[racket] Whats the difference between a predicate and a flat contract?

Yes ..., I think so (well, eq? probably messes things up, as usual).

But that's a funny question! What's really going on there is that
flat-contract is coercing the value into a contract, and flat
contracts also acts as predicate functions (matching what the
contracts match). flat-contract-predicate takes its argument, turns it
into the contract and then returns the predicate. This last step is
useless, as flat-contracts are now predicates without any coercion
(again, not something that was always the case).

I'll add a similar note to the docs for this function too.

Robby

On Tue, Dec 4, 2012 at 8:59 PM, Harry Spier <vasishtha.spier at gmail.com> wrote:
> Are flat-contract and flat-contract-predicate equivalent?
>
>> ((flat-contract 'x) 'x)
> #t
>> ((flat-contract-predicate 'x) 'x)
> #t
>> ((flat-contract 'x) 'y)
> #f
>> ((flat-contract-predicate 'x) 'y)
> #f
>>
>
> Harry
>
> On Tue, Dec 4, 2012 at 9:41 PM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
>> It does that for symbols, but not everything.
>>
>> This is the place you should be looking, I think.
>>
>> http://docs.racket-lang.org/reference/contracts.html
>>
>> Robby
>>
>> On Tue, Dec 4, 2012 at 8:38 PM, Harry Spier <vasishtha.spier at gmail.com> wrote:
>>> OK I see the docs to flat-contract? but not flat-contract
>>> http://docs.racket-lang.org/reference/contract-utilities.html#(def._((lib._racket/contract/private/misc..rkt)._flat-contract~3f))
>>> mention that flat-contracts are more than predicates.  Those docs
>>> don't mention it, but it appears from experimentation that
>>> (flat-contract something) produces a procedure such that
>>> ((flat-contract something) x) is #t if something eq? x .
>>>
>>> Harry Spier
>>>
>>> On Tue, Dec 4, 2012 at 8:57 PM, Robby Findler
>>> <robby at eecs.northwestern.edu> wrote:
>>>> On Tue, Dec 4, 2012 at 7:28 PM, Carl Eastlund <cce at ccs.neu.edu> wrote:
>>>>> On Tue, Dec 4, 2012 at 7:49 PM, Robby Findler <robby at eecs.northwestern.edu>
>>>>> wrote:
>>>>>>
>>>>>> Flat contracts includes more things than contracts. For example:
>>>>>>
>>>>>> [robby at yanpu] ~/git/plt/collects/scribblings/reference$ racket
>>>>>> Welcome to Racket v5.3.1.9.
>>>>>> > (flat-contract? 'x)
>>>>>> #t
>>>>>> > (procedure? 'x)
>>>>>> #f
>>>>>>
>>>>>> The flat-contract function is a holdover from the days when flat contracts
>>>>>> weren't able to be used directly as predicate functions.
>>>>>>
>>>>>>
>>>>>> I'll push a clarification to the docs for flat-contract.
>>>>>
>>>>>
>>>>> Isn't that the wrong way around?  The flat-contract function lets you use a
>>>>> predicate as a contract, not a contract as a predicate.  Presumably it's
>>>>> from before predicates could be used as contracts, although I hadn't
>>>>> realized there was such a time.
>>>>
>>>> There was a time when you had to call 'flat-contract' to turn a
>>>> predicate into a contract, yep. There was a housecleaning (anyone
>>>> remember the days when there were multiple suffixes (not just "/c") on
>>>> the combinators?) and I probably should have gotten rid of it at that
>>>> time, but I didn't.
>>>>
>>>> (Oh and I mean "contracts" where I wrote "preducate functions" above. Oops.)
>>>>
>>>> Robby