suzanne.soy/racket
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

fix queue/stack confusion in the Guide

Browse Source 
closes PR 12896
This commit is contained in:
Robby Findler 2012-07-11 18:14:52 -05:00
parent e430463ea4
commit 0ca54bfe56
1 changed files with 9 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
collects/scribblings/guide/contracts-exists.scrbl
View File

@ -9,7 +9,7 @@ protect abstractions, ensuring that clients of your module
cannot depend on the precise representation choices you make cannot depend on the precise representation choices you make
for your data structures. for your data structures.
@; @ctc-section{Getting Started, with a Stack Example} @; @ctc-section{Getting Started, with a Queue Example}
@margin-note{ @margin-note{
You can type @racket[#:exists] instead of @racket[#:∃] if you You can type @racket[#:exists] instead of @racket[#:∃] if you
@ -21,7 +21,7 @@ The @racket[contract-out] form allows you to write
introduces the variable @racket[_name-of-a-new-contract], binding it to a new introduces the variable @racket[_name-of-a-new-contract], binding it to a new
contract that hides information about the values it protects. contract that hides information about the values it protects.
As an example, consider this (simple) implementation of a stack datastructure: As an example, consider this (simple) implementation of a queue datastructure:
@racketmod[racket @racketmod[racket
(define empty '()) (define empty '())
(define (enq top queue) (append queue (list top))) (define (enq top queue) (append queue (list top)))
@ -42,16 +42,16 @@ data structure (perhaps accidentally) and thus any change in the representation
(say to a more efficient representation that supports amortized constant time (say to a more efficient representation that supports amortized constant time
enqueue and dequeue operations) might break client code. enqueue and dequeue operations) might break client code.
To ensure that the stack representation is abstract, we can use @racket[#:∃] in the To ensure that the queue representation is abstract, we can use @racket[#:∃] in the
@racket[contract-out] expression, like this: @racket[contract-out] expression, like this:
@racketblock[(provide @racketblock[(provide
(contract-out (contract-out
#:∃ stack #:∃ queue
[empty stack] [empty queue]
[enq (-> integer? stack stack)] [enq (-> integer? queue queue)]
[next (-> stack integer?)] [next (-> queue integer?)]
[deq (-> stack (listof integer?))] [deq (-> queue (listof integer?))]
[empty? (-> stack boolean?)]))] [empty? (-> queue boolean?)]))]
Now, if clients of the data structure try to use @racket[car] and @racket[cdr], they Now, if clients of the data structure try to use @racket[car] and @racket[cdr], they
receive an error, rather than mucking about with the internals of the queues. receive an error, rather than mucking about with the internals of the queues.