[plt-scheme] Macro rule selection

Which language?

In 4.1.4 your program plus > a at the repl produces 36 in Module and  
Pretty Big. (You use [ ] so it's not R5.)

-- Matthias



On Jan 30, 2009, at 12:53 PM, James Coglan wrote:

> Hi all,
>
> Just ran into some puzzling macro behaviour that I was hoping  
> someone could explain. I have the following code:
>
> ; Modifies a variable by applying a function to said variable
>
> (define-syntax ! (syntax-rules ()
>   [(! x (fn arg ...))
>     (set! x (fn x arg ...))]
>   [(! x fn)
>     (set! x (fn x))]))
>
> ; for example...
>
> (define-syntax square! (syntax-rules ()
>   [(square! x)
>     (! x (* x))]))
>
> (define a 6)
> (square! a)
> Turns out that when I run this, 'a' turns out as 1296, or 36  
> squared. My understanding of macro rule selection was that the  
> first matching rule is chosen and expanded, and subsequent rules  
> are ignored. In this case, the more specific (! x (fn arg ...))  
> rule appears first so I would assume that would be used to match (!  
> x (* x)) in the (square!) macro. It seems the (! x fn) rule (which  
> would also match but has lower priority) is interfering, since if I  
> remove this rule all is well. I want the second rule so you can  
> simply pass a single argument function without surrounding it in  
> parens.
>
> Any ideas what's going on here?
>
> -- 
> James Coglan
> http://github.com/jcoglan
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