[PATCH] extend map-into (was: new function proposal alist-to-hash)

[Andrea Corallo <akrl@sdf.org>]

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Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Yes, this is a very serious limitation of `map-into`.
> I decided not to try to tackle it when I converted map.el to use
> cl-defmethod, but I'd welcome help on this.
>
>
>         Stefan

Hi,
would a solution like the one in attached patch do the job?
I've added a rest arg so that the current code should not be impacted.

  Andrea

-- 
akrl@sdf.org

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From 2b94c00384f7bd4bfd7b6108e5be3b261e1cf018 Mon Sep 17 00:00:00 2001
From: Andrea Corallo <akrl@sdf.org>
Date: Sat, 5 Oct 2019 09:38:13 +0200
Subject: [PATCH] Extend 'map-into' for better control on hash table creation

---
 etc/NEWS               |  5 +++++
 lisp/emacs-lisp/map.el | 22 ++++++++++++----------
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index c8cc753..caea917 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -24,6 +24,11 @@ applies, and please also update docstrings as needed.
 \f
 * Installation Changes in Emacs 27.1
 
+** Extend function 'map-into'.
+Change 'map-into' lambda-list into (map type &rest args).
+When 'map-into' is used to create hash tables args are now forwarded to
+'make-hash-table' for better control.
+
 ** Emacs now uses GMP, the GNU Multiple Precision library.
 By default, if 'configure' does not find a suitable libgmp, it
 arranges for the included mini-gmp library to be built and used.
diff --git a/lisp/emacs-lisp/map.el b/lisp/emacs-lisp/map.el
index 54e802e..7360705 100644
--- a/lisp/emacs-lisp/map.el
+++ b/lisp/emacs-lisp/map.el
@@ -367,12 +367,12 @@ map-merge-with
                  (pop maps)))
     result))
 
-(cl-defgeneric map-into (map type)
+(cl-defgeneric map-into (map type &rest args)
   "Convert the map MAP into a map of type TYPE.")
 ;; FIXME: I wish there was a way to avoid this η-redex!
-(cl-defmethod map-into (map (_type (eql list))) (map-pairs map))
-(cl-defmethod map-into (map (_type (eql alist))) (map-pairs map))
-(cl-defmethod map-into (map (_type (eql plist)))
+(cl-defmethod map-into (map (_type (eql list)) &rest _) (map-pairs map))
+(cl-defmethod map-into (map (_type (eql alist)) &rest _) (map-pairs map))
+(cl-defmethod map-into (map (_type (eql plist)) &rest _)
   (let ((plist '()))
     (map-do (lambda (k v) (setq plist `(,k ,v ,@plist))) map)
     plist))
@@ -458,12 +458,14 @@ map-do
                      (funcall function index elt))
                    array))
 
-(cl-defmethod map-into (map (_type (eql hash-table)))
-  "Convert MAP into a hash-table."
-  ;; FIXME: Just knowing we want a hash-table is insufficient, since that
-  ;; doesn't tell us the test function to use with it!
-  (let ((ht (make-hash-table :size (map-length map)
-                             :test 'equal)))
+(cl-defmethod map-into (map (_type (eql hash-table)) &rest keyword-args)
+  "Convert MAP into a hash-table.
+If KEYWORD-ARGS are provided they are forwarded to `make-hash-table'.
+If KEYWORD-ARGS is nil hash table size is set to MAP length and test to `equal'."
+  (let* ((h-args (if keyword-args
+                     keyword-args
+                   (list :size (map-length map) :test 'equal)))
+         (ht (apply #'make-hash-table h-args)))
     (map-apply (lambda (key value)
                  (setf (map-elt ht key) value))
                map)
-- 
2.7.4