No, it doesn’t need to be wrapped in an opaque structure. Wrapping it in an 
opaque structure would add a layer of indirection for absolutely no gain. 
Remember, the value itself is already, by definition, opaque. The only way 
typed code can manipulate the value is by passing it to other functions 
imported via require/typed.

This means that contracts only need to be generated wherever those functions 
are called. This can be done without wrapping or unwrapping anything because 
all the information required to generate those contracts is known at 
expansion-time. The typechecker simply needs to insert the relevant contracts 
at the relevant locations.

> On Jan 30, 2015, at 07:27, Alexander D. Knauth <> wrote:
> On Thu, Jan 29, 2015, at 09:03 PM, Alexis King wrote:
>> It isn’t wrapped in an opaque structure. That wasn’t a part of my proposal, 
>> and while I didn’t think of it until you brought it up, I still think it’s 
>> unnecessary and doesn’t add any convenience.
> I think the opaque structures would be necessary for the kind of "sharing 
> wrappers between functions" that you describe just before section 2.1, except 
> that instead of the sub-values being wrapped on the untyped side, the whole 
> thing is wrapped on the typed side, and there is a contract that wraps it and 
> unwraps it when it goes from untyped to typed and back.  
> For parametric types, they have to also work if the type was constrained to 
> the exact set of values that were provided, which means that if you provide 
> two numbers, say 1 and 2, it has to return a posn with not just any two 
> numbers, but values of the type (U 1 2), since A could have been constrained 
> to (U 1 2).  So it has to be wrapped somehow, and I think wrapping it on the 
> typed side makes more sense.  
>> Perhaps I’m not understanding you properly, but your “one-length string” 
>> idea sounds like it has little to do with this opaque type problem and more 
>> to do with the fact that you want refinement types in Typed Racket. I do, 
>> too! But I don’t think hacking the opaque type system is going to help you 
>> with that.
> Well, yeah, refinement types would be the "real" solution for this particular 
> example, but if I do want to constrain it to strings of length 1, opaque 
> types are the only option for now, and they actually work fine.  My point was 
> you couldn't do this type of thing with the opaque structures and you would 
> probably get weird errors if you tried.  (See below because there might be a 
> solution?)
>> (Also, as for the box example, I’m actually a little surprised that doesn’t 
>> contract error. Seems like a bug to me, but perhaps I’m missing some 
>> idiosyncrasies of the type system. Either way, it’s precisely that kind of 
>> craziness I was referring to when I compared casting parametric opaque types 
>> to casting mutable types.)
> There is a bug report for it here, and the solution proposed by Sam 
> Tobin-Hochstadt would be for cast to generate 2 contracts, one for the 
> original type, one for the new type, but that never got implemented.  
> <>
> Actually now that I think about it the two-contract solution might be able to 
> solve the previous problem, since the original contract could unwrap the 
> value before it is passed to the new contract?  I'm not sure though.  The 
> value inside the cast would be from the typed side, then it is passed through 
> the orig contract as if it were going to the typed side, which would unwrap 
> it, and then that unwrapped value would be passed to the new contract as if 
> it were flowing from the untyped side to the typed side.  
>>> On Jan 29, 2015, at 20:50, Alexander D. Knauth < 
>>> <>> wrote:
>>> On Jan 29, 2015, at 11:34 PM, Alexis King < 
>>> <>> wrote:
>>>>> But the problem is that if it’s an opaque type then it can’t unwrap it 
>>>>> once the value is returned from make-posn.
>>>> Yes, that’s precisely the problem. Your point about implementing 
>>>> everything as single-valued structs on the typed side is an interesting 
>>>> one, though I don’t think it ultimately solves any problems. The fact that 
>>>> the typed side knowsnothingabout the contents of the value is what makes 
>>>> this such a tricky problem.
>>>> As for this:
>>>>> But then you couldn’t do any operations on it except those that you use 
>>>>> import with require/typed, right?
>>>> That’s completely correct. That’s why it’s “opaque.”
>>>>> And what happens if you use cast on one of these things?
>>>> That’s a little more interesting. Usingcaston an object of this type would 
>>>> never fail (unless, of course, it didn’t actually satisfy the 
>>>> basicposn?predicate), but it would possibly introduce failures in the 
>>>> future since it would affect the contracts generated forposn-xandposn-y, 
>>>> for example.
>>>> To make that more clear, casting a(Posn Real)to a(Posn String)would work 
>>>> fine until you tried to callposn-xon the instance, in which case it would 
>>>> raise a contract error. Note that this isn’t really any different from 
>>>> casting mutable data types.
>>> But if it were wrapped in an opaque structure, then that structure wouldn’t 
>>> satisfy the posn? predicate, unless of course the posn? predicate has a 
>>> contract that unwraps it.  So all of the operations on it would have to 
>>> have contracts that would unwrap it.  This might actually make sense if the 
>>> type is meant to be actually opaque, but if it’s an opaque type that 
>>> represents a normal non-opaque value, then it will still work as an opaque 
>>> type, but it won’t be a normal non-opaque value anymore on the typed side.  
>>> But the reason I asked about cast was because normally I can use cast with 
>>> a value that has an opaque type, but it’s wrapped on the typed side in this 
>>> opaque structure, then the contracts on the cast would see this opaque 
>>> structure instead of the actual value.  
>>> I’m thinking of an opaque typed representing a string with length 1, which 
>>> I can use as long as I use either (cast x String) or (assert x string?) 
>>> whenever I pass it to a string operation.  But if it were an opaque type, I 
>>> don’t think I could do that.  There could be a 1string->string function 
>>> that could take one of these 1strings and convert it to a string, but that 
>>> seems like it should be unnecessary, but made necessary by this opaque 
>>> structure thing.  
>>> And for “this isn’t really any different from casting mutable data types,” 
>>> look at this:
>>> #lang typed/racket
>>> (: b : (Boxof Number))
>>> (define b (box 1))
>>> (set-box! (cast b (Boxof (U Number String))) "I am a string")
>>> (ann (unbox b) Number) ;"I am a string” ; not a contract error
>>>>> On Jan 29, 2015, at 20:20, Alexander D. Knauth < 
>>>>> <>> wrote:
>>>>> Furthermore, even if the wrappers were shared between functions, untyped 
>>>>> code would recieved wrapped values, which would render them quite useless.
>>>>> If it’s not an opaque type, but something like a list, then this works, 
>>>>> and the untyped code receiving wrapped values isn’t a problem here:
>>>>> #lang typed/racket
>>>>> ; make Posn parametric
>>>>> (define-type (Posn A) (List A A))
>>>>> (provide Posn)
>>>>> (require/typed/provide
>>>>>  "untyped.rkt"
>>>>>  [make-posn (All (A) A A -> (Posn A))]
>>>>>  [posn-x (All (A) (Posn A) -> A)]
>>>>>  [posn-y (All (A) (Posn A) -> A)]
>>>>>  [real-posn? [(Posn Any) -> Boolean]])
>>>>> > (define p (make-posn 1 2))
>>>>> (make-posn #<A6> #<A6>) ; a printf that I put in make-posn from 
>>>>> “untyped.rkt"
>>>>> > p
>>>>> - : (Listof Positive-Byte) [more precisely: (List Positive-Byte 
>>>>> Positive-Byte)]
>>>>> '(1 2) ; unwrapped
>>>>> > (posn-x p)
>>>>> - : Integer [more precisely: Positive-Byte]
>>>>> 1
>>>>> > (posn-y p)
>>>>> - : Integer [more precisely: Positive-Byte]
>>>>> 2
>>>>> > (real-posn? p)
>>>>> - : Boolean
>>>>> #t
>>>>> Even though for a short time it's wrapped, it’s unwrapped as soon as 
>>>>> make-posn returns, and then after that if it flows into untyped code 
>>>>> again it’s not wrapped and functions like real-posn? work fine.  
>>>>> But the problem is that if it’s an opaque type then it can’t unwrap it 
>>>>> once the value is returned from make-posn.
>>>>> And I don’t think parametric opaque types could solve this unless all 
>>>>> posns themselves were wrapped with an opaque struct on the typed side, 
>>>>> which I guess does make sense now that I think about it.  But then you 
>>>>> couldn’t do any operations on it except those that you use import with 
>>>>> require/typed, right?  Or not?  And what happens if you use cast on one 
>>>>> of these things?
>>>>> On Jan 29, 2015, at 9:25 PM, Alexis King < 
>>>>> <>> wrote:
>>>>>> I recently ran into a problem in which opaque types (types imported from 
>>>>>> untyped code) cannot by parameterized by Typed Racket. I initially 
>>>>>> encountered this problem in my attempt to port 2htdp/image to TR 
>>>>>> <>.
>>>>>> After some further consideration, I’m interested in adding support to 
>>>>>> make something like this possible, which would certainly have additional 
>>>>>> benefits beyond this specific use-case. I’ve outlined my proposal here:
>>>>>> <>
>>>>>> Any feedback, suggestions, or advice would be appreciated, especially 
>>>>>> from those who are familiar with Typed Racket’s internals.
>>>>>> Thank you,
>>>>>> Alexis
>>>>>> _________________________
>>>>>>  Racket Developers list:
>>>>>> <>

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