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diff --git a/lisp/emacs-lisp/generator.el b/lisp/emacs-lisp/generator.el new file mode 100644 index 00000000000..284de410580 --- /dev/null +++ b/lisp/emacs-lisp/generator.el @@ -0,0 +1,796 @@ +;;; generator.el --- generators -*- lexical-binding: t -*- + +;;; Copyright (C) 2015 Free Software Foundation, Inc. + +;; Author: Daniel Colascione <dancol@dancol.org> +;; Keywords: extensions, elisp +;; Package: emacs + +;; 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: + +;; This package implements generators for Emacs Lisp through a +;; continuation-passing transformation. It provides essentially the +;; same generator API and iterator facilties that Python and +;; JavaScript ES6 provide. +;; +;; `iter-lambda' and `iter-defun' work like `lambda' and `defun', +;; except that they evaluate to or define, respectively, generator +;; functions. These functions, when called, return an iterator. +;; An iterator is an opaque object that generates a sequence of +;; values. Callers use `iter-next' to retrieve the next value from +;; the sequence; when the sequence is exhausted, `iter-next' will +;; raise the `iter-end-of-sequence' condition. +;; +;; Generator functions are written like normal functions, except that +;; they can invoke `iter-yield' to suspend themselves and return a +;; value to callers; this value becomes the return value of +;; `iter-next'. On the next call to `iter-next', execution of the +;; generator function resumes where it left off. When a generator +;; function returns normally, the `iter-next' raises +;; `iter-end-of-sequence' with the value the function returned. +;; +;; `iter-yield-from' yields all the values from another iterator; it +;; then evaluates to the value the sub-iterator returned normally. +;; This facility is useful for functional composition of generators +;; and for implementing coroutines. +;; +;; `iter-yield' is illegal inside the UNWINDFORMS of an +;; `unwind-protect' for various sordid internal reasons documented in +;; the code. +;; +;; N.B. Each call to a generator function generates a *new* iterator, +;; and each iterator maintains its own internal state. +;; +;; This raw form of iteration is general, but a bit awkward to use, so +;; this library also provides soem convenience functions: +;; +;; `iter-do' is like `cl-do', except that instead of walking a list, +;; it walks an iterator. `cl-loop' is also extended with a new +;; keyword, `iter-by', that iterates over an iterator. +;; + +;;; Implementation: + +;; +;; The internal cps transformation code uses the cps- namespace. +;; Iteration functions use the `iter-' namespace. Generator functions +;; are somewhat less efficient than conventional elisp routines, +;; although we try to avoid CPS transformation on forms that do not +;; invoke `iter-yield'. +;; + +;;; Code: + +(require 'cl-lib) +(require 'pcase) + +(defvar cps--bindings nil) +(defvar cps--states nil) +(defvar cps--value-symbol nil) +(defvar cps--state-symbol nil) +(defvar cps--cleanup-table-symbol nil) +(defvar cps--cleanup-function nil) + +(defmacro cps--gensym (fmt &rest args) + ;; Change this function to use `cl-gensym' if you want the generated + ;; code to be easier to read and debug. + ;; (cl-gensym (apply #'format fmt args)) + `(make-symbol ,fmt)) + +(defvar cps--dynamic-wrappers '(identity) + "List of transformer functions to apply to atomic forms we +evaluate in CPS context.") + +(defconst cps-standard-special-forms + '(setq setq-default throw interactive) + "List of special forms that we treat just like ordinary + function applications." ) + +(defun cps--trace-funcall (func &rest args) + (message "%S: args=%S" func args) + (let ((result (apply func args))) + (message "%S: result=%S" func result) + result)) + +(defun cps--trace (fmt &rest args) + (princ (apply #'format (concat fmt "\n") args))) + +(defun cps--special-form-p (definition) + "Non-nil if and only if DEFINITION is a special form." + ;; Copied from ad-special-form-p + (if (and (symbolp definition) (fboundp definition)) + (setf definition (indirect-function definition))) + (and (subrp definition) (eq (cdr (subr-arity definition)) 'unevalled))) + +(defmacro cps--define-unsupported (function) + `(defun ,(intern (format "cps--transform-%s" function)) + (error "%s not supported in generators" ,function))) + +(defmacro cps--with-value-wrapper (wrapper &rest body) + "Continue generating CPS code with an atomic-form wrapper +to the current stack of such wrappers. WRAPPER is a function that +takes a form and returns a wrapped form. + +Whenever we generate an atomic form (i.e., a form that can't +iter-yield), we first (before actually inserting that form in our +generated code) pass that form through all the transformer +functions. We use this facility to wrap forms that can transfer +control flow non-locally in goo that diverts this control flow to +the CPS state machinery. +" + (declare (indent 1)) + `(let ((cps--dynamic-wrappers + (cons + ,wrapper + cps--dynamic-wrappers))) + ,@body)) + +(defun cps--make-dynamic-binding-wrapper (dynamic-var static-var) + (cl-assert lexical-binding) + (lambda (form) + `(let ((,dynamic-var ,static-var)) + (unwind-protect ; Update the static shadow after evaluation is done + ,form + (setf ,static-var ,dynamic-var)) + ,form))) + +(defmacro cps--with-dynamic-binding (dynamic-var static-var &rest body) + "Evaluate BODY such that generated atomic evaluations run with +DYNAMIC-VAR bound to STATIC-VAR." + (declare (indent 2)) + `(cps--with-value-wrapper + (cps--make-dynamic-binding-wrapper ,dynamic-var ,static-var) + ,@body)) + +(defun cps--add-state (kind body) + "Create a new CPS state with body BODY and return the state's name." + (declare (indent 1)) + (let* ((state (cps--gensym "cps-state-%s-" kind))) + (push (list state body cps--cleanup-function) cps--states) + (push state cps--bindings) + state)) + +(defun cps--add-binding (original-name) + (car (push (cps--gensym (format "cps-binding-%s-" original-name)) + cps--bindings))) + +(defun cps--find-special-form-handler (form) + (let* ((handler-name (format "cps--transform-%s" (car-safe form))) + (handler (intern-soft handler-name))) + (and (fboundp handler) handler))) + +(defvar cps-inhibit-atomic-optimization nil + "When t, always rewrite forms into cps even when they +don't yield.") + +(defvar cps--yield-seen) + +(defun cps--atomic-p (form) + "Return whether the given form never yields." + + (and (not cps-inhibit-atomic-optimization) + (let* ((cps--yield-seen)) + (ignore (macroexpand-all + `(cl-macrolet ((cps-internal-yield + (_val) + (setf cps--yield-seen t))) + ,form) + macroexpand-all-environment)) + (not cps--yield-seen)))) + +(defun cps--make-atomic-state (form next-state) + (let ((tform `(prog1 ,form (setf ,cps--state-symbol ,next-state)))) + (cl-loop for wrapper in cps--dynamic-wrappers + do (setf tform (funcall wrapper tform))) + ;; Bind cps--cleanup-function to nil here because the wrapper + ;; function mechanism is responsible for cleanup here, not the + ;; generic cleanup mechanism. If we didn't make this binding, + ;; we'd run cleanup handlers twice on anything that made it out + ;; to toplevel. + (let ((cps--cleanup-function nil)) + (cps--add-state "atom" + `(setf ,cps--value-symbol ,tform))))) + +(defun cps--transform-1 (form next-state) + (pcase form + + ;; If we're looking at an "atomic" form (i.e., one that does not + ;; iter-yield), just evaluate the form as a whole instead of rewriting + ;; it into CPS. + + ((guard (cps--atomic-p form)) + (cps--make-atomic-state form next-state)) + + ;; Process `and'. + + (`(and) ; (and) -> t + (cps--transform-1 t next-state)) + (`(and ,condition) ; (and CONDITION) -> CONDITION + (cps--transform-1 condition next-state)) + (`(and ,condition . ,rest) + ;; Evaluate CONDITION; if it's true, go on to evaluate the rest + ;; of the `and'. + (cps--transform-1 + condition + (cps--add-state "and" + `(setf ,cps--state-symbol + (if ,cps--value-symbol + ,(cps--transform-1 `(and ,@rest) + next-state) + ,next-state))))) + + ;; Process `catch'. + + (`(catch ,tag . ,body) + (let ((tag-binding (cps--add-binding "catch-tag"))) + (cps--transform-1 tag + (cps--add-state "cps-update-tag" + `(setf ,tag-binding ,cps--value-symbol + ,cps--state-symbol + ,(cps--with-value-wrapper + (cps--make-catch-wrapper + tag-binding next-state) + (cps--transform-1 `(progn ,@body) + next-state))))))) + + ;; Process `cond': transform into `if' or `or' depending on the + ;; precise kind of the condition we're looking at. + + (`(cond) ; (cond) -> nil + (cps--transform-1 nil next-state)) + (`(cond (,condition) . ,rest) + (cps--transform-1 `(or ,condition (cond ,@rest)) + next-state)) + (`(cond (,condition . ,body) . ,rest) + (cps--transform-1 `(if ,condition + (progn ,@body) + (cond ,@rest)) + next-state)) + + ;; Process `condition-case': do the heavy lifting in a helper + ;; function. + + (`(condition-case ,var ,bodyform . ,handlers) + (cps--with-value-wrapper + (cps--make-condition-wrapper var next-state handlers) + (cps--transform-1 bodyform + next-state))) + + ;; Process `if'. + + (`(if ,cond ,then . ,else) + (cps--transform-1 cond + (cps--add-state "if" + `(setf ,cps--state-symbol + (if ,cps--value-symbol + ,(cps--transform-1 then + next-state) + ,(cps--transform-1 `(progn ,@else) + next-state)))))) + + ;; Process `progn' and `inline': they are identical except for the + ;; name, which has some significance to the byte compiler. + + (`(inline) (cps--transform-1 nil next-state)) + (`(inline ,form) (cps--transform-1 form next-state)) + (`(inline ,form . ,rest) + (cps--transform-1 form + (cps--transform-1 `(inline ,@rest) + next-state))) + + (`(progn) (cps--transform-1 nil next-state)) + (`(progn ,form) (cps--transform-1 form next-state)) + (`(progn ,form . ,rest) + (cps--transform-1 form + (cps--transform-1 `(progn ,@rest) + next-state))) + + ;; Process `let' in a helper function that transforms it into a + ;; let* with temporaries. + + (`(let ,bindings . ,body) + (let* ((bindings (cl-loop for binding in bindings + collect (if (symbolp binding) + (list binding nil) + binding))) + (temps (cl-loop for (var value-form) in bindings + collect (cps--add-binding var)))) + (cps--transform-1 + `(let* ,(append + (cl-loop for (var value-form) in bindings + for temp in temps + collect (list temp value-form)) + (cl-loop for (var binding) in bindings + for temp in temps + collect (list var temp))) + ,@body) + next-state))) + + ;; Process `let*' binding: process one binding at a time. Flatten + ;; lexical bindings. + + (`(let* () . ,body) + (cps--transform-1 `(progn ,@body) next-state)) + + (`(let* (,binding . ,more-bindings) . ,body) + (let* ((var (if (symbolp binding) binding (car binding))) + (value-form (car (cdr-safe binding))) + (new-var (cps--add-binding var))) + + (cps--transform-1 + value-form + (cps--add-state "let*" + `(setf ,new-var ,cps--value-symbol + ,cps--state-symbol + ,(if (or (not lexical-binding) (special-variable-p var)) + (cps--with-dynamic-binding var new-var + (cps--transform-1 + `(let* ,more-bindings ,@body) + next-state)) + (cps--transform-1 + (cps--replace-variable-references + var new-var + `(let* ,more-bindings ,@body)) + next-state))))))) + + ;; Process `or'. + + (`(or) (cps--transform-1 nil next-state)) + (`(or ,condition) (cps--transform-1 condition next-state)) + (`(or ,condition . ,rest) + (cps--transform-1 + condition + (cps--add-state "or" + `(setf ,cps--state-symbol + (if ,cps--value-symbol + ,next-state + ,(cps--transform-1 + `(or ,@rest) next-state)))))) + + ;; Process `prog1'. + + (`(prog1 ,first) (cps--transform-1 first next-state)) + (`(prog1 ,first . ,body) + (cps--transform-1 + first + (let ((temp-var-symbol (cps--add-binding "prog1-temp"))) + (cps--add-state "prog1" + `(setf ,temp-var-symbol + ,cps--value-symbol + ,cps--state-symbol + ,(cps--transform-1 + `(progn ,@body) + (cps--add-state "prog1inner" + `(setf ,cps--value-symbol ,temp-var-symbol + ,cps--state-symbol ,next-state)))))))) + + ;; Process `prog2'. + + (`(prog2 ,form1 ,form2 . ,body) + (cps--transform-1 + `(progn ,form1 (prog1 ,form2 ,@body)) + next-state)) + + ;; Process `unwind-protect': If we're inside an unwind-protect, we + ;; have a block of code UNWINDFORMS which we would like to run + ;; whenever control flows away from the main piece of code, + ;; BODYFORM. We deal with the local control flow case by + ;; generating BODYFORM such that it yields to a continuation that + ;; executes UNWINDFORMS, which then yields to NEXT-STATE. + ;; + ;; Non-local control flow is trickier: we need to ensure that we + ;; execute UNWINDFORMS even when control bypasses our normal + ;; continuation. To make this guarantee, we wrap every external + ;; application (i.e., every piece of elisp that can transfer + ;; control non-locally) in an unwind-protect that runs UNWINDFORMS + ;; before allowing the non-local control transfer to proceed. + ;; + ;; Unfortunately, because elisp lacks a mechanism for generically + ;; capturing the reason for an arbitrary non-local control + ;; transfer and restarting the transfer at a later point, we + ;; cannot reify non-local transfers and cannot allow + ;; continuation-passing code inside UNWINDFORMS. + + (`(unwind-protect ,bodyform . ,unwindforms) + ;; Signal the evaluator-generator that it needs to generate code + ;; to handle cleanup forms. + (unless cps--cleanup-table-symbol + (setf cps--cleanup-table-symbol (cps--gensym "cps-cleanup-table-"))) + (let* ((unwind-state + (cps--add-state + "unwind" + ;; N.B. It's safe to just substitute unwindforms by + ;; sexp-splicing: we've already replaced all variable + ;; references inside it with lifted equivalents. + `(progn + ,@unwindforms + (setf ,cps--state-symbol ,next-state)))) + (old-cleanup cps--cleanup-function) + (cps--cleanup-function + (let ((cps--cleanup-function nil)) + (cps--add-state "cleanup" + `(progn + ,(when old-cleanup `(funcall ,old-cleanup)) + ,@unwindforms))))) + (cps--with-value-wrapper + (cps--make-unwind-wrapper unwindforms) + (cps--transform-1 bodyform unwind-state)))) + + ;; Process `while'. + + (`(while ,test . ,body) + ;; Open-code state addition instead of using cps--add-state: we + ;; need our states to be self-referential. (That's what makes the + ;; state a loop.) + (let* ((loop-state + (cps--gensym "cps-state-while-")) + (eval-loop-condition-state + (cps--transform-1 test loop-state)) + (loop-state-body + `(progn + (setf ,cps--state-symbol + (if ,cps--value-symbol + ,(cps--transform-1 + `(progn ,@body) + eval-loop-condition-state) + ,next-state))))) + (push (list loop-state loop-state-body cps--cleanup-function) + cps--states) + (push loop-state cps--bindings) + eval-loop-condition-state)) + + ;; Process various kinds of `quote'. + + (`(quote ,arg) (cps--add-state "quote" + `(setf ,cps--value-symbol (quote ,arg) + ,cps--state-symbol ,next-state))) + (`(function ,arg) (cps--add-state "function" + `(setf ,cps--value-symbol (function ,arg) + ,cps--state-symbol ,next-state))) + + ;; Deal with `iter-yield'. + + (`(cps-internal-yield ,value) + (cps--transform-1 + value + (cps--add-state "iter-yield" + `(progn + (setf ,cps--state-symbol + ,(if cps--cleanup-function + (cps--add-state "after-yield" + `(setf ,cps--state-symbol ,next-state)) + next-state)) + (throw 'cps--yield ,cps--value-symbol))))) + + ;; Catch any unhandled special forms. + + ((and `(,name . ,_) + (guard (cps--special-form-p name)) + (guard (not (memq name cps-standard-special-forms)))) + name ; Shut up byte compiler + (error "special form %S incorrect or not supported" form)) + + ;; Process regular function applications with nontrivial + ;; parameters, converting them to applications of trivial + ;; let-bound parameters. + + ((and `(,function . ,arguments) + (guard (not (cl-loop for argument in arguments + always (atom argument))))) + (let ((argument-symbols + (cl-loop for argument in arguments + collect (if (atom argument) + argument + (cps--gensym "cps-argument-"))))) + + (cps--transform-1 + `(let* ,(cl-loop for argument in arguments + for argument-symbol in argument-symbols + unless (eq argument argument-symbol) + collect (list argument-symbol argument)) + ,(cons function argument-symbols)) + next-state))) + + ;; Process everything else by just evaluating the form normally. + (t (cps--make-atomic-state form next-state)))) + +(defun cps--make-catch-wrapper (tag-binding next-state) + (lambda (form) + (let ((normal-exit-symbol + (cps--gensym "cps-normal-exit-from-catch-"))) + `(let (,normal-exit-symbol) + (prog1 + (catch ,tag-binding + (prog1 + ,form + (setf ,normal-exit-symbol t))) + (unless ,normal-exit-symbol + (setf ,cps--state-symbol ,next-state))))))) + +(defun cps--make-condition-wrapper (var next-state handlers) + ;; Each handler is both one of the transformers with which we wrap + ;; evaluated atomic forms and a state to which we jump when we + ;; encounter the given error. + + (let* ((error-symbol (cps--add-binding "condition-case-error")) + (lexical-error-symbol (cps--gensym "cps-lexical-error-")) + (processed-handlers + (cl-loop for (condition . body) in handlers + collect (cons condition + (cps--transform-1 + (cps--replace-variable-references + var error-symbol + `(progn ,@body)) + next-state))))) + + (lambda (form) + `(condition-case + ,lexical-error-symbol + ,form + ,@(cl-loop + for (condition . error-state) in processed-handlers + collect + `(,condition + (setf ,error-symbol + ,lexical-error-symbol + ,cps--state-symbol + ,error-state))))))) + +(defun cps--replace-variable-references (var new-var form) + "Replace all non-shadowed references to VAR with NEW-VAR in FORM. +This routine does not modify FORM. Instead, it returns a +modified copy." + (macroexpand-all + `(cl-symbol-macrolet ((,var ,new-var)) ,form) + macroexpand-all-environment)) + +(defun cps--make-unwind-wrapper (unwind-forms) + (cl-assert lexical-binding) + (lambda (form) + (let ((normal-exit-symbol + (cps--gensym "cps-normal-exit-from-unwind-"))) + `(let (,normal-exit-symbol) + (unwind-protect + (prog1 + ,form + (setf ,normal-exit-symbol t)) + (unless ,normal-exit-symbol + ,@unwind-forms)))))) + +(put 'iter-end-of-sequence 'error-conditions '(iter-end-of-sequence)) +(put 'iter-end-of-sequence 'error-message "iteration terminated") + +(defun cps--make-close-iterator-form (terminal-state) + (if cps--cleanup-table-symbol + `(let ((cleanup (cdr (assq ,cps--state-symbol ,cps--cleanup-table-symbol)))) + (setf ,cps--state-symbol ,terminal-state + ,cps--value-symbol nil) + (when cleanup (funcall cleanup))) + `(setf ,cps--state-symbol ,terminal-state + ,cps--value-symbol nil))) + +(defun cps-generate-evaluator (body) + (let* (cps--states + cps--bindings + cps--cleanup-function + (cps--value-symbol (cps--gensym "cps-current-value-")) + (cps--state-symbol (cps--gensym "cps-current-state-")) + ;; We make *cps-cleanup-table-symbol** non-nil when we notice + ;; that we have cleanup processing to perform. + (cps--cleanup-table-symbol nil) + (terminal-state (cps--add-state "terminal" + `(signal 'iter-end-of-sequence + ,cps--value-symbol))) + (initial-state (cps--transform-1 + (macroexpand-all + `(cl-macrolet + ((iter-yield (value) + `(cps-internal-yield ,value))) + ,@body) + macroexpand-all-environment) + terminal-state)) + (finalizer-symbol + (when cps--cleanup-table-symbol + (when cps--cleanup-table-symbol + (cps--gensym "cps-iterator-finalizer-"))))) + `(let ,(append (list cps--state-symbol cps--value-symbol) + (when cps--cleanup-table-symbol + (list cps--cleanup-table-symbol)) + (when finalizer-symbol + (list finalizer-symbol)) + (nreverse cps--bindings)) + ;; Order state list so that cleanup states are always defined + ;; before they're referenced. + ,@(cl-loop for (state body cleanup) in (nreverse cps--states) + collect `(setf ,state (lambda () ,body)) + when cleanup + do (cl-assert cps--cleanup-table-symbol) + and collect `(push (cons ,state ,cleanup) ,cps--cleanup-table-symbol)) + (setf ,cps--state-symbol ,initial-state) + + (let ((iterator + (lambda (op value) + (cond + ,@(when finalizer-symbol + `(((eq op :stash-finalizer) + (setf ,finalizer-symbol value)) + ((eq op :get-finalizer) + ,finalizer-symbol))) + ((eq op :close) + ,(cps--make-close-iterator-form terminal-state)) + ((eq op :next) + (setf ,cps--value-symbol value) + (let ((yielded nil)) + (unwind-protect + (prog1 + (catch 'cps--yield + (while t + (funcall ,cps--state-symbol))) + (setf yielded t)) + (unless yielded + ;; If we're exiting non-locally (error, quit, + ;; etc.) close the iterator. + ,(cps--make-close-iterator-form terminal-state))))) + (t (error "unknown iterator operation %S" op)))))) + ,(when finalizer-symbol + `(funcall iterator + :stash-finalizer + (make-finalizer + (lambda () + (iter-close iterator))))) + iterator)))) + +(defun iter-yield (value) + "When used inside a generator, yield control to caller. +The caller of `iter-next' receives VALUE, and the next call to +`iter-next' resumes execution at the previous +`iter-yield' point." + (identity value) + (error "`iter-yield' used outside a generator")) + +(defmacro iter-yield-from (value) + "When used inside a generator function, delegate to a sub-iterator. +The values that the sub-iterator yields are passed directly to +the caller, and values supplied to `iter-next' are sent to the +sub-iterator. `iter-yield-from' evaluates to the value that the +sub-iterator function returns via `iter-end-of-sequence'." + (let ((errsym (cps--gensym "yield-from-result")) + (valsym (cps--gensym "yield-from-value"))) + `(let ((,valsym ,value)) + (unwind-protect + (condition-case ,errsym + (let ((vs nil)) + (while t + (setf vs (iter-yield (iter-next ,valsym vs))))) + (iter-end-of-sequence (cdr ,errsym))) + (iter-close ,valsym))))) + +(defmacro iter-defun (name arglist &rest body) + "Creates a generator NAME. +When called as a function, NAME returns an iterator value that +encapsulates the state of a computation that produces a sequence +of values. Callers can retrieve each value using `iter-next'." + (declare (indent defun)) + (cl-assert lexical-binding) + (let* ((parsed-body (macroexp-parse-body body)) + (declarations (car parsed-body)) + (exps (cdr parsed-body))) + `(defun ,name ,arglist + ,@declarations + ,(cps-generate-evaluator exps)))) + +(defmacro iter-lambda (arglist &rest body) + "Return a lambda generator. +`iter-lambda' is to `iter-defun' as `lambda' is to `defun'." + (declare (indent defun)) + (cl-assert lexical-binding) + `(lambda ,arglist + ,(cps-generate-evaluator body))) + +(defun iter-next (iterator &optional yield-result) + "Extract a value from an iterator. +YIELD-RESULT becomes the return value of `iter-yield` in the +context of the generator. + +This routine raises the `iter-end-of-sequence' condition if the +iterator cannot supply more values." + (funcall iterator :next yield-result)) + +(defun iter-close (iterator) + "Terminate an iterator early. +Run any unwind-protect handlers in scope at the point ITERATOR +is blocked." + (funcall iterator :close nil)) + +(cl-defmacro iter-do ((var iterator) &rest body) + "Loop over values from an iterator. +Evaluate BODY with VAR bound to each value from ITERATOR. +Return the value with which ITERATOR finished iteration." + (declare (indent 1)) + (let ((done-symbol (cps--gensym "iter-do-iterator-done")) + (condition-symbol (cps--gensym "iter-do-condition")) + (it-symbol (cps--gensym "iter-do-iterator")) + (result-symbol (cps--gensym "iter-do-result"))) + `(let (,var + ,result-symbol + (,done-symbol nil) + (,it-symbol ,iterator)) + (while (not ,done-symbol) + (condition-case ,condition-symbol + (setf ,var (iter-next ,it-symbol)) + (iter-end-of-sequence + (setf ,result-symbol (cdr ,condition-symbol)) + (setf ,done-symbol t))) + (unless ,done-symbol ,@body)) + ,result-symbol))) + +(defvar cl--loop-args) + +(defmacro cps--advance-for (conscell) + ;; See cps--handle-loop-for + `(condition-case nil + (progn + (setcar ,conscell (iter-next (cdr ,conscell))) + ,conscell) + (iter-end-of-sequence + nil))) + +(defmacro cps--initialize-for (iterator) + ;; See cps--handle-loop-for + (let ((cs (cps--gensym "cps--loop-temp"))) + `(let ((,cs (cons nil ,iterator))) + (cps--advance-for ,cs)))) + +(defun cps--handle-loop-for (var) + "Support `iter-by' in `loop'. " + ;; N.B. While the cl-loop-for-handler is a documented interface, + ;; there's no documented way for cl-loop-for-handler callbacks to do + ;; anything useful! Additionally, cl-loop currently lexbinds useful + ;; internal variables, so our only option is to modify + ;; cl--loop-args. If we substitute a general-purpose for-clause for + ;; our iterating clause, however, we can't preserve the + ;; parallel-versus-sequential `loop' semantics for for clauses --- + ;; we need a terminating condition as well, which requires us to use + ;; while, and inserting a while would break and-sequencing. + ;; + ;; To work around this problem, we actually use the "for var in LIST + ;; by FUNCTION" syntax, creating a new fake list each time through + ;; the loop, this "list" being a cons cell (val . it). + (let ((it-form (pop cl--loop-args))) + (setf cl--loop-args + (append + `(for ,var + in (cps--initialize-for ,it-form) + by 'cps--advance-for) + cl--loop-args)))) + +(put 'iter-by 'cl-loop-for-handler 'cps--handle-loop-for) + +(eval-after-load 'elisp-mode + (lambda () + (font-lock-add-keywords + 'emacs-lisp-mode + '(("(\\(iter-defun\\)\\_>\\s *\\(\\(?:\\sw\\|\\s_\\)+\\)?" + (1 font-lock-keyword-face nil t) + (2 font-lock-function-name-face nil t)) + ("(\\(iter-\\(?:next\\|lambda\\|yield\\|yield-from\\)\\)\\_>" + (1 font-lock-keyword-face nil t)))))) + +(provide 'generator) + +;;; generator.el ends here |