From 525668310b763ca4ee178d3bc32ebbdff959ffcc Mon Sep 17 00:00:00 2001 From: "Elf M. Sternberg" Date: Wed, 15 Apr 2015 16:01:30 -0700 Subject: [PATCH] Cleanup pass over comments in chapter 1. --- chap1.rkt | 31 ++++++++++++------------------- 1 file changed, 12 insertions(+), 19 deletions(-) diff --git a/chap1.rkt b/chap1.rkt index be4fa23..4f919d3 100644 --- a/chap1.rkt +++ b/chap1.rkt @@ -26,7 +26,7 @@ (define env_global env_init) ; So, this macro places *into the current scope* (i.e. no building of -; a new scope that get's reaped upon exit) the names of variables and +; a new scope that gets reaped upon exit) the names of variables and ; potential initial values. (define-syntax definitial @@ -49,6 +49,9 @@ (apply value values) (wrong-syntax #'here "Incorrect arity ~s" (list 'name values)))))))) +; Sometimes, you do have to define something before you use it. Lesson +; learned. + (define the-false-value (cons "false" "boolean")) (definitial t #t) @@ -68,9 +71,9 @@ ; This function extends the environment so that *at this moment of ; extension* the conslist head points to the old environment, then -; when it's done it points to the new environment. What's really -; interesting is that the conslist head points to the last object -; initialized, not the first. +; when it's done it points to the new environment. What's interesting +; is that the conslist head points to the last object initialized, not +; the first. (define (extend env variables values) (cond ((pair? variables) @@ -84,12 +87,9 @@ (wrong-syntax #'here "Too many values"))) ((symbol? variables) (mcons (mcons variables values) env)))) -; This is interesting. Already we're starting to get some scope here. -; Note that make function provides the environment, not the invoke. -; But we're still only passing the empty init environment. - -; Now a different order, one in which closures aren't present, makes -; the code fragile. So: +; Already we're starting to get some scope here. Note that +; make-function provides the environment, not the invoke. This makes +; this a lexically scoped interpreter. (define (make-function variables body env) (lambda (values) @@ -102,12 +102,6 @@ (fn args) (wrong-syntax #'here "Not an function ~s" fn))) -; The text points out that the global environment is being -; pushed/popped as needed and rebuilt with every function call. In -; this case, the defining envirnoment is no longer being used at all, -; only the current one. This would make this a dynamically scope -; language, no? - ; Iterate through the exps, return the value of the last one. (define (eprogn exps env) @@ -133,8 +127,7 @@ (define-syntax mcadr (syntax-rules () ((_ e) (mcdr (mcar e))))) ; Iterate through the environment, find an ID, return its associated -; value. This is a completely global, resettable environment; we're -; talking BASIC here. +; value. (define (lookup id env) (if (mpair? env) @@ -154,7 +147,7 @@ (update! id (mcdr env) value)) (wrong-syntax #'here "No such binding ~s" id))) -; Different evaluation rules. +; Core evaluation rules. (define (evaluate exp env) (if (atom? exp)