Introduce inf-clojure-completions-fn defcustom

Now the user can provide a custom parsing function for completion and therefore
has complete (pun intended) freedom in what to use for it. Some could use
compliment completion for instance or even use directly what cider provides.
The defcustom defaults to inf-clojure-list-completions, which can only parse
candidates coming as a Lisp list of strings.

Author: arichiardi

inf-clojure.el

@@ -1210,7 +1210,7 @@ prefix argument PROMPT-FOR-NS, it prompts for a namespace name."
 
 (defun inf-clojure-set-ns (prompt-for-ns)
   "Set the ns of the inferior Clojure process to NS.
-See variable `inf-clojure-set-ns-form`.  It defaults to the ns of
+See variable `inf-clojure-set-ns-form'.  It defaults to the ns of
 the current buffer.  When invoked with a prefix argument
 PROMPT-FOR-NS, it prompts for a namespace name."
   (interactive "P")
@@ -1254,14 +1254,52 @@ See variable `inf-clojure-buffer'."
   "Return DATA if and only if it is a list."
   (when (listp data) data))
 
+(defun inf-clojure-list-completions (response-str)
+  "Parse completions from RESPONSE-STR.
+
+Its only ability is to parse a Lisp list of candidate strings,
+every other EXPR will be discarded and nil will be returned."
+  (thread-first
+      response-str
+    (inf-clojure--read-or-nil)
+    (inf-clojure--list-or-nil)))
+
 (defun inf-clojure-completions (expr)
-  "Return a list of completions for the Clojure expression starting with EXPR."
+  "Return completions for the Clojure expression starting with EXPR.
+
+Under the hood it calls the function
+\\[inf-clojure-completions-fn] passing in the result of
+evaluating \\[inf-clojure-completion-form] at the REPL."
   (when (not (string-blank-p expr))
-    (thread-first
-        (format (inf-clojure-completion-form) (substring-no-properties expr))
-      (inf-clojure--process-response (inf-clojure-proc) "(" ")")
-      (inf-clojure--read-or-nil)
-      (inf-clojure--list-or-nil))))
+    (let ((proc (inf-clojure-proc))
+          (completion-form (format (inf-clojure-completion-form) (substring-no-properties expr))))
+      (funcall inf-clojure-completions-fn
+               (inf-clojure--process-response completion-form proc  "(" ")")))))
+
+(defcustom inf-clojure-completions-fn 'inf-clojure-list-completions
+  "The function that parses completion results.
+
+It is a single-arity function that will receive the REPL
+evaluation result of \\[inf-clojure-completion-form] as string and
+should return elisp data compatible with your completion mode.
+
+The easiest possible data passed in input is a list of
+candidates (e.g.: (\"def\" \"defn\")) but more complex libraries
+like `alexander-yakushev/compliment' can return other things like
+edn.
+
+The expected return depends on the mode that you use for
+completion: `company-mode' for instance allows metadata along
+with the candidates completion. See \\[completion-table-dynamic]
+for more details.
+
+The default value is the `inf-clojure-list-completions' function,
+which is able to parse results in list form only.  You can peek
+at its implementation for getting to know some utility functions
+you might want to use in your customization."
+  :type 'function
+  :safe #'functionp
+  :package-version '(inf-clojure . "2.1.0"))
 
 (defconst inf-clojure-clojure-expr-break-chars " \t\n\"\'`><,;|&{()[]")