bug#13126: 24.3.50; (WISH) Document pcase in Info manual

[Jambunathan K <kjambunathan@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Thanks.  `pcase' seemed a good replacement for `case'.
>
> While it's not a plug-in replacement, it provides a superset of the
> features of case, yes.

pcase also reminds me of CL's case.  Btw, you know in what version of
Emacs did pcase made it's appearance.

>> 1. pcase-let, pcase-let*, pcase-dolist (maybe)
>
> pcase-dolist shouldn't be documented (yet?).  Maybe pcase-let and
> pcase-let* should be there, indeed.
>

pcase-let reminds me of `destructuring-bind'.


Here is a real-life example of pcase-let in action from midnight.el.
Good for instructive purposes.

    (defun midnight-next ()
      "Return the number of seconds till the next midnight."
      (pcase-let ((`(,sec ,min ,hrs) (decode-time)))
        (- (* 24 60 60) (* 60 60 hrs) (* 60 min) sec)))

>> 2. It took some effort to understand that there is a U-PATTERN and a
>>    UPATTERN.  We don't read out `*-*', do we?
>>     ,----
>>     |    There are two kinds of patterns involved in `pcase', called
>>     | _U-patterns_ and _Q-patterns_.  The UPATTERN mentioned above are
>>     | U-patterns and can take the following forms:
>>     `----

Provide a BNF and document TERMINALS before NON-TERMINALS. (Currently it
seems other way round).

Docstring for pcase has more info (pred FUNCTION) apropos the arguments
passed to it.  Possibly there are other things...

> What do you suggest instead?

Here is my recommendation.

May be replace the example snippets with a /simple/ and /cohesive/
example.  I found Snippet 2 "too abstract" and leaves a lot to the
imagination of the reader.

I just hacked a working REPL based on pcase.  See below.  We can include
these after 

`repl' will act as a good replacement for Snippet 1.  `repl-eval' will
act as a good replacement for Snippet 2.

M-x repl RET for a reader to toy with.

    ,---- Snippet 1
    | (pcase (get-return-code x)
    |   (`success       (message "Done!"))
    |   (`would-block   (message "Sorry, can't do it now"))
    |   (`read-only     (message "The shmliblick is read-only"))
    |   (`access-denied (message "You do not have the needed rights"))
    |   (code           (message "Unknown return code %S" code)))
    `----

    ,---- Snippet 2
    | (defun evaluate (exp env)
    |   (pcase exp
    |     (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
    |     (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
    |     (`(fn ,arg ,body)     (lambda (val)
    |                             (evaluate body (cons (cons arg val) env))))
    |     ((pred numberp)       exp)
    |     ((pred symbolp)       (cdr (assq exp env)))
    |     (_                    (error "Unknown expression %S" exp))))
    `----

(let ((repl-dictionary '()))
  (repl-eval '((x = "happy")
	       (y = "HACKING")
	       (n = 2013)
	       (z = (upcase-initials x + space +
				     (downcase y) + tab + 2013)))))

(defvar repl-dictionary '()
  "Symbol table for `repl'.")

(defun repl-eval (exp)
  (pcase exp
    ;; In-built constants.
    (`space " ")
    (`tab "\t")
    ;; Add operator.  Concatenate.
    (`(,x + . ,y) (concat (repl-eval x ) (repl-eval y)))
    ;; Assignment operator.  Update dictionary.
    (`(,x = . ,body) (let* ((value (repl-eval body))
			    (entry (assoc x repl-dictionary)))
		       (if (not entry)
			   ;; Add variable & value.
			   (push (cons x value) repl-dictionary)
			 ;; Update value.
			 (setcdr entry value))
		       value))
    ;; Function.  Assume it takes a string as it's only arg.  Call it.
    (`(,(and (pred functionp) f) . ,x) (funcall f (repl-eval x)))
    ;; Last of body forms.  Return it's value.
    (`(,x . nil)
     (repl-eval x))
    ;; Body forms.  Evaluate in sequence.  Return value of last of
    ;; the forms.
    (`(,x . ,y) (repl-eval x) (repl-eval y))
    ;; String, just return it.
    ((pred stringp) exp)
    ;; Number, cast it to string.
    ((pred numberp) (number-to-string exp))
    ;; Symbol, lookup it's value in dictionary.
    ((pred symbolp) (or (cdr (assoc exp repl-dictionary))
			(error "Variable `%s' not bound" exp)))
    (_ (error "Unknown expression %S" exp))))

(defun repl ()
  "Simple REPL for string operations.
Expression syntax:
    In-built Constants  : space
			: tab
    Assignment          : x = \"hello\"
                        : y = \"world\"
    Casting             : n = 2012
    Concatenation       : x + space + y + space + n
    Unary functions     : (upcase-initials x)

Commands:
    exit	=> Quit
    clear	=> Unbind all variables
    examine	=> Examine currently defined variables."
  (interactive)
  (let ((repl-dictionary '())
	(prompt "STRING-REPL> ")
	(result nil))
    (while (pcase (read-from-minibuffer prompt)
	     (input
	      (pcase input
		("exit" (setq result nil))
		("clear" (setq repl-dictionary '()
			       result "[CLEARED]"))
		("examine" (setq result (format "%S" repl-dictionary)))
		(_ (let ((exp (read (format "(%s)" input))))
		     (setq result (condition-case err
				      (repl-eval exp)
				    (error (format "%s" err)))))))
	      (when result
		(minibuffer-message
		 (concat prompt input
			 (propertize "\t" 'display
				     (list 'space :align-to 40))
			 (propertize result 'face 'highlight))))))
      (setq result nil))))