Add tests and documentation for ed hooks

CHANGELOG

@@ -28,6 +28,8 @@
     parameter given to configure script).
 
 * Pending changes since 21.2.1
+** Enhancements
+- Add hook functions for cl:ed via ext:*ed-functions* list
 * 21.2.1 changes since 20.4.24
 ** Announcement
 Dear Community,

src/doc/manual/standards/environment.txi

@@ -9,6 +9,7 @@
 @node Environment - Dictionary
 @subsection Dictionary
 @lspindex disassemble
+@lspindex ed
 @lspindex trace
 
 @defun disassemble function-designator*
@@ -29,6 +30,23 @@ ECL only has a particular difference: it has two different compilers, one based
 @end itemize
 @end defun
 
+@defun ed x?
+Invoke an editor on the file or object specified by @var{x}.
+@paragraph Synopsis
+(ed @var{x?})
+@table @var
+@item x
+@code{nil}, a file path, or an object to edit.
+@end table
+
+@paragraph Description
+Starts the editor (on a file or an object if named).  Functions from the list @var{ext:*ed-functions*} are called in order with @var{x} as an argument until one of them returns non-@code{nil}; these functions are responsible for signalling a @code{file-error} to indicate failure to perform an operation on the file system. If no function returns a non-@code{nil} value or @var{ext:*ed-functions*} is @code{nil} then a @code{simple-error} will be signalled.
+
+The Common Lisp specification states that the @var{x} argument is either @code{nil}, a function name, or an instance of @var{string} or @var{pathname} and that a @var{type-error} may be signalled if is not one of these types. ECL does not check the type of @var{x} and thus permits any object to be passed to the hook functions. This allows for the possibility of editing other objects that have a representation in source code such as class definitions. Therefore, the hook functions should not make any assumptions about the type of @var{x} and should instead return @code{nil} if there is not an approriate edit method for a specific value of @var{x}.
+
+By default @var{ext:*ed-functions*} contains a single function that attempts to run the program named in the environment variable @code{EDITOR}. If this environment variable is not set then the fallback program is @code{vi}.
+@end defun
+
 @defmac trace function-name*
 Follow the execution of functions
 @paragraph Synopsis

src/tests/normal-tests/mixed.lsp

@@ -378,3 +378,20 @@
       (setf stream-var inner-stream-var)
       (is (open-stream-p stream-var)))
     (is (not (open-stream-p stream-var)))))
+
+;;;; Author:   Tarn W. Burton
+;;;; Created:  2021-06-05
+;;;; Contains: *ed-functions* tests
+(test mix.0020.ed-functions
+  (let ((ext:*ed-functions* (list (lambda (x)
+                                    (equal x "foo"))
+                                  (lambda (x)
+                                    (equal x "bar"))
+                                  (lambda (x)
+                                    (when (equal x "baz")
+                                      (error 'file-error :pathname x))))))
+    (is (ed "foo"))
+    (is (ed "bar"))
+    (signals simple-error (ed "qux"))
+    (signals file-error (ed "baz"))))
+