I just realized that typed racket seems to ignore the guard anyway, so I could do something like this and convince the type-checker that any value I want has any type that I want:
(struct (a) foo ([a : a]) #:guard (lambda (a _) "this value")) (: x : (foo 1)) (define x (foo 1)) > (foo-a x) - : Integer [more precisely: One] "this value” On Sep 28, 2014, at 10:41 PM, Benjamin Greenman <bl...@cornell.edu> wrote: > I was able to get something very similar to your example program to compile. > > (struct (a) foo ([a : a]) #:transparent > #:guard (lambda (a _) > (define a* a) ;; a* not "polluted" by occurrence typing > (unless (exact-integer? a) > (error 'foo "expected Integer, given ~v" a)) > a*)) ;; we return a value of type a, so the typechecker is > happy > > From here, you can safely cast a foo-a into an Integer. It's not type safe to > ann because we (deliberately) don't have the occurrence typing information, > but we the programmers can be sure that the casts will never fail. > > (cast (foo (ann 1 Any)) (foo Integer)) > > (foo 1) > > > > On Sun, Sep 28, 2014 at 10:29 PM, Matthias Felleisen <matth...@ccs.neu.edu> > wrote: > > You might be able to unwrap such things for a local context, but then you > need to wrap it again on the way out. That cost might become quite high if it > happens in a loop. Then again Racket programs rely on this a lot too (as do > many dynamically typed programs) so it might not be a cost that kills > performance completely. But do be aware of how others deal with this issue -- > Matthias > > > > > > > > On Sep 28, 2014, at 10:03 PM, Alexander D. Knauth wrote: > >> >> >> On Sep 28, 2014, at 8:25 PM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: >> >>> >>> Your message points out where gradual typing badly fails. >>> >>> Type systems combine two nearly orthogonal ideas: (1) proving theorems >>> about your program and (2) bridging the gap between abstract linguistic >>> constructs and concrete machine implementations. In a sense, the design of >>> a type system should always have the goal to eliminate as much run-time >>> overhead as possible. >>> >>> In this specific case, you have two aspects of dimensionality: dimension >>> per se (I am sure there is a word for it) and the chosen unit to represent >>> it. So if you know that a number denotes a distance, you can choose mm, m, >>> and km to represent it. If you want to represent an area, you choose mm^2, >>> m^2, km^2. Now if you want to compute the area of a rectangle, you write >>> >>> area-of-rectangle : Real [Distance in m] * Real [Distance in m] -> Real >>> [Area in m^2] >>> >>> If someone writes (area-of-rectangle 1 [mm] 1 [km]), there is nothing wrong >>> with it -- except that the type checker should insert a coercion from mm to >>> m and from km to m (multiplying/dividing by 1,000). That is, a type system >>> for this application ought to use the type elaboration process idea and >>> translate code as it goes. >>> >>> This is quite comparable to the idea that a machine-oriented type system >>> needs to insert a coercion from integer to real (for example) as Algol 60 >>> and its degraded successors do. >> >> What I had in mind was for the structs to be available at run-time, but that >> ideally the optimizer could take them out for the intermediate operations >> and put them back for the final result. >> >> And then if this value goes into untyped code, then it knows at run-time >> that it has the unit (u* millimeter kilometer), which is unit=? to >> square-meter. >> >> Is this sort of like what happens with flonum unboxing? >> >> >> >>> >>> Our gradual type system lacks some (intensiona) expressive power, which is >>> why you can't get close to this ideal. >>> >>> -- Matthias >>> >>> >>> >>> >>> >>> >>> >>> >>> On Sep 28, 2014, at 8:14 PM, Alexander D. Knauth wrote: >>> >>>> Yes I do want run-time residual of the dimensions, and yes I want a unit >>>> struct. >>>> >>>> Also if there’s run-time residual, then I can use predicates and >>>> occurrence typing, and it can also be used in untyped code with contracts >>>> instead of types. >>>> >>>> And also the type-checker would check if it is dimension-correct, but at >>>> run-time it would would still convert between different units of the same >>>> dimension, multiply and divide units, etc. >>>> >>>> On Sep 28, 2014, at 6:58 PM, Matthias Felleisen <matth...@ccs.neu.edu> >>>> wrote: >>>> >>>>> >>>>> If you want the type checker to ensure your program is unit-correct, I >>>>> assume you also want no run-time residual of the dimensions but that is >>>>> in conflict with wanting a structure because it imposes a run-time cost. >>>>> Are you sure you want a unit? -- Matthias >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> On Sep 28, 2014, at 1:37 PM, Alexander D. Knauth wrote: >>>>> >>>>>> No because I want the unit to be a struct that has a dimension field, >>>>>> not a symbol with various dimensions defined as unions of units. >>>>>> I want the unit to be based on the dimension, not the other way around, >>>>>> so that new units can be made that have the same dimension. >>>>>> >>>>>> I have something like the number+unit struct (I called it measure), but >>>>>> I’ll work on more that after I have the unit struct figured out. >>>>>> >>>>>> On Sep 28, 2014, at 12:13 PM, Spencer florence >>>>>> <spencerflore...@gmail.com> wrote: >>>>>> >>>>>>> would something like this work? >>>>>>> >>>>>>> #lang typed/racket >>>>>>> >>>>>>> (struct (U) number+unit ([amount : Real] [unit : U])) >>>>>>> >>>>>>> (define-type Weight-Unit (U 'kg 'g 'mg 'μg)) >>>>>>> (define-type Weight (number+unit Weight-Unit)) >>>>>>> (define-predicate weight? Weight) >>>>>>> >>>>>>> (: make-weight : Real Weight-Unit -> Weight) >>>>>>> (define (make-weight n u) >>>>>>> (number+unit n u)) >>>>>>> >>>>>>> (: +/weight : Weight Weight -> Weight) >>>>>>> ;; something something needs unit conversion >>>>>>> (define (+/weight w1 w2) >>>>>>> (number+unit (+ (number+unit-amount w1) >>>>>>> (number+unit-amount w1)) >>>>>>> (number+unit-unit w1))) >>>>>>> >>>>>>> (+/weight (make-weight 1 'kg) (make-weight 1 'kg)) >>>>>>> >>>>>>> >>>>>>> >>>>>>> On Sun, Sep 28, 2014 at 11:03 AM, Alexander D. Knauth >>>>>>> <alexan...@knauth.org> wrote: >>>>>>> >>>>>>> Because the struct is representing a unit (kilograms, meters, seconds, >>>>>>> etc.), and a unit has a dimension (mass, length, time, etc.) and I want >>>>>>> the type-checker to be able to know what the dimension of a unit is so >>>>>>> that the types of functions can specify the dimension that something >>>>>>> should have. >>>>>>> The real solution to this would probably be bounded polymorphism, but I >>>>>>> was wondering if there was some other way to do it with occurrence >>>>>>> typing in the guard or something like that. >>>>>>> >>>>>>> On Sep 28, 2014, at 11:48 AM, Sam Tobin-Hochstadt >>>>>>> <sa...@cs.indiana.edu> wrote: >>>>>>> >>>>>>> > Why not do this with the type, instead of making this polymorphic? >>>>>>> > >>>>>>> > Sam >>>>>>> > >>>>>>> > On Fri, Sep 26, 2014 at 7:35 PM, Alexander D. Knauth >>>>>>> > <alexan...@knauth.org> wrote: >>>>>>> >> Is it possible to have a struct that does certain things according >>>>>>> >> to the >>>>>>> >> guard? >>>>>>> >> #lang typed/racket >>>>>>> >> >>>>>>> >> (struct (a) foo ([a : a]) #:transparent >>>>>>> >> #:guard (lambda (a _) >>>>>>> >> (unless (exact-integer? a) >>>>>>> >> (error 'foo "expected Integer, given ~v" a)) >>>>>>> >> a)) >>>>>>> >> >>>>>>> >> (ann (foo (ann 1 Any)) (foo Integer)) >>>>>>> >> >>>>>>> >> (: x : (foo Any)) >>>>>>> >> (define x (foo 1)) >>>>>>> >> >>>>>>> >> (ann (foo-a x) Integer) >>>>>>> >> >>>>>>> >> ;. Type Checker: Polymorphic function `foo1' could not be applied to >>>>>>> >> arguments: >>>>>>> >> ;Argument 1: >>>>>>> >> ; Expected: a >>>>>>> >> ; Given: Any >>>>>>> >> ; >>>>>>> >> ;Result type: (foo a) >>>>>>> >> ;Expected result: (foo Integer) >>>>>>> >> ; in: (foo (ann 1 Any)) >>>>>>> >> ;. Type Checker: Polymorphic function `foo-a' could not be applied >>>>>>> >> to >>>>>>> >> arguments: >>>>>>> >> ;Argument 1: >>>>>>> >> ; Expected: (foo a) >>>>>>> >> ; Given: (foo Any) >>>>>>> >> ; >>>>>>> >> ;Result type: (a : ....) >>>>>>> >> ;Expected result: Integer >>>>>>> >> ; in: (foo-a x) >>>>>>> >> >>>>>>> >> On Sep 25, 2014, at 9:42 PM, Alexander D. Knauth >>>>>>> >> <alexan...@knauth.org> >>>>>>> >> wrote: >>>>>>> >> >>>>>>> >> What I’m trying to accomplish is something more like this: >>>>>>> >> #lang typed/racket >>>>>>> >> >>>>>>> >> (require "dimensions.rkt") >>>>>>> >> >>>>>>> >> (struct (d) unit ([name : Any] [scalar : Positive-Real] [dimension : >>>>>>> >> d]) >>>>>>> >> #:transparent >>>>>>> >> #:guard (lambda (name scalar dimension _) >>>>>>> >> (unless (dimension? dimension) >>>>>>> >> (error 'unit "expected Dimension, given ~v" dimension)) >>>>>>> >> (values name scalar dimension))) >>>>>>> >> >>>>>>> >> (define-type (Unitof d) (unit d)) >>>>>>> >> >>>>>>> >> (define-type Unit (Unitof Dimension)) >>>>>>> >> >>>>>>> >> (define Unit? (make-predicate Unit)) >>>>>>> >> >>>>>>> >> (define-type Unitish >>>>>>> >> (U (Unitof Any) >>>>>>> >> Dimension >>>>>>> >> Positive-Real)) >>>>>>> >> >>>>>>> >> (: ->unit : (All (d) (case-> [(Unitof d) -> (Unitof d)] >>>>>>> >> [Unitish -> Unit]))) >>>>>>> >> (define (->unit u) >>>>>>> >> (cond [(unit? u) >>>>>>> >> (unless (Unit? u) ; this should never happen anyway because of the >>>>>>> >> guard >>>>>>> >> (error '->unit "expected (Unitof Dimension), given ~v" u)) >>>>>>> >> u] >>>>>>> >> [(dimension? u) (unit u 1 u)] >>>>>>> >> [(positive-real? u) (unit u u dimensionless-dimension)])) >>>>>>> >> >>>>>>> >> >>>>>>> >> On Sep 25, 2014, at 6:19 PM, Sam Tobin-Hochstadt >>>>>>> >> <sa...@cs.indiana.edu> >>>>>>> >> wrote: >>>>>>> >> >>>>>>> >> No, I don't think you can do this. Can you say more about what >>>>>>> >> you're >>>>>>> >> trying to accomplish? >>>>>>> >> >>>>>>> >> Sam >>>>>>> >> >>>>>>> >> On Thu, Sep 25, 2014 at 6:15 PM, Alexander D. Knauth >>>>>>> >> <alexan...@knauth.org> wrote: >>>>>>> >> >>>>>>> >> Do any of you have any advice for getting a function like this to >>>>>>> >> type-check? >>>>>>> >> #lang typed/racket >>>>>>> >> >>>>>>> >> (: check-int : (All (a) (case-> [a -> a] >>>>>>> >> [Any -> Integer]))) >>>>>>> >> (define (check-int int) >>>>>>> >> (unless (exact-integer? int) >>>>>>> >> (error 'check-int "expected Integer, given ~v" int)) >>>>>>> >> int) >>>>>>> >> >>>>>>> >> ;. Type Checker: type mismatch >>>>>>> >> ; expected: a >>>>>>> >> ; given: Integer in: int >>>>>>> >> >>>>>>> >> >>>>>>> >> >>>>>>> >> ____________________ >>>>>>> >> Racket Users list: >>>>>>> >> http://lists.racket-lang.org/users >>>>>>> >> >>>>>>> >> >>>>>>> >> ____________________ >>>>>>> >> Racket Users list: >>>>>>> >> http://lists.racket-lang.org/users >>>>>>> >> >>>>>>> >> >>>>>>> >>>>>>> >>>>>>> ____________________ >>>>>>> Racket Users list: >>>>>>> http://lists.racket-lang.org/users >>>>>>> >>>>>>> >>>>>> >>>>>> ____________________ >>>>>> Racket Users list: >>>>>> http://lists.racket-lang.org/users >>>>> >>>> >>> >> > >
____________________ Racket Users list: http://lists.racket-lang.org/users
