Re: TR issue with the type of expt

Matthew Eric Bassett <[email protected]> Tue, 27 Sep 2016 14:36:43 +0100

Newsgroups	gmane.comp.lang.racket.user,gmane.lisp.scheme.plt
Message-ID	<[email protected]>

I do not believe this is an oversight.  The function expt has type
signature (-> Number Number Number).  Mathematically, yes, a^b is a real
when a and b are reals.  But the implementation of expt is not aware of
it (it is, in principle, possible to do case-by-case types for a
function, but expt does not appear to take advantage of this).

Because of this, (expt a b) is a Number, but your function definition
requires it to be the more restrictive Real.  Since you know that a and
b are reals, you could probably get away with a cast to Real.

On 09/27/2016 02:23 PM, Tim Brown wrote:
> The following snippet:
> 
> --------------------------
> #lang typed/racket
> (: ** [Real Real -> Real])
> (define (** a b)
>   (expt a b))
> --------------------------
> 
> Throws:
> unsaved editor:5:2: Type Checker: type mismatch expected: Real given: Number in: (expt a b)
>   #(67 10)
> 
> 
> I believe (because I can’t think of a counterexample) that
> (expt Real Real) is never Complex; and therefore expt can be of type
> (Real Real -> Real).
> 
> First off, is the statement above true?
> 
> Is there a technical reason why my example HAS TO fall back to
> (Number Number -> Number)? Or is this simply an oversight?
> 
> Regards,
> 
> Tim
> 
> Version: 6.6.0.4--2016-09-08(-/f) [3m].
> 

-- 
Matthew Eric Bassett | http://mebassett.info

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