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#lang racket
(require "lib.rkt")
;; the untyped lambda calculus
(define (interpret expr [ctx #hash()])
(match expr
[val #:when (or (number? val) (string? val)) val]
[val #:when (symbol? val)
(with-handlers
([exn:fail? (λ (exn) val)]) ; note: unbound identifiers are supported
(interpret (dict-ref ctx val) ctx))]
[`(λ ,id ,body) `(λ ,id ,body)]
[`(,body ,arg)
(match (interpret body ctx)
[`(λ ,id ,body) (interpret body (dict-set ctx id (interpret arg ctx)))]
[expr `(,(interpret expr ctx) ,(interpret arg ctx))])]
[expr (err (format "unknown expression ~a" expr))]))
(require rackunit)
(check-equal? (interpret '(λ x x)) '(λ x x))
(check-equal? (interpret '((λ a a) (x y))) '(x y))
(check-equal? (interpret '((λ a (x y)) (λ z z))) '(x y))
(check-equal? (interpret '((λ a (a y)) x)) '(x y))
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