From: Jim Pryor <profjim@jimpryor.net>
Date: Sat, 18 Sep 2010 21:47:44 +0000 (-0400)
Subject: tweaked week3
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=2b8a6578f434e632ed029ca4963cd1f619ada69e;ds=sidebyside

tweaked week3

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

diff --git a/week3.mdwn b/week3.mdwn
index 29a058c0..a9ca3cd5 100644
--- 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))