Re: Lazy evaluation

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 19 Sep 2014 09:51:21 -0400, Peter Chapin wrote:

> On 2014-09-18 15:15, Victor Porton wrote:
> 
>> Can we incorporate into Ada some kind of lazy evaluation (as in Haskell), 
>> without breaking backward compatibility?
>> 
>> If yes, would this be much useful?
> 
> Speaking in generalities lazy evaluation tends to make it difficult to
> reason about the performance characteristics of a program. It is my
> understand this is sometimes a problem for Haskell programs that have
> significant performance requirements. Thus it appears that lazy
> evaluation would be an anathema as far as Ada is concerned.

Consider the problem of evaluation order:

   X : String := String'Input (File) & String'Input (File); -- Ill defined!

Now, let "&" be lazy in the right argument:

   function "&" (Left : String; Right : lazy String) return String;

In my opinion, it is not laziness which causes problems, but more the
closures. If closures were limited in some reasonable (from Ada's POV) way,
then laziness would be not an issue.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de