Typed Racket won't generate type-dispatching code for you
automatically. However, if you write such code the same way you'd
write it in regular Racket, Typed Racket will be ok with that. For
example,
#lang typed/racket
(: id : (case-> (One One -> Number)
((Immutable-Vectorof One) (Immutable-Vectorof One) ->
(Vectorof Number))))
(define id (lambda (x y)
(if (number? x)
(+ x y)
(for/vector : (Vectorof Number) ([i x] [j y]) (+ i j)))))
(id 1 1)
(id #(1 1) #(1 1))
On Mon, Oct 14, 2019 at 9:13 AM Raoul Schorer <raoul.scho...@gmail.com> wrote:
>
> Hi,
>
> I am trying to write an APL compiler (a typed J implementation) with a typed
> racket backend. APLs have first-class functions that apply to both scalars
> and arrays, so there are 2 cases for functions of 2 arguments:
>
> the array -- array case
> the scalar -- scalar case
>
> Scalar -- array and array -- scalar cases get their scalar argument lifted to
> arrays, and are therefore equivalent to the array -- array case.
> Now, with an interpreter if I assign a function to an identifier, it can use
> just textual replacement to evaluate the function glyph in the runtime
> context (that's the way J does it). For example:
>
> id =: + ;;assign the 'add' function to 'id'
>
> 1 id 1 ;;evaluate to '2' using the scalar context on replacement with
> '+'
>
> 1 1 id 1 1 ;;evaluates to '2 2' using the array context on replacement
> with '+'
>
>
> But at compile-time I can't use that trick, can I? So I end up with something
> like:
> #lang typed/racket
>
> (define id (case-lambda
> [([x : One] [y : One]) (+ y)]
> [([x : (Immutable-Vectorof One)] [y : (Immutable-Vectorof One)])
> (for/vector ([i x] [j y]) (+ i j))]))
>
> (id 1 1)
> (id #(1 1) #(1 1))
>
> Which fails although the case-lambda is actually legal typed racket, because
> the second clause is considered dead code and one cannot dispatch on type.
> In my (hobbyist) understanding, the solution seems to require multiple
> dispatch? Or is there another way to do that in typed racket?
>
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