[plt-dev] abstract contracts

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Sat Sep 5 20:58:31 EDT 2009

On Sep 5, 2009, at 6:04 PM, Robby Findler wrote:

> No. As we discussed, there can't be with this meaning of #:exists. If
> there were, you'd run into the same problem as with number? (for those
> that don't know what I'm talking about, see the new second gotcha
> section in the contracts documentation in the guide).
> But, if you want to protect yourself by shifting blame, you can still
> do that. You need to use the 'stack' contract in your own code to do
> so instead of using a predicate test.

So the client can write additional contracts involving stacks, e.g.,

   [stack-append (-> stack stack stack)]

but it is impossible to write data definitions such as

  A collection is one of:
    -- stack
    -- queue

and then use it because you can't distinguish a stack from a queue.  
(Of course, if both stacks and queues support the same operations, you  
can do this.)

-- Matthias

> Robby
> On Sat, Sep 5, 2009 at 10:02 AM, Matthias Felleisen<matthias at ccs.neu.edu 
> > wrote:
>> Is there a stack? predicate afterwards?
>> On Sep 4, 2009, at 2:00 PM, Robby Findler wrote:
>>> I've added #:exists to provide/contract, as a way to hide  
>>> information
>>> ala "type t" declarations on ML signatures.
>>> See the contracts section in the Guide for a worked example and a
>>> discussion of a gotcha. The short version is that you can now write
>>> things like this:
>>> (provide/contract
>>>  #:exists stack
>>>  [new stack]
>>>  [push (-> int stack stack)]
>>>  [pop (-> (and/c stack non-empty?) int)]
>>>  [non-empty? (-> stack boolean?)])
>>> and have the contract system enforce data abstraction, even if your
>>> stack operations are simply these:
>>>  (define new '())
>>>  (define push cons)
>>>  (define pop car)
>>>  (define non-empty? pair?)
>>> That is, clients of your module will not be able to treat your  
>>> stacks
>>> as if they were lists, even though they really are lists.
>>> Robby
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