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Re: [Scheme-reports] practical matters - CSAN

 | Date: Mon, 29 Aug 2011 12:59:15 -0400
 | From: John Cowan <cowan@x>
 | 
 | Aubrey Jaffer scripsit:
 | 
 | > LET-VALUES, LET*-VALUES, and (srfi-8) RECEIVE expect the number
 | > of bindings to match the number of return values.  They can't be
 | > used if the number of return values is not known in advance.
 | 
 | In fact they can:
 | 
 |         (let-value (let (((x y . z) (some-mv-form))) <body>)
 | 
 | will bind the first two values of (some-mv-form) to x and y, and any
 | remaining values as a list to z.

Does r7rs-draft-3 allow this?  Even if it does, it is an error in your
example if fewer than 2 values are returned.

Scanf is used for reading non-uniform data.  Here is a snippet from
<http://cvs.savannah.gnu.org/viewvc/slib/slib/mkclrnam.scm?view=markup>
Procedures like this are tried in sequence until one matches the line
argument.

  (lambda (line)
    (case (sscanf line " %24[a-zA-Z0-9_ ] %d %d %d %e %e %e %s"
		  name r g b ri gi bi junk)
      ((7)
       (set! method-id 'm7)
       (list (check-match line (color:sRGB r g b) (floats->rgb ri gi bi))
	     (color-name:canonicalize name)))
      (else #f)))

The variables NAME R, G, B, RI, GI, BI, and JUNK are bound in the
containing procedure.  When there is no match, sscanf returns 0; for
partial matches sscanf returns between 1 and 6; reading 8 items means
that there was extra stuff in line.

 | > CASE-LAMBDA can be used if the desired behavior changes radically with
 | > different numbers of values; but often there is a lot of commonality
 | > in the procedure.  A shared internal definition can't be called
 | > because CASE-LAMBDA is the outer binding of the definition.  So either
 | > code must be replicated in each clause or each clause must call an
 | > external procedure with all the arguments.
 | 
 | A case-lambda can tail-call itself:
 | 
 |         (define x (case-lambda
 |           ((a b) ...)
 |           ((a) (x a 0))))
 | 
 | A compiler can detect this case and optimize it.  In addition, the
 | case-lambda can be wrapped in a letrec containing internal definitions
 | visible to the case-lambda.

The example use of scanf above treats its first matched item quite
differently from subsequent items.  The recursive CASE-LAMBDA is no
help to this application.

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