Path: utzoo!attcan!uunet!pyrdc!netsys!rutgers!ucla-cs!pierce
From: pierce@lanai.cs.ucla.edu
Newsgroups: comp.lang.scheme
Subject: case-lambda syntax in Chez Scheme
Message-ID: <16934@shemp.CS.UCLA.EDU>
Date: 18 Oct 88 19:04:12 GMT
Sender: news@CS.UCLA.EDU
Reply-To: pierce@CS.UCLA.EDU ()
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Organization: UCLA Computer Science Department
Lines: 37

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Some good person replied to my example in Re: Limitation with Lambda.
This person pointed out that the "case-lambda" syntax is not standard
Scheme.  My apologies, case-lambda is from Chez Scheme Version 2.0,
and I have found it so useful (I use it a *lot*) that I just forgot
that users of other Scheme systems might not know what it meant.

An excerpt from the Chez Scheme Version 2.0 release notes follows.  It's
a simple idea that not only makes code easier to write and to understand,
but it also runs faster than the old way.  Maybe Kent Dybvig could
be encouraged to post a little more info about case-lambda?

-- Brad Pierce

------------------------------------------------------------------------
(below Copyright Kent Dybvig)

Optional Arguments

    Procedures with a variable number of arguments could only be defined 
    in earlier versions using the "lambda dot" interface, which requires 
    allocation  of  a  list  to hold all but  the  required  parameters.  
    Version  2.0 supports a variant of "lambda",  called  "case-lambda", 
    that  allows  procedures with variable numbers of  arguments  to  be 
    defined efficiently.  "case-lambda" has the form:

         (case-lambda [idspec-a exp1-a exp2-a ...]
                      [idspec-b exp1-b exp2-b ...] ...)

    where the idspecs are normal "lambda" parameter lists.   In essence, 
    each  bracketted item (parens may be used in place of the  brackets) 
    represents  a different "lambda" expression;  which one is evaluated 
    depends upon the number of arguments.  If the number of arguments is 
    a correct number for the first idspec,  evaluation proceeds with the 
    first body within the bindings implied by the first idspec.  If not, 
    then  if the number of arguments is a correct number for the  second 
    idspec, evaluation proceeds with the second body, etc.