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clarify comments in eval.scm

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* module/ice-9/eval.scm: Add some more comments.
This commit is contained in:
Andy Wingo 2009-12-03 00:15:02 +01:00
parent 5f1611640a
commit b2b554efd3
1 changed files with 24 additions and 4 deletions
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28
module/ice-9/eval.scm
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@ -22,14 +22,27 @@
;;; Commentary: ;;; Commentary:
;;; Scheme eval, written in Scheme! ;;; Scheme eval, written in Scheme.
;;;
;;; Expressions are first expanded, by the syntax expander (i.e.
;;; psyntax), then memoized into internal forms. The evaluator itself
;;; only operates on the internal forms ("memoized expressions").
;;;
;;; Environments are represented as linked lists of the form (VAL ... .
;;; MOD). If MOD is #f, it means the environment was captured before
;;; modules were booted. If MOD is the literal value '(), we are
;;; evaluating at the top level, and so should track changes to the
;;; current module.
;;;
;;; Evaluate this in Emacs to make code indentation work right:
;;;
;;; (put 'memoized-expression-case 'scheme-indent-function 1)
;;; ;;;
;;; Code: ;;; Code:
;; (put 'memoized-expression-case 'scheme-indent-function 1)
(eval-when (compile) (eval-when (compile)
(define-syntax capture-env (define-syntax capture-env
(syntax-rules () (syntax-rules ()
@ -37,11 +50,13 @@
(if (null? env) (if (null? env)
(current-module) (current-module)
(if (not env) (if (not env)
;; the and current-module checks that modules are booted ;; the and current-module checks that modules are booted,
;; and thus the-root-module is defined
(and (current-module) the-root-module) (and (current-module) the-root-module)
env))))) env)))))
;; could be more straightforward if we had better copy propagation ;; This macro could be more straightforward if the compiler had better
;; copy propagation. As it is we do some copy propagation by hand.
(define-syntax mx-bind (define-syntax mx-bind
(lambda (x) (lambda (x)
(syntax-case x () (syntax-case x ()
@ -63,6 +78,8 @@
#'(let ((v data)) #'(let ((v data))
body))))) body)))))
;; The resulting nested if statements will be an O(n) dispatch. Once
;; we compile `case' effectively, this situation will improve.
(define-syntax mx-match (define-syntax mx-match
(lambda (x) (lambda (x)
(syntax-case x (quote) (syntax-case x (quote)
@ -85,6 +102,7 @@
(define primitive-eval (define primitive-eval
(let () (let ()
;; The "engine". EXP is a memoized expression.
(define (eval exp env) (define (eval exp env)
(memoized-expression-case exp (memoized-expression-case exp
(('begin (first . rest)) (('begin (first . rest))
@ -197,7 +215,9 @@
(memoize-variable-access! exp #f)) (memoize-variable-access! exp #f))
(eval x env))))) (eval x env)))))
;; primitive-eval
(lambda (exp) (lambda (exp)
"Evaluate @var{exp} in the current module."
(eval (eval
(memoize-expression ((or (module-transformer (current-module)) (memoize-expression ((or (module-transformer (current-module))
(lambda (x) x)) (lambda (x) x))