diff options
Diffstat (limited to 'lc.rkt')
| -rw-r--r-- | lc.rkt | 31 |
1 files changed, 18 insertions, 13 deletions
@@ -1,21 +1,26 @@ -#lang racket +#lang racket ; note: do NOT use racket/base (require "lib.rkt") -;; the untyped lambda calculus +;; The Untyped Lambda Calculus -(define (interpret expr [ctx #hash()]) +; note: default arguments MUST all be at the end +(define (interpret expr [ctx '()]) (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)] + [`(λ ,id ,body) + `(λ ,id ,(interpret body ctx))] + [`((λ ,id ,body) ,arg) + (interpret body (dict-set ctx id arg))] + [`(,id ,arg) #:when (dict-has-key? ctx id) + (interpret `(,(interpret (dict-ref ctx id) ctx) ,arg))] [`(,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))])) + (let ([reduced (interpret body ctx)]) + (if (equal? body reduced) + `(,reduced ,(interpret arg ctx)) + (interpret `(,reduced ,arg) ctx)))] + [id #:when (dict-has-key? ctx id) + (interpret (dict-ref ctx id) ctx)] + [val #:when (symbol? val) val] + [_ (err (format "unknown expression ~a" expr))])) (require rackunit) (check-equal? (interpret '(λ x x)) '(λ x x)) |