda would need to use an uninterned symbol as parameter to avoid
> collisions with the involved expressions.
> 
> 
> Michael.

Please see the attached file. I have re-worded the doc string and added 
a wrapper macro to re-order the arguments to `map-elt` for the `app` 
pattern.

v2-0001-Allow-default-values-in-map-let-and-the-pcase-map.patch
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