Re: conditional bindings

[Jamison Hope <jrh@theptrgroup.com>]

On Dec 9, 2014, at 12:48 AM, Per Bothner <per@bothner.com> wrote:

> The syntax (? NAME::TYPE VALUE) can be used in conditional forms:
> if, and, cond, etc.  First VALUE is evaluated.  Then if the result
> is an compatible with TYPE, then NAME is bound to the result
> (possibly after conversion).  The result is #t, but with the "side effect"
> that NAME is lexically bound in "following forms".  If VALUE doesn't
> match TYPE then the ?-form is false.

Is the result actually #t, or some unspecified true value?  Is something
like this supposed to work?

#|kawa:1|# (define (f x) (cond ((? r ::real x) => (lambda (y) y)) (else #f)))
/dev/stdin:1:22: '?' is only allowed in a conditional e.g. 'if' or 'and'


Types backed by Java primitives seem to be troublesome.  I get VerifyErrors
("Bad type on operand stack") with the following:

(define (f x) (if (? b ::boolean x) b #f))

(define (f x) (if (? c ::character x) c #\?))


What types of implicit conversions are to be expected?  I find the
difference between these to be slightly unintuitive.

#|kawa:5|# (map (lambda (x) (if (? i ::int x) i -1)) '(3 "hello" 3.5 #\?))
(3 -1 3 -1)
#|kawa:6|# (map (lambda (x) (if (? i ::integer x) i -1)) '(3 "hello" 3.5 #\?))
(3 -1 -1 -1)

Note that 3.5 becomes int 3, but not integer 3.


#|kawa:7|# (map (lambda (x) (if (? s ::string x) s "N/A")) '(3 "hello" 3.5 #\?))
(N/A hello N/A N/A)

but

#|kawa:8|# (map (lambda (x) (if (? s ::String x) s "N/A")) '(3 "hello" 3.5 #\?))
Exception in thread "main" java.lang.NoClassDefFoundError: String



--
Jamison Hope
The PTR Group
www.theptrgroup.com