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

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