tweaked week3

Signed-off-by: Jim Pryor <profjim@jimpryor.net>

diff --git a/week3.mdwn b/week3.mdwn
index 29a058c..a9ca3cd 100644 (file)
--- a/week3.mdwn
+++ b/week3.mdwn
@@ -14,7 +14,7 @@ In OCaml, you'd define that like this:
 
 In Scheme you'd define it like this:
 
-       (letrec [(get_length (lambda (lst) (if (null? lst) 0 (+ 1 (get_length (cdr lst))))))]
+       (letrec [(get_length (lambda (lst) (if (null? lst) 0 [+ 1 (get_length (cdr lst))] )) )]
                ... ; here you go on to use the function "get_length"
        )
 
@@ -24,6 +24,8 @@ Some comments on this:
 
 2. `cdr` is function that gets the tail of a Scheme list. (By definition, it's the function for getting the second member of an ordered pair. It just turns out to return the tail of a list because of the particular way Scheme implements lists.)
 
+3.     I alternate between `[ ]`s and `( )`s in the Scheme code just to make it more readable. These have no syntactic difference.
+
 What is the `let rec` in the OCaml code and the `letrec` in the Scheme code? These work like the `let` expressions we've already seen, except that they let you use the variable `get_length` *inside* the body of the function being bound to it---with the understanding that it will there refer to the same function that you're then in the process of binding to `get_length`. So our recursively-defined function works the way we'd expect it to. In OCaml:
 
        let rec get_length = fun lst ->
@@ -34,8 +36,8 @@ What is the `let rec` in the OCaml code and the `letrec` in the Scheme code? The
 In Scheme:
 
        > (letrec [(get_length 
-                       (lambda (lst) (if (null? lst) 0 (+ 1 (get_length (cdr lst))) ) )i
-               )] (get_length (list 20 30)))
+                               (lambda (lst) (if (null? lst) 0 [+ 1 (get_length (cdr lst))] )) )]
+                       (get_length (list 20 30)))
        ; this evaluates to 2
        
 If you instead use an ordinary `let` (or `let*`), here's what would happen, in OCaml:
@@ -48,8 +50,8 @@ If you instead use an ordinary `let` (or `let*`), here's what would happen, in O
 Here's Scheme:
 
        > (let* [(get_length 
-                       (lambda (lst) (if (null? lst) 0 (+ 1 (get_length (cdr lst))) ) )
-               )] (get_length (list 20 30)))
+                               (lambda (lst) (if (null? lst) 0 [+ 1 (get_length (cdr lst))] )) )]
+                       (get_length (list 20 30)))
        ; fails with error "reference to undefined identifier: get_length"
 
 Why? Because we said that constructions of this form:
@@ -98,7 +100,7 @@ So how could we do it? And how do OCaml and Scheme manage to do it, with their `
 2.     If you tried this in Scheme:
 
                > (define get_length 
-                               (lambda (lst) (if (null? lst) 0 (+ 1 (get_length (cdr lst))) ) ))
+                               (lambda (lst) (if (null? lst) 0 [+ 1 (get_length (cdr lst))] )) )
 
                > (get_length (list 20 30))