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Article contents:== Outdated egg! This is an egg for CHICKEN 3, the unsupported old release. You're almost certainly looking for [[/eggref/4/varsubst|the CHICKEN 4 version of this egg]], if it exists. If it does not exist, there may be equivalent functionality provided by another egg; have a look at the [[https://wiki.call-cc.org/chicken-projects/egg-index-4.html|egg index]]. Otherwise, please consider porting this egg to the current version of CHICKEN. [[tags: egg]] == varsubst === Introduction The {{varsubst}} library provides support for variable substition in user-defined languages. It contains a collection of routines for managing substitution environments and a generalized substitution procedure. === Library procedures ==== {{subst?:: OBJECT -> BOOL}} A predicate that returns true if the given argument is a substitution environment. ==== {{subst-empty:: () -> SUBST}} Returns an empty substitution environment. ==== {{subst-empty?:: SUBST -> BOOL}} Returns true if the given argument is an empty substitution environment, false otherwise. ==== {{subst-includes?:: K * SUBST -> BOOL}} Returns true if the given symbol {{K}} is contained in the given substitution environment, false otherwise. ==== {{subst-lookup:: K * SUBST -> TERM}} Looks up symbol {{K}} in the given substitution environment, and returns the term associated with it. ==== {{subst-extend:: K * V * SUBST -> SUBST}} Adds the binding {{K * V}} to the given substitution environment, and returns the resulting substitution environment. ==== {{subst-map:: PROC * SUBST -> SUBST}} For each binding {{K * V}} in the given substitution environment, applies procedure {{PROC}} to {{V}} and adds the new binding {{K * (PROC V)}} to the environment. Returns the resulting substitution environment. ==== {{subst-driver:: VAR? * BIND? * VAR-PROC * BIND-PROC * SUBST-PROC [* PREFIX] -> (LAMBDA T SUBST)}} Generalized substitution procedure. Predicates {{VAR?}} and {{BIND?}} are used to determine if a given term is a variable or a binding, respectively. Procedure {{VAR-PROC}} takes a symbol and creates a variable term. Procedure {{SUBST-PROC}} substitues variables in the terms of the user-defined language. Optional variable {{PREFIX}} specifies the prefix for fresh variable names. === Example <enscript highlight="scheme"> (use syntax-case matchable varsubst) (define (subst-term t subst k) (match t (('if c t e) `(if ,(k c subst) ,(k t subst) ,(k e subst))) (('let bs e) (k `(let ,(map (lambda (b) `(,(car b) ,(k (cadr b) subst))) bs) ,e) subst)) ((f . es) (cons (k f subst) (map (lambda (e) (k e subst)) es))) ((? atom? ) t))) (define (binding? t) (and (list? t) (eq? 'let (car t)) (cdr t))) (define (bind ks vs e) `(let ,(zip ks vs) ,e)) (define driver (subst-driver (lambda (x) (and (symbol? x) x)) binding? identity bind subst-term)) (print (driver `(let ((a 1) (b 2)) (+ a (let ((a (+ b 5))) (+ a b)))) subst-empty)) </enscript> === Authors Ivan Raikov === Version ; 1.1 : Using string comparison because equal? on the same symbols sometimes fails. ; 1.0 : Initial version === License Copyright 2008 Ivan Raikov and the Okinawa Institute of Science and Technology. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. A full copy of the GPL license can be found at <http://www.gnu.org/licenses/>. Description of your changes:

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