Add racket-doc-index-predicate-function

Allows user to configure showing subset of search results, such as for
a certain language family.

doc/generate.el

@@ -216,6 +216,7 @@
     racket-repl-command-file
     "Other variables"
     racket-doc-index-directory
+    racket-doc-index-predicate-function
     racket-indent-curly-as-sequence
     racket-indent-sequence-depth
     racket-pretty-lambda

doc/racket-mode.texi

@@ -242,6 +242,7 @@ REPL variables
 Other variables
 
 * racket-doc-index-directory::
+* racket-doc-index-predicate-function::
 * racket-indent-curly-as-sequence::
 * racket-indent-sequence-depth::
 * racket-pretty-lambda::
@@ -3567,6 +3568,7 @@ Name of the file used by @ref{racket-repl}.
 
 @menu
 * racket-doc-index-directory::
+* racket-doc-index-predicate-function::
 * racket-indent-curly-as-sequence::
 * racket-indent-sequence-depth::
 * racket-pretty-lambda::
@@ -3582,6 +3584,21 @@ Name of the file used by @ref{racket-repl}.
 
 Directory for @ref{racket-describe-search} doc index files.
 
+@node racket-doc-index-predicate-function
+@subsection racket-doc-index-predicate-function
+
+A function used by @ref{racket-describe-search} to filter results.
+
+The default value, the @code{always} function, filters nothing.
+
+The function is given four string arguments -- TERM, WHAT,
+FROM-LIBS, and FAMILIES -- and should return whether to include
+the item in the list of completion candidates. An example that
+limits candidates to the ``Rhombus'' family:
+
+(lambda (@math{_term} _what _from-libs families)
+  (string-equal families ``Rhombus'')
+
 @node racket-indent-curly-as-sequence
 @subsection racket-indent-curly-as-sequence
 

racket-custom.el

@@ -227,6 +227,23 @@ Scribble text, use the face `racket-hash-lang-text'."
   "Directory for `racket-describe-search' doc index files."
   :type 'file)
 
+(defcustom racket-doc-index-predicate-function
+  'always
+  "A function used by `racket-describe-search' to filter results.
+
+The default value, the `always' function, filters nothing.
+
+The function is given four string arguments -- TERM, WHAT,
+FROM-LIBS, and FAMILIES -- and should return whether to include
+the item in the list of completion candidates. An example that
+limits candidates to the \"Rhombus\" family:
+
+    (lambda (_term _what _from-libs families)
+      (string-equal families \"Rhombus\")
+"
+  :type 'function
+  :risky t)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; racket-repl group
 

racket-describe.el

@@ -687,7 +687,9 @@ the symbol at point into the minibuffer."
                (racket--doc-index)
                `((category . ,racket--identifier-category)
                  (affixation-function . ,#'racket--doc-index-affixator)))
-              nil                       ;predicate
+              (lambda (v)
+                (apply racket-doc-index-predicate-function
+                       (get-text-property 0 'racket-affix (car v))))
               t                         ;require-match
               nil                       ;initial-input
               nil                       ;hist