Merge from origin/emacs-24

[emacs.git] / lisp / emacs-lisp / pcase.el

;;; pcase.el --- ML-style pattern-matching macro for Elisp -*- lexical-binding: t; coding: utf-8 -*-

;; Copyright (C) 2010-2015 Free Software Foundation, Inc.

;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
;; Keywords:

;; This file is part of GNU Emacs.

;; GNU Emacs 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.

;; GNU Emacs 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 GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; ML-style pattern matching.
;; The entry points are autoloaded.

;; Todo:

;; - (pcase e (`(,x . ,x) foo)) signals an "x unused" warning if `foo' doesn't
;;   use x, because x is bound separately for the equality constraint
;;   (as well as any pred/guard) and for the body, so uses at one place don't
;;   count for the other.
;; - provide ways to extend the set of primitives, with some kind of
;;   define-pcase-matcher.  We could easily make it so that (guard BOOLEXP)
;;   could be defined this way, as a shorthand for (pred (lambda (_) BOOLEXP)).
;;   But better would be if we could define new ways to match by having the
;;   extension provide its own `pcase--split-<foo>' thingy.
;; - along these lines, provide patterns to match CL structs.
;; - provide something like (setq VAR) so a var can be set rather than
;;   let-bound.
;; - provide a way to fallthrough to subsequent cases (not sure what I meant by
;;   this :-()
;; - try and be more clever to reduce the size of the decision tree, and
;;   to reduce the number of leaves that need to be turned into function:
;;   - first, do the tests shared by all remaining branches (it will have
;;     to be performed anyway, so better do it first so it's shared).
;;   - then choose the test that discriminates more (?).
;; - provide Agda's `with' (along with its `...' companion).
;; - implement (not UPAT).  This might require a significant redesign.
;; - ideally we'd want (pcase s ((re RE1) E1) ((re RE2) E2)) to be able to
;;   generate a lex-style DFA to decide whether to run E1 or E2.

;;; Code:

(require 'macroexp)

;; Macro-expansion of pcase is reasonably fast, so it's not a problem
;; when byte-compiling a file, but when interpreting the code, if the pcase
;; is in a loop, the repeated macro-expansion becomes terribly costly, so we
;; memoize previous macro expansions to try and avoid recomputing them
;; over and over again.
;; FIXME: Now that macroexpansion is also performed when loading an interpreted
;; file, this is not a real problem any more.
(defconst pcase--memoize (make-hash-table :weakness 'key :test 'eq))
;; (defconst pcase--memoize-1 (make-hash-table :test 'eq))
;; (defconst pcase--memoize-2 (make-hash-table :weakness 'key :test 'equal))

(defconst pcase--dontcare-upats '(t _ pcase--dontcare))

(defvar pcase--dontwarn-upats '(pcase--dontcare))

(def-edebug-spec
  pcase-UPAT
  (&or symbolp
       ("or" &rest pcase-UPAT)
       ("and" &rest pcase-UPAT)
       ("`" pcase-QPAT)
       ("guard" form)
       ("let" pcase-UPAT form)
       ("pred"
        &or lambda-expr
        ;; Punt on macros/special forms.
        (functionp &rest form)
        sexp)
       sexp))

(def-edebug-spec
  pcase-QPAT
  (&or ("," pcase-UPAT)
       (pcase-QPAT . pcase-QPAT)
       sexp))

;;;###autoload
(defmacro pcase (exp &rest cases)
  "Perform ML-style pattern matching on EXP.
CASES is a list of elements of the form (UPATTERN CODE...).

UPatterns can take the following forms:
  _             matches anything.
  SELFQUOTING   matches itself.  This includes keywords, numbers, and strings.
  SYMBOL        matches anything and binds it to SYMBOL.
  (or UPAT...)  matches if any of the patterns matches.
  (and UPAT...) matches if all the patterns match.
  'VAL          matches if the object is `equal' to VAL
  `QPAT         matches if the QPattern QPAT matches.
  (pred FUN)    matches if FUN applied to the object returns non-nil.
  (guard BOOLEXP)       matches if BOOLEXP evaluates to non-nil.
  (let UPAT EXP)        matches if EXP matches UPAT.
  (app FUN UPAT)        matches if FUN applied to the object matches UPAT.
If a SYMBOL is used twice in the same pattern (i.e. the pattern is
\"non-linear\"), then the second occurrence is turned into an `eq'uality test.

QPatterns can take the following forms:
  (QPAT1 . QPAT2)       matches if QPAT1 matches the car and QPAT2 the cdr.
  [QPAT1 QPAT2..QPATn]  matches a vector of length n and QPAT1..QPATn match
                           its 0..(n-1)th elements, respectively.
  ,UPAT                 matches if the UPattern UPAT matches.
  STRING                matches if the object is `equal' to STRING.
  ATOM                  matches if the object is `eq' to ATOM.

FUN can take the form
  SYMBOL or (lambda ARGS BODY)  in which case it's called with one argument.
  (F ARG1 .. ARGn) in which case F gets called with an n+1'th argument
                        which is the value being matched.
So a FUN of the form SYMBOL is equivalent to one of the form (FUN).
FUN can refer to variables bound earlier in the pattern.
FUN is assumed to be pure, i.e. it can be dropped if its result is not used,
and two identical calls can be merged into one.
E.g. you can match pairs where the cdr is larger than the car with a pattern
like `(,a . ,(pred (< a))) or, with more checks:
`(,(and a (pred numberp)) . ,(and (pred numberp) (pred (< a))))"
  (declare (indent 1) (debug (form &rest (pcase-UPAT body))))
  ;; We want to use a weak hash table as a cache, but the key will unavoidably
  ;; be based on `exp' and `cases', yet `cases' is a fresh new list each time
  ;; we're called so it'll be immediately GC'd.  So we use (car cases) as key
  ;; which does come straight from the source code and should hence not be GC'd
  ;; so easily.
  (let ((data (gethash (car cases) pcase--memoize)))
    ;; data = (EXP CASES . EXPANSION)
    (if (and (equal exp (car data)) (equal cases (cadr data)))
        ;; We have the right expansion.
        (cddr data)
      ;; (when (gethash (car cases) pcase--memoize-1)
      ;;   (message "pcase-memoize failed because of weak key!!"))
      ;; (when (gethash (car cases) pcase--memoize-2)
      ;;   (message "pcase-memoize failed because of eq test on %S"
      ;;            (car cases)))
      (when data
        (message "pcase-memoize: equal first branch, yet different"))
      (let ((expansion (pcase--expand exp cases)))
        (puthash (car cases) `(,exp ,cases ,@expansion) pcase--memoize)
        ;; (puthash (car cases) `(,exp ,cases ,@expansion) pcase--memoize-1)
        ;; (puthash (car cases) `(,exp ,cases ,@expansion) pcase--memoize-2)
        expansion))))

;;;###autoload
(defmacro pcase-exhaustive (exp &rest cases)
  "The exhaustive version of `pcase' (which see)."
  (declare (indent 1) (debug pcase))
  (let* ((x (make-symbol "x"))
         (pcase--dontwarn-upats (cons x pcase--dontwarn-upats)))
    (pcase--expand
     ;; FIXME: Could we add the FILE:LINE data in the error message?
     exp (append cases `((,x (error "No clause matching `%S'" ,x)))))))

(defun pcase--let* (bindings body)
  (cond
   ((null bindings) (macroexp-progn body))
   ((pcase--trivial-upat-p (caar bindings))
    (macroexp-let* `(,(car bindings)) (pcase--let* (cdr bindings) body)))
   (t
    (let ((binding (pop bindings)))
      (pcase--expand
       (cadr binding)
       `((,(car binding) ,(pcase--let* bindings body))
         ;; We can either signal an error here, or just use `pcase--dontcare'
         ;; which generates more efficient code.  In practice, if we use
         ;; `pcase--dontcare' we will still often get an error and the few
         ;; cases where we don't do not matter that much, so
         ;; it's a better choice.
         (pcase--dontcare nil)))))))

;;;###autoload
(defmacro pcase-let* (bindings &rest body)
  "Like `let*' but where you can use `pcase' patterns for bindings.
BODY should be an expression, and BINDINGS should be a list of bindings
of the form (UPAT EXP)."
  (declare (indent 1)
           (debug ((&rest (pcase-UPAT &optional form)) body)))
  (let ((cached (gethash bindings pcase--memoize)))
    ;; cached = (BODY . EXPANSION)
    (if (equal (car cached) body)
        (cdr cached)
      (let ((expansion (pcase--let* bindings body)))
        (puthash bindings (cons body expansion) pcase--memoize)
        expansion))))

;;;###autoload
(defmacro pcase-let (bindings &rest body)
  "Like `let' but where you can use `pcase' patterns for bindings.
BODY should be a list of expressions, and BINDINGS should be a list of bindings
of the form (UPAT EXP)."
  (declare (indent 1) (debug pcase-let*))
  (if (null (cdr bindings))
      `(pcase-let* ,bindings ,@body)
    (let ((matches '()))
      (dolist (binding (prog1 bindings (setq bindings nil)))
        (cond
         ((memq (car binding) pcase--dontcare-upats)
          (push (cons (make-symbol "_") (cdr binding)) bindings))
         ((pcase--trivial-upat-p (car binding)) (push binding bindings))
         (t
          (let ((tmpvar (make-symbol (format "x%d" (length bindings)))))
            (push (cons tmpvar (cdr binding)) bindings)
            (push (list (car binding) tmpvar) matches)))))
      `(let ,(nreverse bindings) (pcase-let* ,matches ,@body)))))

(defmacro pcase-dolist (spec &rest body)
  (declare (indent 1) (debug ((pcase-UPAT form) body)))
  (if (pcase--trivial-upat-p (car spec))
      `(dolist ,spec ,@body)
    (let ((tmpvar (make-symbol "x")))
      `(dolist (,tmpvar ,@(cdr spec))
         (pcase-let* ((,(car spec) ,tmpvar))
           ,@body)))))


(defun pcase--trivial-upat-p (upat)
  (and (symbolp upat) (not (memq upat pcase--dontcare-upats))))

(defun pcase--expand (exp cases)
  ;; (message "pid=%S (pcase--expand %S ...hash=%S)"
  ;;          (emacs-pid) exp (sxhash cases))
  (macroexp-let2 macroexp-copyable-p val exp
    (let* ((defs ())
           (seen '())
           (codegen
            (lambda (code vars)
              (let ((prev (assq code seen)))
                (if (not prev)
                    (let ((res (pcase-codegen code vars)))
                      (push (list code vars res) seen)
                      res)
                  ;; Since we use a tree-based pattern matching
                  ;; technique, the leaves (the places that contain the
                  ;; code to run once a pattern is matched) can get
                  ;; copied a very large number of times, so to avoid
                  ;; code explosion, we need to keep track of how many
                  ;; times we've used each leaf and move it
                  ;; to a separate function if that number is too high.
                  ;;
                  ;; We've already used this branch.  So it is shared.
                  (let* ((code (car prev))         (cdrprev (cdr prev))
                         (prevvars (car cdrprev))  (cddrprev (cdr cdrprev))
                         (res (car cddrprev)))
                    (unless (symbolp res)
                      ;; This is the first repeat, so we have to move
                      ;; the branch to a separate function.
                      (let ((bsym
                             (make-symbol (format "pcase-%d" (length defs)))))
                        (push `(,bsym (lambda ,(mapcar #'car prevvars) ,@code))
                              defs)
                        (setcar res 'funcall)
                        (setcdr res (cons bsym (mapcar #'cdr prevvars)))
                        (setcar (cddr prev) bsym)
                        (setq res bsym)))
                    (setq vars (copy-sequence vars))
                    (let ((args (mapcar (lambda (pa)
                                          (let ((v (assq (car pa) vars)))
                                            (setq vars (delq v vars))
                                            (cdr v)))
                                        prevvars)))
                      ;; If some of `vars' were not found in `prevvars', that's
                      ;; OK it just means those vars aren't present in all
                      ;; branches, so they can be used within the pattern
                      ;; (e.g. by a `guard/let/pred') but not in the branch.
                      ;; FIXME: But if some of `prevvars' are not in `vars' we
                      ;; should remove them from `prevvars'!
                      `(funcall ,res ,@args)))))))
           (used-cases ())
           (main
            (pcase--u
             (mapcar (lambda (case)
                       `(,(pcase--match val (pcase--macroexpand (car case)))
                         ,(lambda (vars)
                            (unless (memq case used-cases)
                              ;; Keep track of the cases that are used.
                              (push case used-cases))
                            (funcall
                             (if (pcase--small-branch-p (cdr case))
                                 ;; Don't bother sharing multiple
                                 ;; occurrences of this leaf since it's small.
                                 #'pcase-codegen codegen)
                             (cdr case)
                             vars))))
                     cases))))
      (dolist (case cases)
        (unless (or (memq case used-cases)
                    (memq (car case) pcase--dontwarn-upats))
          (message "Redundant pcase pattern: %S" (car case))))
      (macroexp-let* defs main))))

(defun pcase--macroexpand (pat)
  "Expands all macro-patterns in PAT."
  (let ((head (car-safe pat)))
    (cond
     ((null head)
      (if (pcase--self-quoting-p pat) `',pat pat))
     ((memq head '(pred guard quote)) pat)
     ((memq head '(or and)) `(,head ,@(mapcar #'pcase--macroexpand (cdr pat))))
     ((eq head 'let) `(let ,(pcase--macroexpand (cadr pat)) ,@(cddr pat)))
     ((eq head 'app) `(app ,(nth 1 pat) ,(pcase--macroexpand (nth 2 pat))))
     (t
      (let* ((expander (get head 'pcase-macroexpander))
             (npat (if expander (apply expander (cdr pat)))))
        (if (null npat)
            (error (if expander
                       "Unexpandable %s pattern: %S"
                     "Unknown %s pattern: %S")
                   head pat)
          (pcase--macroexpand npat)))))))

;;;###autoload
(defmacro pcase-defmacro (name args &rest body)
  "Define a pcase UPattern macro."
  (declare (indent 2) (debug (def-name sexp def-body)) (doc-string 3))
  `(put ',name 'pcase-macroexpander
        (lambda ,args ,@body)))

(defun pcase--match (val upat)
  "Build a MATCH structure, hoisting all `or's and `and's outside."
  (cond
   ;; Hoist or/and patterns into or/and matches.
   ((memq (car-safe upat) '(or and))
    `(,(car upat)
      ,@(mapcar (lambda (upat)
                  (pcase--match val upat))
                (cdr upat))))
   (t
    `(match ,val . ,upat))))

(defun pcase-codegen (code vars)
  ;; Don't use let*, otherwise macroexp-let* may merge it with some surrounding
  ;; let* which might prevent the setcar/setcdr in pcase--expand's fancy
  ;; codegen from later metamorphosing this let into a funcall.
  `(let ,(mapcar (lambda (b) (list (car b) (cdr b))) vars)
     ,@code))

(defun pcase--small-branch-p (code)
  (and (= 1 (length code))
       (or (not (consp (car code)))
           (let ((small t))
             (dolist (e (car code))
               (if (consp e) (setq small nil)))
             small))))

;; Try to use `cond' rather than a sequence of `if's, so as to reduce
;; the depth of the generated tree.
(defun pcase--if (test then else)
  (cond
   ((eq else :pcase--dontcare) then)
   ((eq then :pcase--dontcare) (debug) else) ;Can/should this ever happen?
   (t (macroexp-if test then else))))

;; Note about MATCH:
;; When we have patterns like `(PAT1 . PAT2), after performing the `consp'
;; check, we want to turn all the similar patterns into ones of the form
;; (and (match car PAT1) (match cdr PAT2)), so you naturally need conjunction.
;; Earlier code hence used branches of the form (MATCHES . CODE) where
;; MATCHES was a list (implicitly a conjunction) of (SYM . PAT).
;; But if we have a pattern of the form (or `(PAT1 . PAT2) PAT3), there is
;; no easy way to eliminate the `consp' check in such a representation.
;; So we replaced the MATCHES by the MATCH below which can be made up
;; of conjunctions and disjunctions, so if we know `foo' is a cons, we can
;; turn (match foo . (or `(PAT1 . PAT2) PAT3)) into
;; (or (and (match car . `PAT1) (match cdr . `PAT2)) (match foo . PAT3)).
;; The downside is that we now have `or' and `and' both in MATCH and
;; in PAT, so there are different equivalent representations and we
;; need to handle them all.  We do not try to systematically
;; canonicalize them to one form over another, but we do occasionally
;; turn one into the other.

(defun pcase--u (branches)
  "Expand matcher for rules BRANCHES.
Each BRANCH has the form (MATCH CODE . VARS) where
CODE is the code generator for that branch.
VARS is the set of vars already bound by earlier matches.
MATCH is the pattern that needs to be matched, of the form:
  (match VAR . UPAT)
  (and MATCH ...)
  (or MATCH ...)"
  (when (setq branches (delq nil branches))
    (let* ((carbranch (car branches))
           (match (car carbranch)) (cdarbranch (cdr carbranch))
           (code (car cdarbranch))
           (vars (cdr cdarbranch)))
      (pcase--u1 (list match) code vars (cdr branches)))))

(defun pcase--and (match matches)
  (if matches `(and ,match ,@matches) match))

(defconst pcase-mutually-exclusive-predicates
  '((symbolp . integerp)
    (symbolp . numberp)
    (symbolp . consp)
    (symbolp . arrayp)
    (symbolp . vectorp)
    (symbolp . stringp)
    (symbolp . byte-code-function-p)
    (integerp . consp)
    (integerp . arrayp)
    (integerp . vectorp)
    (integerp . stringp)
    (integerp . byte-code-function-p)
    (numberp . consp)
    (numberp . arrayp)
    (numberp . vectorp)
    (numberp . stringp)
    (numberp . byte-code-function-p)
    (consp . arrayp)
    (consp . vectorp)
    (consp . stringp)
    (consp . byte-code-function-p)
    (arrayp . byte-code-function-p)
    (vectorp . byte-code-function-p)
    (stringp . vectorp)
    (stringp . byte-code-function-p)))

(defun pcase--mutually-exclusive-p (pred1 pred2)
  (or (member (cons pred1 pred2)
              pcase-mutually-exclusive-predicates)
      (member (cons pred2 pred1)
              pcase-mutually-exclusive-predicates)))

(defun pcase--split-match (sym splitter match)
  (cond
    ((eq (car-safe match) 'match)
     (if (not (eq sym (cadr match)))
         (cons match match)
       (let ((res (funcall splitter (cddr match))))
         (cons (or (car res) match) (or (cdr res) match)))))
    ((memq (car-safe match) '(or and))
     (let ((then-alts '())
           (else-alts '())
           (neutral-elem (if (eq 'or (car match))
                             :pcase--fail :pcase--succeed))
           (zero-elem (if (eq 'or (car match)) :pcase--succeed :pcase--fail)))
       (dolist (alt (cdr match))
         (let ((split (pcase--split-match sym splitter alt)))
           (unless (eq (car split) neutral-elem)
             (push (car split) then-alts))
           (unless (eq (cdr split) neutral-elem)
             (push (cdr split) else-alts))))
       (cons (cond ((memq zero-elem then-alts) zero-elem)
                   ((null then-alts) neutral-elem)
                   ((null (cdr then-alts)) (car then-alts))
                   (t (cons (car match) (nreverse then-alts))))
             (cond ((memq zero-elem else-alts) zero-elem)
                   ((null else-alts) neutral-elem)
                   ((null (cdr else-alts)) (car else-alts))
                   (t (cons (car match) (nreverse else-alts)))))))
    ((memq match '(:pcase--succeed :pcase--fail)) (cons match match))
    (t (error "Uknown MATCH %s" match))))

(defun pcase--split-rest (sym splitter rest)
  (let ((then-rest '())
        (else-rest '()))
    (dolist (branch rest)
      (let* ((match (car branch))
             (code&vars (cdr branch))
             (split
              (pcase--split-match sym splitter match)))
        (unless (eq (car split) :pcase--fail)
          (push (cons (car split) code&vars) then-rest))
        (unless (eq (cdr split) :pcase--fail)
          (push (cons (cdr split) code&vars) else-rest))))
    (cons (nreverse then-rest) (nreverse else-rest))))

(defun pcase--split-equal (elem pat)
  (cond
   ;; The same match will give the same result.
   ((and (eq (car-safe pat) 'quote) (equal (cadr pat) elem))
    '(:pcase--succeed . :pcase--fail))
   ;; A different match will fail if this one succeeds.
   ((and (eq (car-safe pat) 'quote)
         ;; (or (integerp (cadr pat)) (symbolp (cadr pat))
         ;;     (consp (cadr pat)))
         )
    '(:pcase--fail . nil))
   ((and (eq (car-safe pat) 'pred)
         (symbolp (cadr pat))
         (get (cadr pat) 'side-effect-free))
    (ignore-errors
      (if (funcall (cadr pat) elem)
          '(:pcase--succeed . nil)
        '(:pcase--fail . nil))))))

(defun pcase--split-member (elems pat)
  ;; FIXME: The new pred-based member code doesn't do these optimizations!
  ;; Based on pcase--split-equal.
  (cond
   ;; The same match (or a match of membership in a superset) will
   ;; give the same result, but we don't know how to check it.
   ;; (???
   ;;  '(:pcase--succeed . nil))
   ;; A match for one of the elements may succeed or fail.
   ((and (eq (car-safe pat) 'quote) (member (cadr pat) elems))
    nil)
   ;; A different match will fail if this one succeeds.
   ((and (eq (car-safe pat) 'quote)
         ;; (or (integerp (cadr pat)) (symbolp (cadr pat))
         ;;     (consp (cadr pat)))
         )
    '(:pcase--fail . nil))
   ((and (eq (car-safe pat) 'pred)
         (symbolp (cadr pat))
         (get (cadr pat) 'side-effect-free)
         (ignore-errors
           (let ((p (cadr pat)) (all t))
             (dolist (elem elems)
               (unless (funcall p elem) (setq all nil)))
             all)))
    '(:pcase--succeed . nil))))

(defun pcase--split-pred (vars upat pat)
  (let (test)
    (cond
     ((and (equal upat pat)
           ;; For predicates like (pred (> a)), two such predicates may
           ;; actually refer to different variables `a'.
           (or (and (eq 'pred (car upat)) (symbolp (cadr upat)))
               ;; FIXME: `vars' gives us the environment in which `upat' will
               ;; run, but we don't have the environment in which `pat' will
               ;; run, so we can't do a reliable verification.  But let's try
               ;; and catch at least the easy cases such as (bug#14773).
               (not (pcase--fgrep (mapcar #'car vars) (cadr upat)))))
      '(:pcase--succeed . :pcase--fail))
     ((and (eq 'pred (car upat))
           (let ((otherpred
                  (cond ((eq 'pred (car-safe pat)) (cadr pat))
                        ((not (eq 'quote (car-safe pat))) nil)
                        ((consp (cadr pat)) #'consp)
                        ((vectorp (cadr pat)) #'vectorp)
                        ((byte-code-function-p (cadr pat))
                         #'byte-code-function-p))))
             (pcase--mutually-exclusive-p (cadr upat) otherpred)))
      '(:pcase--fail . nil))
     ((and (eq 'pred (car upat))
           (eq 'quote (car-safe pat))
           (symbolp (cadr upat))
           (or (symbolp (cadr pat)) (stringp (cadr pat)) (numberp (cadr pat)))
           (get (cadr upat) 'side-effect-free)
           (ignore-errors
             (setq test (list (funcall (cadr upat) (cadr pat))))))
      (if (car test)
          '(nil . :pcase--fail)
        '(:pcase--fail . nil))))))

(defun pcase--fgrep (vars sexp)
  "Check which of the symbols VARS appear in SEXP."
  (let ((res '()))
    (while (consp sexp)
      (dolist (var (pcase--fgrep vars (pop sexp)))
        (unless (memq var res) (push var res))))
    (and (memq sexp vars) (not (memq sexp res)) (push sexp res))
    res))

(defun pcase--self-quoting-p (upat)
  (or (keywordp upat) (numberp upat) (stringp upat)))

(defun pcase--app-subst-match (match sym fun nsym)
  (cond
   ((eq (car-safe match) 'match)
    (if (and (eq sym (cadr match))
             (eq 'app (car-safe (cddr match)))
             (equal fun (nth 1 (cddr match))))
        (pcase--match nsym (nth 2 (cddr match)))
      match))
   ((memq (car-safe match) '(or and))
    `(,(car match)
      ,@(mapcar (lambda (match)
                  (pcase--app-subst-match match sym fun nsym))
                (cdr match))))
   ((memq match '(:pcase--succeed :pcase--fail)) match)
   (t (error "Uknown MATCH %s" match))))

(defun pcase--app-subst-rest (rest sym fun nsym)
  (mapcar (lambda (branch)
            `(,(pcase--app-subst-match (car branch) sym fun nsym)
              ,@(cdr branch)))
          rest))

(defsubst pcase--mark-used (sym)
  ;; Exceptionally, `sym' may be a constant expression rather than a symbol.
  (if (symbolp sym) (put sym 'pcase-used t)))

(defmacro pcase--flip (fun arg1 arg2)
  "Helper function, used internally to avoid (funcall (lambda ...) ...)."
  (declare (debug (sexp body)))
  `(,fun ,arg2 ,arg1))

(defun pcase--funcall (fun arg vars)
  "Build a function call to FUN with arg ARG."
  (if (symbolp fun)
      `(,fun ,arg)
    (let* (;; `vs' is an upper bound on the vars we need.
           (vs (pcase--fgrep (mapcar #'car vars) fun))
           (env (mapcar (lambda (var)
                          (list var (cdr (assq var vars))))
                        vs))
           (call (progn
                   (when (memq arg vs)
                     ;; `arg' is shadowed by `env'.
                     (let ((newsym (make-symbol "x")))
                       (push (list newsym arg) env)
                       (setq arg newsym)))
                   (if (functionp fun)
                       `(funcall #',fun ,arg)
                     `(,@fun ,arg)))))
      (if (null vs)
          call
        ;; Let's not replace `vars' in `fun' since it's
        ;; too difficult to do it right, instead just
        ;; let-bind `vars' around `fun'.
        `(let* ,env ,call)))))

(defun pcase--eval (exp vars)
  "Build an expression that will evaluate EXP."
  (let* ((found (assq exp vars)))
    (if found (cdr found)
      (let* ((vs (pcase--fgrep (mapcar #'car vars) exp))
             (env (mapcar (lambda (v) (list v (cdr (assq v vars))))
                          vs)))
        (if env (macroexp-let* env exp) exp)))))

;; It's very tempting to use `pcase' below, tho obviously, it'd create
;; bootstrapping problems.
(defun pcase--u1 (matches code vars rest)
  "Return code that runs CODE (with VARS) if MATCHES match.
Otherwise, it defers to REST which is a list of branches of the form
\(ELSE-MATCH ELSE-CODE . ELSE-VARS)."
  ;; Depending on the order in which we choose to check each of the MATCHES,
  ;; the resulting tree may be smaller or bigger.  So in general, we'd want
  ;; to be careful to chose the "optimal" order.  But predicate
  ;; patterns make this harder because they create dependencies
  ;; between matches.  So we don't bother trying to reorder anything.
  (cond
   ((null matches) (funcall code vars))
   ((eq :pcase--fail (car matches)) (pcase--u rest))
   ((eq :pcase--succeed (car matches))
    (pcase--u1 (cdr matches) code vars rest))
   ((eq 'and (caar matches))
    (pcase--u1 (append (cdar matches) (cdr matches)) code vars rest))
   ((eq 'or (caar matches))
    (let* ((alts (cdar matches))
           (var (if (eq (caar alts) 'match) (cadr (car alts))))
           (simples '()) (others '()) (memq-ok t))
      (when var
        (dolist (alt alts)
          (if (and (eq (car alt) 'match) (eq var (cadr alt))
                   (let ((upat (cddr alt)))
                     (eq (car-safe upat) 'quote)))
              (let ((val (cadr (cddr alt))))
                (unless (or (integerp val) (symbolp val))
                  (setq memq-ok nil))
                (push (cadr (cddr alt)) simples))
            (push alt others))))
      (cond
       ((null alts) (error "Please avoid it") (pcase--u rest))
       ;; Yes, we can use `memq' (or `member')!
       ((> (length simples) 1)
        (pcase--u1 (cons `(match ,var
                                 . (pred (pcase--flip
                                          ,(if memq-ok #'memq #'member)
                                          ',simples)))
                         (cdr matches))
                   code vars
                   (if (null others) rest
                     (cons (cons
                            (pcase--and (if (cdr others)
                                            (cons 'or (nreverse others))
                                          (car others))
                                        (cdr matches))
                            (cons code vars))
                           rest))))
       (t
        (pcase--u1 (cons (pop alts) (cdr matches)) code vars
                   (if (null alts) (progn (error "Please avoid it") rest)
                     (cons (cons
                            (pcase--and (if (cdr alts)
                                            (cons 'or alts) (car alts))
                                        (cdr matches))
                            (cons code vars))
                           rest)))))))
   ((eq 'match (caar matches))
    (let* ((popmatches (pop matches))
           (_op (car popmatches))      (cdrpopmatches (cdr popmatches))
           (sym (car cdrpopmatches))
           (upat (cdr cdrpopmatches)))
      (cond
       ((memq upat '(t _)) (pcase--u1 matches code vars rest))
       ((eq upat 'pcase--dontcare) :pcase--dontcare)
       ((memq (car-safe upat) '(guard pred))
        (if (eq (car upat) 'pred) (pcase--mark-used sym))
        (let* ((splitrest
                (pcase--split-rest
                 sym (lambda (pat) (pcase--split-pred vars upat pat)) rest))
               (then-rest (car splitrest))
               (else-rest (cdr splitrest)))
          (pcase--if (if (eq (car upat) 'pred)
                         (pcase--funcall (cadr upat) sym vars)
                       (pcase--eval (cadr upat) vars))
                     (pcase--u1 matches code vars then-rest)
                     (pcase--u else-rest))))
       ((symbolp upat)
        (pcase--mark-used sym)
        (if (not (assq upat vars))
            (pcase--u1 matches code (cons (cons upat sym) vars) rest)
          ;; Non-linear pattern.  Turn it into an `eq' test.
          (pcase--u1 (cons `(match ,sym . (pred (eq ,(cdr (assq upat vars)))))
                           matches)
                     code vars rest)))
       ((eq (car-safe upat) 'let)
        ;; A upat of the form (let VAR EXP).
        ;; (pcase--u1 matches code
        ;;            (cons (cons (nth 1 upat) (nth 2 upat)) vars) rest)
        (macroexp-let2
            macroexp-copyable-p sym
            (pcase--eval (nth 2 upat) vars)
          (pcase--u1 (cons (pcase--match sym (nth 1 upat)) matches)
                     code vars rest)))
       ((eq (car-safe upat) 'app)
        ;; A upat of the form (app FUN UPAT)
        (pcase--mark-used sym)
        (let* ((fun (nth 1 upat))
               (nsym (make-symbol "x"))
               (body
                ;; We don't change `matches' to reuse the newly computed value,
                ;; because we assume there shouldn't be such redundancy in there.
                (pcase--u1 (cons (pcase--match nsym (nth 2 upat)) matches)
                           code vars
                           (pcase--app-subst-rest rest sym fun nsym))))
          (if (not (get nsym 'pcase-used))
              body
            (macroexp-let*
             `((,nsym ,(pcase--funcall fun sym vars)))
             body))))
       ((eq (car-safe upat) 'quote)
        (pcase--mark-used sym)
        (let* ((val (cadr upat))
               (splitrest (pcase--split-rest
                           sym (lambda (pat) (pcase--split-equal val pat)) rest))
               (then-rest (car splitrest))
               (else-rest (cdr splitrest)))
          (pcase--if (cond
                      ((null val) `(null ,sym))
                      ((or (integerp val) (symbolp val))
                       (if (pcase--self-quoting-p val)
                           `(eq ,sym ,val)
                         `(eq ,sym ',val)))
                      (t `(equal ,sym ',val)))
                     (pcase--u1 matches code vars then-rest)
                     (pcase--u else-rest))))
       ((eq (car-safe upat) 'not)
        ;; FIXME: The implementation below is naive and results in
        ;; inefficient code.
        ;; To make it work right, we would need to turn pcase--u1's
        ;; `code' and `vars' into a single argument of the same form as
        ;; `rest'.  We would also need to split this new `then-rest' argument
        ;; for every test (currently we don't bother to do it since
        ;; it's only useful for odd patterns like (and `(PAT1 . PAT2)
        ;; `(PAT3 . PAT4)) which the programmer can easily rewrite
        ;; to the more efficient `(,(and PAT1 PAT3) . ,(and PAT2 PAT4))).
        (pcase--u1 `((match ,sym . ,(cadr upat)))
                   ;; FIXME: This codegen is not careful to share its
                   ;; code if used several times: code blow up is likely.
                   (lambda (_vars)
                     ;; `vars' will likely contain bindings which are
                     ;; not always available in other paths to
                     ;; `rest', so there' no point trying to pass
                     ;; them down.
                     (pcase--u rest))
                   vars
                   (list `((and . ,matches) ,code . ,vars))))
       (t (error "Unknown internal pattern `%S'" upat)))))
   (t (error "Incorrect MATCH %S" (car matches)))))

(pcase-defmacro \` (qpat)
  (cond
   ((eq (car-safe qpat) '\,) (cadr qpat))
   ((vectorp qpat)
    `(and (pred vectorp)
          (app length ,(length qpat))
          ,@(let ((upats nil))
              (dotimes (i (length qpat))
                (push `(app (pcase--flip aref ,i) ,(list '\` (aref qpat i)))
                      upats))
              (nreverse upats))))
   ((consp qpat)
    `(and (pred consp)
          (app car ,(list '\` (car qpat)))
          (app cdr ,(list '\` (cdr qpat)))))
   ((or (stringp qpat) (integerp qpat) (symbolp qpat)) `',qpat)))


(provide 'pcase)
;;; pcase.el ends here