dorai@titan.rice.edu (Dorai Sitaram) (10/21/88)
Since there's been some talk of case-lambda, thought I'd say something
about a simple use of this syntax to get a nice implementation of
interpreter environments. The definition of the initial environment
(which doesn't associate any identifier with any value) goes as
follows:
(define init-env
(letrec ([extend (lambda (r x v)
(rec r1
(case-lambda
[(y) (if (eq? y x) v (r y))]
[(y w) (extend r1 y w)])))])
(rec r
(case-lambda
[(x) (error "unbound ~a" x)]
[(x v) (extend r x v)]))))
No other function is required! When called with a single argument (an
identifier), the environment yields the value of the identifier; when
called with two arguments (an identifier and a value) a new
environment is produced which is the extension of the original one
associating the new identifier with the new value.
One attraction (fatal? |-]) with this is that the use of the
environment closely mimics denotational semantics notation, viz.,
r[x] ;for looking up identifier x in environment r; and
r[x/v] ;for extending environment r to associate x with v.
--dorai