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[Chicken-janitors] #505: permit user-defined read-syntax to restart the

From: Chicken Trac
Subject: [Chicken-janitors] #505: permit user-defined read-syntax to restart the reader
Date: Thu, 17 Feb 2011 21:23:26 -0000

#505: permit user-defined read-syntax to restart the reader
 Reporter:  zbigniew               |       Owner:  zbigniew
     Type:  enhancement            |      Status:  new     
 Priority:  minor                  |   Milestone:  4.7.0   
Component:  core libraries         |     Version:  4.6.x   
 Keywords:  don't fear the reader  |  
 Currently, user-defined read-syntax must return a value to the reader; it
 cannot simply discard input completely.  A proposed solution, derived from
 CL reader macros (see CLHS 2.2 no.4) allows a zero-value {{{(values)}}}
 return to reinvoke the reader in tail position, discarding any consumed
 input forever.

 This change permits user implementation of line comments, block comments,
 s-expr comments, read-time feature testing, read-time evaluation, and
 more; all with the existing user API.

 Though a zero-value return is admittedly ugly and inefficient -- one noted
 luminary has rightfully referred to it as a "festering CL pustule on the
 buttocks of Scheme" -- it has the advantages of being compatible with the
 current API and being virtually impossible to trigger accidentally.

Ticket URL: <>
Chicken Scheme <>
Chicken Scheme is a compiler for the Scheme programming language.

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