From 4cad341a5d6551d3a5b6fdfa3aed9f5e2cf23e6b Mon Sep 17 00:00:00 2001
From: Isaac Shoebottom <ir.shoebottom@gmail.com>
Date: Thu, 6 Feb 2025 12:56:32 -0400
Subject: [PATCH] Add A1 basics

---
 Assignments/01.rkt | 161 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 161 insertions(+)
 create mode 100644 Assignments/01.rkt

diff --git a/Assignments/01.rkt b/Assignments/01.rkt
new file mode 100644
index 0000000..2171dac
--- /dev/null
+++ b/Assignments/01.rkt
@@ -0,0 +1,161 @@
+#lang plait
+(define-type Exp
+  [numE (n : Number)]
+  [plusE (left : Exp) (right : Exp)]
+  [timesE (left : Exp) (right : Exp)]
+  [minusE (left : Exp) (right : Exp)]
+  [lamE (var : Symbol) (body : Exp)]
+  [appE (fun : Exp) (arg : Exp)]
+  [varE (name : Symbol)]
+  [if0E (check : Exp) (zero : Exp) (non-zero : Exp)]
+  [let1E (var : Symbol) (value : Exp) (body : Exp)]
+  [recE (var : Symbol) (value : Exp) (body : Exp)])
+
+(define-type Value
+  [numV (the-number : Number)]
+  [funV (var : Symbol) (body : Exp) (nv : Env)]
+  [undefV])
+
+(define (parse s)
+  (local
+      [(define (sx n) (list-ref (s-exp->list s) n))
+       (define (px n) (parse (sx n)))
+       (define (? pat) (s-exp-match? pat s))
+       (define (parse-let)
+         (let* ([def (sx 1)]
+                [parts (s-exp->list def)]
+                [var (s-exp->symbol (list-ref parts 0))]
+                [val (parse (list-ref parts 1))]
+                [body (px 2)])
+           (values var val body)))]
+    (cond
+      [(? `SYMBOL) (varE (s-exp->symbol s))]
+      [(? `NUMBER) (numE (s-exp->number s))]
+      [(? `(+ ANY ANY)) (plusE (px 1) (px 2))]
+      [(? `(- ANY ANY)) (minusE (px 1) (px 2))]
+      [(? `(* ANY ANY)) (timesE (px 1) (px 2))]
+      [(? `(if0 ANY ANY ANY))
+       (if0E (px 1) (px 2) (px 3))]
+      [(? `(rec (SYMBOL ANY) ANY))
+       (local [(define-values (var val body) (parse-let))]
+         (recE var val body))]
+      [(? `(let1 (SYMBOL ANY) ANY))
+       (local [(define-values (var val body) (parse-let))]
+         (let1E var val body))]
+      [(? `(lam SYMBOL ANY))
+       (lamE (s-exp->symbol (sx 1)) (px 2))]
+      [(? `(ANY ANY)) (appE (px 0) (px 1))]
+      [else (error 'parse (to-string s))])))
+
+(define (num-op op expr1 expr2)
+  (local [(define (unwrap v)
+            (type-case Value v
+              [(numV n) n]
+              [else (error 'num-op "NaN")]))]
+    (numV (op (unwrap expr1)
+              (unwrap expr2)))))
+
+(define-type-alias Env (Hashof Symbol (Boxof Value)))
+
+(define mt-env (hash empty)) ;; "empty environment"
+
+(define (extend old-env new-name value)
+  (hash-set old-env new-name (box value)))
+
+(define (lookup (s : Symbol) (n : Env))
+  (type-case (Optionof (Boxof Value)) (hash-ref n s)
+    [(none) (error s "not bound")]
+    [(some b) (unbox b)]))
+
+(test/exn (lookup 'x mt-env) "not bound")
+
+; Needs to return new environment, with the boxed value containing the function, and a recursive reference to the same environment
+(define (extend-rec [env : Env] [sym : Symbol] [exp : Exp])
+  (local [(define self (extend env sym exp)]
+    (begin
+      (set! (funV-nv self) env)
+      (hash-set self sym (box(interp exp self))))))
+
+#;(let* ([exp (parse `{lam x {f 0}})]
+       [env (extend-rec mt-env 'f exp)]
+       [fun (lookup 'f env)])
+  (begin
+    (display exp)
+    (display "\n")
+    (display env)
+    (display "\n")
+    (display fun)
+    (display "\n")
+    (display (funV-nv fun))
+    (display "\n")         
+    (test (funV-nv fun) env)))
+
+(interp : (Exp Env -> Value))
+(define (interp e nv)
+  (type-case Exp e
+    [(numE n) (numV n)]
+    [(varE s) (lookup s nv)]
+    [(plusE l r) (num-op + (interp l nv) (interp r nv))]
+    [(minusE l r) (num-op - (interp l nv) (interp r nv))]
+    [(timesE l r) (num-op * (interp l nv) (interp r nv))]
+    [(lamE v b) (funV v b nv)]
+    [(if0E c z nz)
+     (if (equal? (numV 0) (interp c nv))
+         (interp z nv)
+         (interp nz nv))]
+     [(appE f a)
+     (let ([fv (interp f nv)]
+           [av (interp a nv)])
+       (type-case Value fv
+         [(funV v b f-env)
+          (interp b (extend f-env v av))]  ;; changed
+         [else (error 'app "not a function")]))]
+    [(let1E var val body)
+     (let ([new-env (extend nv
+                            var
+                            (interp val nv))])
+       (interp body new-env))]
+    [(recE var val body) ....]))
+
+(run : (S-Exp -> Value))
+(define (run s)
+  (interp (parse s) mt-env))
+
+(test (run `{let1 {f {lam x {+ x 1}}} {f 8}}) (numV 9))
+(test (run `{let1 {y 1} {let1 {f {lam x {+ x y}}}
+                              {f 8}}})
+      (numV 9))
+(test (run `{let1 {y 1} {let1 {f {lam x {+ x y}}}
+                              {let1 {y 2} {f 8}}}})
+      (numV 9))
+(test (run `{{let1 {x 3} {lam y {+ x y}}} 4})
+      (numV 7))
+(test (run `{{let1 {y 3} {lam y {+ y 1}}} 5})
+      (numV 6))
+(test (run `{if0 0 (* 1 2) 1}) (numV 2))
+
+(test (run `{rec {f {lam x {+ x 1}}} {f 8}}) (numV 9))
+
+(test (run `{rec {f {let1 {y 7} {lam x {+ x y}}}} {f 8}}) (numV 15))
+
+(test
+ (run `{rec {fact {lam n {if0 n 1 {* n {fact {- n 1}}}}}}
+            {fact 10}})
+ (numV 3628800))
+
+(test
+ (run
+  `{rec
+    {fib
+     {lam n
+          {if0 n 1
+               {if0 {- n 1} 1
+                    {+ {fib {- n 1}}
+                       {fib {- n 2}}}}}}}
+    {fib 6}})
+ (numV 13))
+
+(test
+ (run `{rec {sum {lam n {if0 n 0 {+ n {sum {- n 1}}}}}}
+            {sum 10}})
+ (numV 55))
\ No newline at end of file