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fix queue/stack confusion in the Guide

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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
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18
collects/scribblings/guide/contracts-exists.scrbl
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@ -9,7 +9,7 @@ protect abstractions, ensuring that clients of your module
cannot depend on the precise representation choices you make
for your data structures.
@; @ctc-section{Getting Started, with a Stack Example}
@; @ctc-section{Getting Started, with a Queue Example}
@margin-note{
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
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
(define empty '())
(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
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:
@racketblock[(provide
(contract-out
#:∃ stack
[empty stack]
[enq (-> integer? stack stack)]
[next (-> stack integer?)]
[deq (-> stack (listof integer?))]
[empty? (-> stack boolean?)]))]
#:∃ queue
[empty queue]
[enq (-> integer? queue queue)]
[next (-> queue integer?)]
[deq (-> queue (listof integer?))]
[empty? (-> queue boolean?)]))]
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.