New function 'hash-table-contains-p'

This function tests whether a given key is present in a hash table.
Emacs Lisp has long lacked a standard way to do this, leading users to
write one of:

    (not (eq (gethash key table 'missing) 'missing))
or
    (gethash key table)

This idiom is error-prone (when 'missing' or 'nil' are valid values),
and it obscures intent.  The new function avoids such pitfalls,
improves readability, and makes the intent explicit:

    (hash-table-contains-p key table)

The name 'hash-table-contains-p' exists in other Lisp dialects (e.g.,
SRFI-125), and follows the precedent of 'seq-contains-p'.  Other
alternatives considered include `hash-table-has-key-p` and
`hash-table-key-exists-p`, but none were clearly better.

This was previously discussed in 2018, and all comments were positive,
but the proposed patch (implementing it in C) was never pushed:
https://lists.gnu.org/r/emacs-devel/2018-02/msg00424.html

* lisp/subr.el (hash-table-contains-p): New function.
* lisp/emacs-lisp/shortdoc.el (hash-table):
* doc/lispref/hash.texi (Other Hash): Document the new function.
* test/lisp/subr-tests.el (hash-table-contains-p): New test.

doc/lispref/hash.texi

@@ -347,6 +347,11 @@ itself is copied---the keys and values are shared.
 This function returns the actual number of entries in @var{table}.
 @end defun
 
+@defun hash-table-contains-p key table
+This returns non-@code{nil} if there is an association for @var{key} in
+@var{table}.
+@end defun
+
 @defun hash-table-test table
 This returns the @var{test} value that was given when @var{table} was
 created, to specify how to hash and compare keys.  See

etc/NEWS

@@ -1786,6 +1786,10 @@ Optional arguments are provided to produce human-readable time-duration
 strings in a variety of formats, for example "6 months 3 weeks" or "5m
 52.5s".
 
++++
+** New function 'hash-table-contains-p'.
+This function returns non-nil if a given key is present in a hash table.
+
 +++
 ** The function 'purecopy' is now an obsolete alias for 'identity'.
 

lisp/emacs-lisp/shortdoc.el

@@ -710,6 +710,8 @@ A FUNC form can have any number of `:no-eval' (or `:no-value'),
   "Other Hash Table Functions"
   (hash-table-p
    :eval (hash-table-p 123))
+  (hash-table-contains-p
+   :no-eval (hash-table-contains-p 'key table))
   (copy-hash-table
    :no-eval (copy-hash-table table)
    :result-string "#s(hash-table ...)")

lisp/subr.el

@@ -7392,6 +7392,13 @@ TRIM-LEFT and TRIM-RIGHT default to \"[ \\t\\n\\r]+\"."
   (declare (important-return-value t))
   (string-trim-left (string-trim-right string trim-right) trim-left))
 
+(defsubst hash-table-contains-p (key table)
+  "Return non-nil if TABLE has an element with KEY."
+  (declare (side-effect-free t)
+           (important-return-value t))
+  (let ((missing (make-symbol "missing")))
+    (not (eq (gethash key table missing) missing))))
+
 ;; The initial anchoring is for better performance in searching matches.
 (defconst regexp-unmatchable "\\`a\\`"
   "Standard regexp guaranteed not to match any string at all.")

test/lisp/subr-tests.el

@@ -1489,5 +1489,17 @@ final or penultimate step during initialization."))
                  (props-out (object-intervals out)))
             (should (equal props-out props-in))))))))
 
+(ert-deftest hash-table-contains-p ()
+  (let ((h (make-hash-table)))
+    (should-not (hash-table-contains-p 'problems h))
+    (should-not (hash-table-contains-p 'cookie h))
+    (should-not (hash-table-contains-p 'milk h))
+    (puthash 'problems 99 h)
+    (puthash 'cookie nil h)
+    (puthash 'milk 'missing h)
+    (should (hash-table-contains-p 'problems h))
+    (should (hash-table-contains-p 'cookie h))
+    (should (hash-table-contains-p 'milk h))))
+
 (provide 'subr-tests)
 ;;; subr-tests.el ends here