Still having trouble though, can't get struct/dc to work in the way I'd
like. I tried to simplify my example:
(define Result/c
(struct/dc Result
(a number?)
(b boolean?)
; in other words, if a is 0, then b must be
; false or the contract is broken
#:inv (a b) (if (= a 0)
(eq? b #f)
#t)))
(provide (contract-out [Result Result/c]))
The documentation on #:inv says it must return a non-#f value, which I
assume means it acts like a predicate? #f if the contract fails, something
else otherwise? In any case, this doesn't work because it says:
Result: broke its own contract
promised: Result?
produced: #<procedure:Result>
in: (struct/dc
Result
(a number?)
(b boolean?)
#:inv
(a b)
...)
I also tried specifying all of the struct functions that are created with
their own contracts, like Result-a, Result-b, Result? and then using
Result/c for Result, and this fails too.
I think I understand how this is being used in the example in the
documentation with #:lazy, but it's not quite what I'm looking for: I'm
looking to use a contract on the struct constructor (in particular, which
is why I thought #:inv would be the way to go) where one field
initialization depends on another.
As an aside, I also tried the following:
(provide (contract-out [Result (or/c (number? boolean? . -> . Result?)
(0 #f . -> . Result?))]))
But the contract system complained that both might match (I mean I can see
that one is more specific, but I understand it's not a theorem prover,
haha).
Thoughts?
Chris
On Fri, Feb 15, 2019 at 7:56 AM Chris GauthierDickey <chris...@gmail.com>
wrote:
> I believe it should be since bst/c is supposed to be a contract. Also I
> noticed the docs at
> https://docs.racket-lang.org/reference/data-structure-contracts.html show
> a similar example:
>
> ; bst-between : number number -> contract
> ; builds a contract for binary search trees
> ; whose values are between low and high
> (define (bst-between/c low high)
> (or/c null?
> (struct/dc node [val (between/c low high)]
> [left (val) #:lazy (bst-between/c low val)]
> [right (val) #:lazy (bst-between/c val high)])))
>
>
> On Fri, Feb 15, 2019 at 1:31 AM David Storrs <david.sto...@gmail.com>
> wrote:
>
>> On Fri, Feb 15, 2019 at 12:18 AM David Storrs <david.sto...@gmail.com>
>> wrote:
>> >
>> > On Thu, Feb 14, 2019 at 9:08 PM Robby Findler
>> > <ro...@eecs.northwestern.edu> wrote:
>> > >
>> > > This is what struct/dc is for. Let me know if the docs let you down!
>> > >
>> > > Robby
>> >
>> > This is good to hear about, because I'm actually in the middle of
>> > writing something that it will help with. Thanks, Robby.
>> >
>> > One question: Is there a typo in the example code or am I missing
>> something?
>> >
>> > (struct bt (val left right))
>> > (define (bst/c lo hi)
>> > (or/c #f
>> > (struct/dc bt
>> > [val (between/c lo hi)]
>> > [left (val) #:lazy (bst lo val)] ; should that be (bt
>> lo val)?
>> > [right (val) #:lazy (bst val hi)]))) ; and this be (bt val
>> hi) ?
>>
>> Ugh. I hate it when I make significant typos in code questions. I
>> should have asked if it was supposed to (bst/c lo val) and (bst/c val
>> hi). (i.e., is there a missing /c in the code?) If not, where is bst
>> defined?
>>
>> >
>> > Also, am I correct that the contract is not actually attached to the
>> > bt structure at this point? That's a thing that would be done
>> > elsewhere, probably in the provide statement. Yes?
>> >
>> > >
>> > > On Thu, Feb 14, 2019 at 7:57 PM Chris GauthierDickey <
>> chris...@gmail.com> wrote:
>> > >>
>> > >> I'm wondering if it's possible to have a contract for a struct that
>> only allows certain kinds of initializations. For example, I have this:
>> > >>
>> > >> (provide (contract-out
>> > >> [struct Result ((name (or/c Temp? Label?)) (global?
>> boolean?) (value (or/c VarValue? #f)))]))
>> > >>
>> > >> But it's too general. What I'd really like to have is a contract
>> that combines them like an or/c (I can see what this would cause problems,
>> just wondering if there's a clever way around it):
>> > >> (provide (contract-out
>> > >> [or/c (struct Result ((name Temp?) (global? #f) (value
>> (or/c VarValue? #f))))
>> > >> (struct Result ((name Label?) (global? boolean?)
>> (value (or/c (VarValue #f)))))]))
>> > >>
>> > >> In this case, if name is Temp? then global? has to be #f. If name is
>> Label? then global? can be #t or #f. I'd like to use struct cause it
>> provides all the contracts for accessors and such. Note, I can use a
>> #:guard on the struct, but wondered if it was possible to do something
>> along these lines with a contract.
>> > >>
>> > >> Thanks!
>> > >> Chris
>> > >>
>> > >>
>> > >> --
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