无法获得Y组合器的实现工作 [英] Unable to get implementation of Y combinator working
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问题描述
以下是代码(。
Here's the code (also here):
#lang racket
(define poorY
((lambda length
(lambda (ls)
(cond
[(null? ls) 0]
[else (add1 ((length length) (cdr ls)))])))
(lambda length
(lambda (ls)
(cond
[(null? ls) 0]
[else (add1 ((length length) (cdr ls)))])))))
When I run it:
> (poorY '(9 7 8))
. . application: not a procedure;
expected a procedure that can be applied to arguments
given: '(#<procedure>)
arguments...:
'(#<procedure>)
The screenshot looks like this:
I'm using DrRacket as the repl. What's wrong with the code?
解决方案
There should be parentheses around length
:
(define poorY
((lambda (length) ;; here
(lambda (ls)
(cond
[(null? ls) 0]
[else (add1 ((length length) (cdr ls)))])))
(lambda (length) ;; and here
(lambda (ls)
......
Instead of typing the same long lambda expression twice, you can also try
(define poorY
((lambda (f) (f f))
(lambda (length)
(lambda (ls)
(cond
[(null? ls) 0]
[else (add1 ((length length) (cdr ls)))])))))
See also Y combinator discussion in "The Little Schemer" .
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