Re: [Scheme-reports] editorial awkwardness for syntax-rules pattern-matching

[Per Bothner <per@x>]

On 09/03/2014 04:40 PM, John Cowan wrote:
> Per Bothner scripsit:
>
>> Much clearer to write:
>>
>>     P is of the form (P_1 . . . P_k P_e ellipsis P_k+1 ... P_k+l)
>>     where E is a proper list of n elements, the first
>>     k of which match P_1 through P_k , respectively, whose
>>     next n−k-l elements each match P_e, whose remaining
>>     l elements match P_k+1 through P_k+l
>
> I don't see where the aditional clarity comes in;

Well, in my opinion 'P is of the form (P_1 . . . P_k P_e ellipsis P_m+1 ... P_n)'
is somewhat nonsensical.  Where do m and n come from?  n is the length of E, but m
is complicated to describe as it depends on both n and the number of patterns
in P following the ellipsis.  This seems a rather unusual way to enumerate items in
a list.

Using l (or m) for the number of sub-patterns following the ellipsis is much
easier to understand: l is just the number of sub-patterns following the ellipsis,
and this is obvious from the form P_k+1 ... P_k+l.

> also, having both 1 and l in a formula is asking for trouble.

That occurred to me.  Using m in place of l is one option.

Even better if we're willing to use other letters beside P for sub-patterns:

   P is of the form (P_1 . . . P_k Q ellipsis R_1 ... R_m)
   where E is a proper list of n elements, the first
   k of which match P_1 through P_k , respectively, whose
   next n−k-m elements each match Q, whose remaining
   m elements match R_1 through R_m

-- 
	--Per Bothner
per@x   http://per.bothner.com/

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