I tried this approach with Racket 6.5 and still get the type checker error when
trying to call the continuation after it has been set.
#lang typed/racket
(define-type EmptySet (U))
(: d-or-s (U False (-> Number EmptySet)))
(define d-or-s #f)
(: double-or-same (-> Number Number))
(define (double-or-same x)
(call/cc (lambda ({c : (-> Number EmptySet)})
(set! d-or-s c)
(+ x x))))
> (double-or-same 10)
- : Number
20
> (d-or-s 1)
. Type Checker: Cannot apply expression of type (U False (-> Number Nothing)),
since it is not a function type in: (d-or-s 1)
> d-or-s
- : (U False (-> Number Nothing))
#<continuation>
Not sure why the type checker is complaining that d-or-s is not a function type.
On Tuesday, August 30, 2016 at 2:02:18 AM UTC+5:30, Matthias Felleisen wrote:
> Continuations don’t return. In a set-oriented type system this means their
> result type is the empty set:
>
> #lang typed/racket
>
> (define-type EmptySet (U))
>
> (: d-or-s (U False (-> Number EmptySet)))
> (define d-or-s #f)
>
> (: double-or-same (-> Number Number))
> (define (double-or-same x)
> (call/cc (lambda ({c : (-> Number EmptySet)})
> (set! d-or-s c)
> (+ x x))))
>
>
> > On Aug 29, 2016, at 2:18 PM, Sourav Datta <soura.ja...@gmail.com> wrote:
> >
> > Hey everyone,
> >
> > I am a beginner in Racket and recently learned the basic concepts of
> > continuations. I like Racket's support of multiple types of continuations
> > as opposed one type in Scheme. Recently I also started learning about typed
> > Racket. My problem is, I am not sure how I can annotate a continuation in a
> > typed racket program. For example, when trying to annotate the below code:
> >
> > #lang racket
> >
> > (define d-or-s #f)
> >
> > (define (double-or-same x)
> > (call/cc (lambda (c)
> > (set! d-or-s c)
> > (+ x x))))
> >
> > in typed racket, I can see that double-or-same function can have type
> > (Integer -> Integer). But what should be the type of the variable do-or-s?
> > If it is Any, then it cannot be called like a function after the
> > continuation is set to it. So basically my question is how can we type
> > annotate different types of continuations starting from a simple one like
> > the above? Also a pointer to the relevant section in typed racket docs
> > would be great.
> >
> > Thanks!
> >
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