Have you considered this:
#lang typed/racket
(define-type Myopt (U String Boolean Integer))
(define-type OptList (Listof (Pairof Symbol Myopt)))
(: olist OptList)
(define olist
(list '(dir . ".")
'(verbose . #t)
'(size . 12)))
; This is a fake type, where the "filter" flows into the result
; (: optval (-> Symbol OptList (-> Any Boolean : Type) Type))
(define-syntax-rule
(optval v ol type?)
(let ([val (assoc v ol)])
(if (and val (type? (cdr val)))
(cdr val)
(error "Unexpected option encountered"))))
(: process (-> OptList Void))
(define (process ol)
(define s (optval 'size ol integer?))
(if (< s 0)
(displayln "wrong value")
(displayln "ok")))
(process olist)
On Nov 30, 2014, at 10:38 AM, Manfred Lotz wrote:
> On Sun, 30 Nov 2014 09:25:57 -0500
> Sam Tobin-Hochstadt <sa...@cs.indiana.edu>
> wrote:
>
>> I think this is exactly what you need. You could write (< (assert s
>> integer?) 0) or (and s (integer? s) (< s 0)), which mostly do the same
>> thing.
>>
>> But the fundamental issue is that this program is well-typed:
>>
>> (process (list (cons 'size "hi")))
>>
>> since that's a perfectly valid `OptList`. If you didn't have a dynamic
>> check in `process`, you'd end up with (< "hi" 0) which is wrong.
>>
>> If that's not what you want, what should the restrictions on
>> `OptList` be?
>>
>
> In my real program I have an OptList with 15 options or so, all known
> at compile time. Don't know if this helps to make it simpler.
>
> Actually, I had hoped to be able to put the cast into the optval
> procedure but I wasn't successful in doing this.
>
> Or factoring it out in some way so that I do not need to write the cast
> or assert? each time I use an option.
>
> I played a bit and found this:
>
>
> (define-syntax-rule (optval v ol)
> (let ([vv (_optval v ol)])
> (case v
> ['size (cast vv Integer)]
> ['dir (cast vv String)]
> ['verbose (cast vv Boolean)]
> [else (error "bad option")])))
>
> by renaming previous optval to _optval. This works fine but I have two
> questions:
>
> 1. You offered assert? as an alternative. Is this preferable in
> comparison to cast?
>
> 2. My solution seems to be bad because the decision about the type
> happens at runtime.
>
> It would be better I could redefine optval in a way that this will be
> done at compile time as I always call optval with an explicit option
> symbol.
>
> Is this possible? I'm afraid y knowledge about define-syntax-rule isn't
> really good.
>
>
> Thanks.
>
>
> --
> Manfred
>
>
>
>
>
>> Sam
>>
>>
>>
>> On Sun, Nov 30, 2014 at 8:46 AM, Manfred Lotz
>> <manfred.l...@arcor.de> wrote:
>>> Well, I should have added that I've found this:
>>>
>>> (: process (-> OptList Void))
>>> (define (process ol)
>>> (let ([s (optval 'size ol)])
>>> (if (and s (< (cast s Integer) 0))
>>> (displayln "wrong value")
>>> (displayln "ok")))
>>> )
>>>
>>>
>>> but I didn't find this really nice. Actually, I was hoping for
>>> something more concise.
>>>
>>> --
>>> Manfred
>>>
>>>
>>> On Sun, 30 Nov 2014 10:13:48 +0100
>>> Manfred Lotz <manfred.l...@arcor.de> wrote:
>>>
>>>> Hi there,
>>>> I've got another problem with typed racket.
>>>>
>>>> Let us say I have an assoc list with options for a program. These
>>>> options could be of type String, Boolean, or Integer.
>>>>
>>>> Now in my program I want to check certain options but I don't know
>>>> how to do without Typed Racket screaming at me.
>>>>
>>>> Here a minimal example:
>>>>
>>>> #lang typed/racket
>>>>
>>>> (define-type Myopt (U String Boolean Integer))
>>>> (define-type OptList (Listof (Pairof Symbol Myopt)))
>>>>
>>>>
>>>> (: olist OptList)
>>>> (define olist (list '(dir . ".")
>>>> '(verbose . #t)
>>>> '(size . 12)))
>>>>
>>>> (: optval (-> Symbol OptList Myopt))
>>>> (define (optval v ol)
>>>> (let ([val (assoc v ol)])
>>>> (if val
>>>> (cdr val)
>>>> (error "Unexpected option encountered"))))
>>>>
>>>> (: process (-> OptList Void))
>>>> (define (process ol)
>>>> (if (< (optval 'size ol) 0)
>>>> (displayln "wrong value")
>>>> (displayln "ok")))
>>>>
>>>>
>>>> I get an error:
>>>> Type Checker: type mismatch
>>>> expected: Real
>>>> given: Myopt
>>>> in: (optval (quote size) ol)
>>>>
>>>>
>>>> How could I solve this?
>>>>
>>>>
>>>
>>>
>>>
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>
>
>
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