Re: Contracts question on structs

On Fri, Feb 15, 2019 at 12:18 AM David Storrs <[email protected]> wrote:
>
> On Thu, Feb 14, 2019 at 9:08 PM Robby Findler
> <[email protected]> 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 <[email protected]> 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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