From 2b8a6578f434e632ed029ca4963cd1f619ada69e Mon Sep 17 00:00:00 2001
From: Jim Pryor
Date: Sat, 18 Sep 2010 17:47:44 -0400
Subject: [PATCH] tweaked week3
Signed-off-by: Jim Pryor
---
week3.mdwn | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
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))
--
2.11.0