Grab the completion prefix correctly

cider-completion.el

@@ -181,16 +181,25 @@ performed by `cider-annotate-completion-function'."
 
 (defun cider-complete-at-point ()
   "Complete the symbol at point."
-  (when-let* ((bounds (bounds-of-thing-at-point 'symbol)))
+  (when-let* ((bounds (or (bounds-of-thing-at-point 'symbol)
+                          (cons (point) (point))))
+              (bounds-string (buffer-substring (car bounds) (cdr bounds))))
     (when (and (cider-connected-p)
                (not (or (cider-in-string-p) (cider-in-comment-p))))
-      (let* (last-prefix
+      (let* (last-bounds-string
              last-result
              (complete
-              (lambda (prefix)
-                (unless (string-equal last-prefix prefix)
-                  (setq last-prefix prefix)
-                  (setq last-result (cider-complete prefix)))
+              (lambda ()
+                ;; We are Not using the prefix extracted within the (prefix pred action)
+                ;; lambda.  In certain completion styles, the prefix might be an empty
+                ;; string, which is unreliable. A more dependable method is to use the
+                ;; string defined by the bounds of the symbol at point.
+                ;;
+                ;; Caching just within the function is sufficient. Keeping it local
+                ;; ensures that it will not extend across different CIDER sessions.
+                (unless (string= bounds-string last-bounds-string)
+                  (setq last-bounds-string bounds-string)
+                  (setq last-result (cider-complete bounds-string)))
                 last-result)))
         (list (car bounds) (cdr bounds)
               (lambda (prefix pred action)
@@ -205,8 +214,7 @@ performed by `cider-annotate-completion-function'."
                 (cond ((eq action 'metadata) `(metadata (category . cider))) ;; defines a completion category named 'cider, used later in our `completion-category-overrides` logic.
                       ((eq (car-safe action) 'boundaries) nil)
                       (t (with-current-buffer (current-buffer)
-                           (complete-with-action action
-                                                 (funcall complete prefix) prefix pred)))))
+                           (complete-with-action action (funcall complete) prefix pred)))))
               :annotation-function #'cider-annotate-symbol
               :company-kind #'cider-company-symbol-kind
               :company-doc-buffer #'cider-create-compact-doc-buffer
@@ -264,12 +272,6 @@ in the buffer."
                cider-company-unfiltered-candidates
                "CIDER backend-driven completion style."))
 
-;; Currently CIDER completions only work for `basic`, and not `initials`, `partial-completion`, `orderless`, etc.
-;; So we ensure that those other styles aren't used with CIDER, otherwise one would see bad or no completions at all.
-;; This `add-to-list` call can be removed once we implement the other completion styles.
-;; (When doing that, please refactor `cider-enable-flex-completion' as well)
-(add-to-list 'completion-category-overrides '(cider (styles basic)))
-
 (defun cider-company-enable-fuzzy-completion ()
   "Enable backend-driven fuzzy completion in the current buffer.
 
@@ -292,6 +294,8 @@ Only affects the `cider' completion category.`"
     (setq completion-category-overrides (seq-remove (lambda (x)
                                                       (equal 'cider (car x)))
                                                     completion-category-overrides))
+    (unless found-styles
+      (setq found-styles '(styles basic)))
     (unless (member 'flex found-styles)
       (setq found-styles (append found-styles '(flex))))
     (add-to-list 'completion-category-overrides (apply #'list 'cider found-styles (when found-cycle

doc/modules/ROOT/pages/usage/code_completion.adoc

@@ -37,18 +37,27 @@ is already properly indented.
 
 == Completion styles
 
-CIDER defines (via the `completion-styles-alist` Elisp variable) a completion category named `cider`.
+CIDER defines a specialized completion category through the `cider-complete-at-point` function,
+added to `completion-at-point-functions`, establishing a dedicated completion category named
+`cider`.
 
-The `cider` completion category currently only supports `basic` completion styles (and not `partial-completion`, `orderless`, etc),
-and `flex`, optionally (read more below).
+The CIDER completion at point function supports most completion styles, including
+`partial-completion`, `orderless` and `flex` (read more below).
 
-CIDER declares so in this fashion:
+
+Sometimes the user may want to use a different completion style just for the CIDER
+complete at point function. That can be achieved by setting
+`completion-category-overrides`, overriting the completion style of the CIDER
+complete at point function. The following snippet accomplishes that:
 
 [source,lisp]
 ----
 (add-to-list 'completion-category-overrides '(cider (styles basic)))
 ----
 
+This specifies that the `cider` completion category should employ the basic completion style by
+default.
+
 You can also enable the `flex` completion style by activating xref:usage/code_completion.adoc#fuzzy-candidate-matching[fuzzy candidate matching].
 
 == Auto-completion
@@ -131,13 +140,14 @@ without needing to hit an extra key, please customize:
 
 === Fuzzy candidate matching
 
-By default, CIDER will provide completion candidates with the
-assumption that whatever you've typed so far is a prefix of what
-you're really trying to type. For example, if you type `map-` then
-you'll only get completion candidates that have `map-` as the
-beginning of their names.  Sometimes, you don't know the exact prefix
-for the item you want to type. In this case, you can get
-CIDER-specific "fuzzy completion" by setting up in your Emacs init file:
+By default, CIDER will use the completion styles defined in
+`completion-styles`, the defaults being `(basic partial-completion
+emacs22)` since Emacs 23. For a better description of how those
+completion styles operates, refer to the official Emacs manual on
+https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Styles.html[how completion alternatives are chosen].
+
+CIDER provides a function to enable the `flex` completion style for CIDER-specific
+completions. If you wish to enable that, you can add this to your config:
 
 [source,lisp]
 ----
@@ -146,11 +156,11 @@ CIDER-specific "fuzzy completion" by setting up in your Emacs init file:
 
 This adds the `flex` completion style, as introduced in Emacs 27.
 
-Now, `company-mode` will accept certain fuzziness when matching
-candidates against the prefix. For example, typing `mi` will show you
-`map-indexed` as one of the possible completion candidates and `cji`
-will complete to `clojure.java.io`. Different completion examples are
-shown
+Now, `company-mode` (and other completion packages like `corfu`) will
+accept certain fuzziness when matching candidates against the
+prefix. For example, typing `mi` will show you `map-indexed` as one of
+the possible completion candidates and `cji` will complete to
+`clojure.java.io`. Different completion examples are shown
 https://github.com/alexander-yakushev/compliment/wiki/Examples[here].
 
 NOTE: `cider-company-enable-fuzzy-completion` (now deprecated) should be used for Emacs < 27. 
@@ -189,18 +199,6 @@ keys to cancel the prompt by customizing:
                 (funcall f a b))))
 ----
 
-=== Changing the completion style
-
-Sometimes the user may want to use a different completion style just for the CIDER
-complete at point function. That can be achieved by setting
-`completion-category-defaults`, overriting the completion style of the CIDER
-complete at point function. The following snippet accomplishes that:
-
-[source,lisp]
-----
-(add-to-list 'completion-category-defaults '(cider (styles basic)))
-----
-
 === Updating stale classes and methods cache
 
 Sometimes, the completion fails to recognize new classes that came with

test/cider-completion-tests.el

@@ -36,15 +36,27 @@
     (let ((old-value completion-category-overrides))
       (unwind-protect
           (progn
-            (it "adds `flex'"
+            (it "adds `flex' and `basic' as a fallback"
               (cider-enable-flex-completion)
               (expect (member 'flex (assq 'styles (assq 'cider completion-category-overrides)))
+                      :to-be-truthy)
+              (expect (member 'basic (assq 'styles (assq 'cider completion-category-overrides)))
                       :to-be-truthy))
 
             (it "doesn't add `cycle'"
               (expect (assq 'cycle (assq 'cider completion-category-overrides))
                       :to-be nil))
 
+            (it "adds just `flex' if there is another style present"
+              (setq completion-category-overrides '((cider (styles partial-completion))))
+              (cider-enable-flex-completion)
+              (expect (member 'flex (assq 'styles (assq 'cider completion-category-overrides)))
+                      :to-be-truthy)
+              (expect (member 'partial-completion (assq 'styles (assq 'cider completion-category-overrides)))
+                      :to-be-truthy)
+              (expect (member 'basic (assq 'styles (assq 'cider completion-category-overrides)))
+                      :to-be nil))
+
             (it "doesn't re-add `flex' if already present, preserving `cycle' as well"
               (let ((with-flex-and-cycle '((cider (styles basic flex)
                                                   (cycle t)))))