Re: [Scheme-reports] Draft 3 Comments: Chapter 4

[John Cowan <cowan@x>]

Denis Washington scripsit:

> This is a nice trick indeed, but does suffer from some subtle  
> complications. Consider the usage example on KeywordArgumentsArcfide:
>
>   (define/optional (blah a b c (x: 0) (y: 7))
>     (list a b c x: y:))
>
> What if you don't want to call "list", but another procedure which  
> itself accepts "x:" and "y:" keyword arguments? 

Good point.  This is a consequence of a basic Scheme limitation: once
a binding has been shadowed by a more local binding, there is no way
to retrieve the outer binding.  A better approach for define/optional
might be to have it take triples rather than doubles: a local variable,
a keyword (always globally bound), and a default value.


-- 
May the hair on your toes never fall out!       John Cowan
        --Thorin Oakenshield (to Bilbo)         cowan@x

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