Add 'external' completion style for completions from external tools

This completion style has been in use in Eglot and other third-party
extensions for a long time.  It's meant for libraries that interface
with an external tool which provides completions, such as a shell
utility, an inferior process, an http server.

The new file lisp/external-completion.el provides a helper function,
external-completion-table which is given an interfacing function to
the external tool and returns a "programmed completion" table that is
bound to the 'external' completion style.  That table can then be used
with completing-read or any other facility expecting a completion
table.

When the 'external' is in use, the usual styles configured by the user
or other in 'completion-styles' are completely overriden.  This
relatively minor inconvenience is the price to pay for responsive
completion where the full set of completion candidates doesn't need to
be transfered into Emacs's address space.

* lisp/external-completion.el: New file.

diff --git a/lisp/external-completion.el b/lisp/external-completion.el
new file mode 100644 (file)
index 0000000..5962011
--- /dev/null
+++ b/lisp/external-completion.el
@@ -0,0 +1,174 @@
+;;; external-completion.el --- Let external tools control completion style  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2018-2022 Free Software Foundation, Inc.
+
+;; Version: 0.1
+;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
+;; Maintainer: João Távora <joaotavora@gmail.com>
+;; Keywords:
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Written by Stefan Monnier circa 2016.  Variants of this code have
+;; been working stably in SLY and other packages for a long time.
+
+;; The `external' completion style is used with a "programmable
+;; completion" table that gathers completions from an external tool
+;; such as a shell utility, an inferior process, an http server.
+
+;; The table and external tool are fully in control of the matching of
+;; the pattern string to the potential candidates of completion.  When
+;; `external' is in use, the usual styles configured by the user or
+;; other in `completion-styles' are ignored.
+;;
+;; This compromise is for speed: all other styles need the full data
+;; set to be available in Emacs' addressing space, which is often slow
+;; if not completely unfeasible.
+;;
+;; To make use of the `external' style the function
+;; `external-completion-table' should be used.  See its docstring.
+
+;;; Code:
+(require 'cl-lib)
+
+(add-to-list 'completion-styles-alist
+             '(external
+               external-completion--try-completion
+               external-completion--all-completions
+               "Ad-hoc completion style provided by the completion table."))
+
+(defun external-completion-table (category lookup
+                                           &optional metadata
+                                           try-completion-function)
+  "Make completion table using the `external' completion style.
+
+The `external' style is particularly useful when the caller
+interfaces with an external tool that provides completions.  This
+may be a shell utility, an inferior process, an http server, etc.
+Given a pattern string, the external tool matches it to an
+arbitrarily large set of candidates.  Since the full set doesn't
+need to be transferred to Emacs's address space, this often
+results in much faster overall experience, at the expense of the
+convenience of offered by other completion styles.
+
+CATEGORY is a symbol uniquely naming the external tool.  This
+function links CATEGORY to the style `external', by modifying
+`completion-category-defaults', overriding any styles normally
+set in `completion-styles'.
+
+LOOKUP is a function taking a string PATTERN and a number
+POINT. The function should contact the backend and return a list
+of strings representing the completions for PATTERN given that
+POINT is the location of point within it.  LOOKUP decides if
+PATTERN is interpreted as a substring, a regular expression, or
+any other type of matching method.  The strings returned may be
+propertized with `completions-common-part' to illustrate the
+specific method used.  LOOKUP may ignore POINT if it doesn't
+meaningfully alter the results.
+
+LOOKUP is a synchronous blocking function.  Since it contacts an
+external tool, it's possible that it takes significant time to
+return results.  To maintain Emacs's responsiveness, LOOKUP
+should detect pending user input using `while-no-input' or
+`sit-for' (which see).  In those cases, LOOKUP should attempt to
+cancel the request (if possible) and immediately return any
+non-list.
+
+METADATA is an alist of additional properties such as
+`cycle-sort-function' to associate with CATEGORY.  This means
+that the caller may still retain control the sorting of the
+candidates while the tool controls the matching.
+
+Optional TRY-COMPLETION-FUNCTION helps some frontends partially
+or fully expand PATTERN before finishing the completion
+operation.  If supplied, it is a function taking a (PATTERN POINT
+ALL-COMPLETIONS), where PATTERN and POINT are as described above
+and ALL-COMPLETIONS are gathered by LOOKUP for these
+arguments (this function ensures LOOKUP isn't called more than
+needed).  If you know the matching method that the external tool
+using, TRY-COMPLETION-FUNCTION may return a cons
+cell (EXPANDED-PATTERN . NEW-POINT).  For example, if the tool is
+completing by prefix, one could call `try-completion' to find the
+largest common prefix in ALL-COMPLETIONS and then return that as
+EXPANDED-PATTERN."
+  (let ((probe (alist-get category completion-category-defaults)))
+    (if probe
+        (cl-assert (equal '(external) (alist-get 'styles probe))
+                   nil "Category `%s' must only use `external' style" category)
+        (push `(,category (styles external))
+              completion-category-defaults)))
+  (let ((cache (make-hash-table :test #'equal)))
+    (cl-flet ((lookup-internal (string point)
+                (let* ((key (cons string point))
+                       (probe (gethash key cache 'external--notfound)))
+                  (if (eq probe 'external--notfound)
+                      (puthash key (funcall lookup string point) cache)
+                    probe))))
+      (lambda (string pred action)
+        (pcase action
+          (`metadata
+           `(metadata (category . ,category) . ,metadata))
+          ;; Note: the `--tryc' `--allc' suffixes are made akward on
+          ;; purpose, so it's easy to pick them apart from the jungle
+          ;; of combinations of "try" and "all" and "completion" that
+          ;; inhabit Emacs's completion logic.
+          (`(external-completion--tryc . ,point)
+           ;; FIXME: Obey `pred'?  Pass it to `try-completion-function'?
+           `(external-completion--tryc
+             . ,(if try-completion-function
+                    (funcall try-completion-function
+                             string
+                             point
+                             (lookup-internal string point))
+                  (cons string point))))
+          (`(external-completion--allc . ,point)
+           (let ((all (lookup-internal string point)))
+             `(external-completion--allc
+               . ,(if pred (cl-remove-if-not pred all) all))))
+          (`(boundaries . ,_) nil)
+          (_method
+           (let ((all (lookup-internal string (length string))))
+             ;; This branch might be taken:
+             ;;
+             ;; * when users work around
+             ;;   `completion-category-defaults' (via
+             ;;   `completion-category-overrides') and access this
+             ;;   table with another completion style.  We assume
+             ;;   these users know what they are doing, but it might
+             ;;   not work very well, as this whatever is in `all'
+             ;;   very often doesn't equate the full set of candidates
+             ;;   (many tools cap to sth like 100-1000 results).
+             ;;
+             ;; * when `_method' is `nil' or `lambda' which some
+             ;;   frontends will invoke.  Here, `all' should be
+             ;;   suficient information for `complete-with-action' to
+             ;;   do the job correctly.
+             (complete-with-action action all string pred))))))))
+
+(defun external-completion--call (op string table pred point)
+  (when (functionp table)
+    (let ((res (funcall table string pred (cons op point))))
+      (when (eq op (car-safe res))
+        (cdr res)))))
+
+(defun external-completion--try-completion (string table pred point)
+  (external-completion--call 'external-completion--tryc string table pred point))
+
+(defun external-completion--all-completions (string table pred point)
+  (external-completion--call 'external-completion--allc string table pred point))
+
+(provide 'external-completion)
+;;; external-completion.el ends here