From 28f43d4282638de846735da0d0e68cace1029220 Mon Sep 17 00:00:00 2001
From: Enrico Lumetti <enrico.lumetti@gmail.com>
Date: Thu, 11 Jun 2020 02:53:57 +0200
Subject: [PATCH] Add R1 language interpretation

---
 c2.scm | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 c2.scm

diff --git a/c2.scm b/c2.scm
new file mode 100644
index 0000000..0f6ce15
--- /dev/null
+++ b/c2.scm
@@ -0,0 +1,63 @@
+#lang racket
+
+(require racket/fixnum)
+
+
+(define (interp-R1 env)
+  (lambda (p)
+    (match p
+       [`(program ,data ,exp) ((interp-exp env) exp)])))
+
+(define (interp-exp env)
+  (lambda (sexp)
+    (match sexp
+      [(? fixnum?) sexp]
+      [(? symbol?) (eval-symbol env sexp)]
+      [`(read) (read-fixnum)]
+      [`(- ,e) (fx- 0 ((interp-exp env) e))]
+      [`(+ ,e1 ,e2) (fx+ ((interp-exp env) e1) ((interp-exp env) e2))]
+      [`(let ([,var ,rexp]) ,body)
+        (let ([value ((interp-exp env) rexp)])
+          ((interp-exp (add-binding env (list var value))) body))])))
+
+(define (read-fixnum)
+  (let ([r (read)])
+    (cond
+       [(fixnum? r) r]
+       [else (error "invalid fixnum in input: " r)])))
+
+(define (env-head-symbol env)
+  (car (car env)))
+
+(define (env-head-value env)
+  (cadr (car env)))
+
+(define (add-binding env binding)
+  (cons binding env))
+    
+(define (eval-symbol env s)
+  (cond
+    [(equal? env `())                 (error "Symbol " s " not found")]
+    [(equal? (env-head-symbol env) s) (env-head-value env)]
+    [else                             (eval-symbol (cdr env) s)]))
+  
+
+(define (test-eq a b)
+   (if (equal? a b) #t (error "assert failed: " a " != " b)))
+
+(define (ppexp exp) (begin (print (exp)) (newline)))
+
+(test-eq (env-head-value `((a 1))) 1)
+(test-eq (env-head-symbol `((a 1))) `a)
+(test-eq (eval-symbol `((a 1)) `a) 1)
+(test-eq
+  (let ([env `((a 1) (b 2))])
+    ((interp-exp env) `(+ a (- b))))
+  -1)
+(test-eq
+  ((interp-exp `()) `(let ([a (+ 1 2)]) (+ a 3)))
+  6)
+
+(test-eq
+  ((interp-R1 `()) `(program `() (let ([a (+ 1 2)]) (+ a 3))))
+  6)