Add info-lookup-symbol support

Provide a command to register the Dash Info index under
emacs-lisp-mode in info-lookup-alist for the benefit of 'C-h S', and
provide an unloader function for unload-feature to undo this.

* dash.el (dash--info-doc-spec, dash--info-elisp-docs)
(dash-register-info-lookup, dash-unload-function): New definitions.
* dash-template.texi (Info symbol lookup):
* readme-template.md (Info symbol lookup): New sections.

* README.md:
* dash.texi: Regenerate docs.

Fixes #326.

README.md

@@ -58,6 +58,21 @@ either interactively or from your `user-init-file`:
 (global-dash-fontify-mode)
 ```
 
+## Info symbol lookup
+
+While editing Elisp files, you can use `C-h S` (`info-lookup-symbol`)
+to look up Elisp symbols in the relevant Info manuals (see [`(emacs)
+Info
+Lookup`](https://gnu.org/software/emacs/manual/html_node/emacs/Info-Lookup.html)).
+To enable the same for Dash symbols, use the command
+`dash-register-info-lookup`.  It can be called directly when needed,
+or automatically from your `user-init-file`.  For example:
+
+```el
+(with-eval-after-load 'info-look
+  (dash-register-info-lookup))
+```
+
 ## Functions
 
 All functions and constructs in the library use a dash (`-`) prefix.

dash-template.texi

@@ -64,6 +64,7 @@ Installation
 
 * Using in a package::  Listing Dash as a package dependency.
 * Fontification of special variables::  Font Lock of anaphoric macro variables.
+* Info symbol lookup::  Looking up Dash symbols in this manual.
 
 Functions
 
@@ -101,6 +102,7 @@ Alternatively, you can just dump @file{dash.el} or
 @menu
 * Using in a package::  Listing Dash as a package dependency.
 * Fontification of special variables::  Font Lock of anaphoric macro variables.
+* Info symbol lookup::  Looking up Dash symbols in this manual.
 @end menu
 
 @node Using in a package
@@ -142,6 +144,23 @@ call its autoloaded global counterpart
 (global-dash-fontify-mode)
 @end lisp
 
+@node Info symbol lookup
+@section Info symbol lookup
+
+@findex dash-register-info-lookup
+While editing Elisp files, you can use @kbd{C-h S}
+(@code{info-lookup-symbol}) to look up Elisp symbols in the relevant
+Info manuals (@pxref{Info Lookup,,, emacs, The GNU Emacs Manual}).  To
+enable the same for Dash symbols, use the command
+@code{dash-register-info-lookup}.  It can be called directly when
+needed, or automatically from your @code{user-init-file}.  For
+example:
+
+@lisp
+(with-eval-after-load 'info-look
+  (dash-register-info-lookup))
+@end lisp
+
 @node Functions
 @chapter Functions
 

dash.el

@@ -2894,7 +2894,7 @@ replaced with new ones.  This is useful when you need to clone a
 structure such as plist or alist."
   (declare (pure t) (side-effect-free t))
   (-tree-map 'identity list))
-
+
 ;;; Font lock
 
 (defvar dash--keywords
@@ -3050,6 +3050,39 @@ See also `dash-fontify-mode-lighter' and
 
 (define-obsolete-function-alias
   'dash-enable-font-lock #'global-dash-fontify-mode "2.18.0")
+
+;;; Info
+
+(defvar dash--info-doc-spec '("(dash) Index" nil "^ -+ .*: " "\\( \\|$\\)")
+  "The Dash :doc-spec entry for `info-lookup-alist'.
+It is based on that for `emacs-lisp-mode'.")
+
+(defun dash--info-elisp-docs ()
+  "Return the `emacs-lisp-mode' symbol docs from `info-lookup-alist'.
+Specifically, return the cons containing their
+`info-lookup->doc-spec' so that we can modify it."
+  (defvar info-lookup-alist)
+  (nthcdr 3 (assq #'emacs-lisp-mode (cdr (assq 'symbol info-lookup-alist)))))
+
+;;;###autoload
+(defun dash-register-info-lookup ()
+  "Register the Dash Info manual with `info-lookup-symbol'.
+This allows Dash symbols to be looked up with \\[info-lookup-symbol]."
+  (interactive)
+  (require 'info-look)
+  (let ((docs (dash--info-elisp-docs)))
+    (setcar docs (append (car docs) (list dash--info-doc-spec)))
+    (info-lookup-reset)))
+
+(defun dash-unload-function ()
+  "Remove Dash from `info-lookup-alist'.
+Used by `unload-feature', which see."
+  (let ((docs (and (featurep 'info-look)
+                   (dash--info-elisp-docs))))
+    (when (member dash--info-doc-spec (car docs))
+      (setcar docs (remove dash--info-doc-spec (car docs)))
+      (info-lookup-reset)))
+  nil)
 
 (provide 'dash)
 ;;; dash.el ends here

dash.texi

@@ -64,6 +64,7 @@ Installation
 
 * Using in a package::  Listing Dash as a package dependency.
 * Fontification of special variables::  Font Lock of anaphoric macro variables.
+* Info symbol lookup::  Looking up Dash symbols in this manual.
 
 Functions
 
@@ -116,6 +117,7 @@ Alternatively, you can just dump @file{dash.el} or
 @menu
 * Using in a package::  Listing Dash as a package dependency.
 * Fontification of special variables::  Font Lock of anaphoric macro variables.
+* Info symbol lookup::  Looking up Dash symbols in this manual.
 @end menu
 
 @node Using in a package
@@ -157,6 +159,23 @@ call its autoloaded global counterpart
 (global-dash-fontify-mode)
 @end lisp
 
+@node Info symbol lookup
+@section Info symbol lookup
+
+@findex dash-register-info-lookup
+While editing Elisp files, you can use @kbd{C-h S}
+(@code{info-lookup-symbol}) to look up Elisp symbols in the relevant
+Info manuals (@pxref{Info Lookup,,, emacs, The GNU Emacs Manual}).  To
+enable the same for Dash symbols, use the command
+@code{dash-register-info-lookup}.  It can be called directly when
+needed, or automatically from your @code{user-init-file}.  For
+example:
+
+@lisp
+(with-eval-after-load 'info-look
+  (dash-register-info-lookup))
+@end lisp
+
 @node Functions
 @chapter Functions
 

readme-template.md

@@ -58,6 +58,21 @@ either interactively or from your `user-init-file`:
 (global-dash-fontify-mode)
 ```
 
+## Info symbol lookup
+
+While editing Elisp files, you can use `C-h S` (`info-lookup-symbol`)
+to look up Elisp symbols in the relevant Info manuals (see [`(emacs)
+Info
+Lookup`](https://gnu.org/software/emacs/manual/html_node/emacs/Info-Lookup.html)).
+To enable the same for Dash symbols, use the command
+`dash-register-info-lookup`.  It can be called directly when needed,
+or automatically from your `user-init-file`.  For example:
+
+```el
+(with-eval-after-load 'info-look
+  (dash-register-info-lookup))
+```
+
 ## Functions
 
 All functions and constructs in the library use a dash (`-`) prefix.