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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.
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.
(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))
- Using string comparison because equal? on the same symbols sometimes fails.
- Initial version
Copyright 2008 Ivan Raikov and the Okinawa Institute of Science and Technology.
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