[racket] typed racket, filters, and polymorphism

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 <spencerflorence at 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 <alexander at 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 <samth at 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 
> > <alexander at 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 <alexander at 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 <samth at 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 
> >> <alexander at 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 
> >> 
> >> 
> >> 
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> >> 
> >> 
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> >> 
> >> 
> 
> 
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