From ad19de6be6b9a9d620d94e3f162dcee1bfda2cf7 Mon Sep 17 00:00:00 2001
From: JJ
Date: Wed, 23 Oct 2024 18:10:03 -0700
Subject: restructure code, put implementations into broadly categorical
 folders

---
 simple/ref.rkt | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 simple/ref.rkt

(limited to 'simple/ref.rkt')

diff --git a/simple/ref.rkt b/simple/ref.rkt
new file mode 100644
index 0000000..aff983f
--- /dev/null
+++ b/simple/ref.rkt
@@ -0,0 +1,119 @@
+#lang racket
+(require "../lib.rkt")
+(provide (all-defined-out))
+
+;; The Simply-Typed Lambda Calculus with references
+
+; todo: rewrite to use call-by-reference or call-by-value or call-by-name explicitly
+
+;; Checks an expression for syntactic well-formedness.
+(define (stlc-ref/expr? expr)
+  (match expr
+    [x #:when (symbol? x) #t]
+    [n #:when (natural? n) #t]
+    [(or `(new ,e) `(! ,e)) (stlc-ref/expr? e)]
+    [(or `(set ,e1 ,e2) `(,e1 ,e2)) (and (stlc-ref/expr? e1) (stlc-ref/expr? e2))]
+    [`(λ (,x : ,t) ,e) (and (symbol? x) (stlc-ref/type? t) (stlc-ref/expr? e))]
+    [_ #f]))
+
+;; Checks a type for syntactic well-formedness.
+(define (stlc-ref/type? type)
+  (match type
+    [t #:when (symbol? t) #t]
+    [`(Ref ,t) (stlc-ref/type? t)]
+    [`(,t1 → ,t2) (and (stlc-ref/type? t1) (stlc-ref/type? t2))]
+    [_ #f]))
+
+;; Checks a value for syntactic well-formedness.
+(define (stlc-ref/value? value)
+  (match value
+    [x #:when (symbol? x) #t]
+    [n #:when (natural? n) #t]
+    [`(,v1 ,v2) (and (stlc-ref/value? v1) (stlc-ref/value? v2))]
+    [`(λ ,x ,e ,env) (and (symbol? x) (stlc-ref/expr? e) (dict? env))]
+    [_ #f]))
+
+;; Interprets an expression down to a value, in a given context.
+(define (interpret expr)
+  (interpret/core (desugar expr) #hash() (make-hash)))
+(define/contract (interpret/core expr Γ Σ)
+  (-> stlc-ref/expr? dict? dict? stlc-ref/value?)
+  (match expr
+    [r #:when (dict-has-key? Σ r) r]
+    [x #:when (dict-has-key? Γ x) (dict-ref Γ x)]
+    [f #:when (symbol? f) f]
+
+    [`(new ,e)
+      (let ([r (gensym)])
+      (dict-set! Σ r e) r)]
+    [`(! ,e)
+      (let ([r (interpret/core e Γ Σ)])
+      (if (dict-has-key? Σ r)
+        (interpret/core (dict-ref Σ r) Γ Σ)
+        (err (format "attempting to deref unknown reference ~a" r))))]
+    [`(set ,e1 ,e2)
+      (let ([r (interpret/core e1 Γ Σ)])
+      (if (dict-has-key? Σ r) (dict-set! Σ r (interpret/core e2 Γ Σ))
+        (err (format "attempting to update unknown reference ~a" r))))
+      'sole]
+
+    [`(λ (,x : ,t) ,e) `(λ ,x ,e ,Γ)]
+    [`(,e1 ,e2)
+      (match (interpret/core e1 Γ Σ)
+        [`(λ ,x ,e1 ,env)
+          (interpret/core e1 (dict-set env x (interpret/core e2 Γ Σ)) Σ)]
+        [e1 (err (format "attempting to interpret arg ~a applied to unknown expression ~a" e2 e1))])]))
+
+;; Checks an expression against some type, in a given context.
+(define (check expr with)
+  (check/core (desugar expr) with #hash()))
+(define/contract (check/core expr with Γ)
+  (-> stlc-ref/expr? stlc-ref/type? dict? boolean?)
+  (match expr
+    [`(new ,e)
+      (match with
+        [`(Ref ,t) (check/core e t Γ)]
+        [_ #f])]
+    [`(! ,e) (check/core e `(Ref ,with) Γ)]
+
+    [`(λ (,x : ,t) ,e)
+      (match with
+        [`(,t1 → ,t2)
+          (and (equal? t1 t) (check/core e t2 (dict-set Γ x t1)))]
+        [_ #f])]
+
+    [_ (equal? (infer/core expr Γ) with)]))
+
+;; Infers a type from some expression, in a given context.
+(define (infer expr)
+  (infer/core (desugar expr) #hash()))
+(define/contract (infer/core expr Γ)
+  (-> stlc-ref/expr? dict? stlc-ref/type?)
+  (match expr
+    ['sole 'Unit]
+    [x #:when (dict-has-key? Γ x) (dict-ref Γ x)]
+    [n #:when (natural? n) n]
+    [f #:when (symbol? f)
+      (err (format "attempting to infer type of free variable ~a" f))]
+
+    [`(new ,e) `(Ref ,(infer/core e Γ))]
+    [`(! ,e)
+      (match (infer/core e Γ)
+        [`(Ref ,t) t]
+        [t (err "attempting to deref term not of Ref type!")])]
+    [`(set ,e1 ,e2)
+      (match (infer/core e1 Γ)
+        [`(Ref ,t)
+          (if (check/core e2 t Γ) 'Unit
+            (err (format "attempting to update ~a: ~a with term ~a: ~a of differing type"
+              e1 t e2 (infer/core e2 Γ))))]
+        [t (err (format "attempting to update non-reference ~a: ~a" e1 t))])]
+
+    [`(λ (,x : ,t) ,e)
+      `(,t → ,(infer/core e (dict-set Γ x t)))]
+    [`(,e1 ,e2)
+      (match (infer/core e1 Γ)
+        [`(,t1 → ,t2)
+          (if (check/core e2 t1 Γ) t2
+            (err (format "inferred argument type ~a does not match arg ~a" t1 e2)))]
+        [t (err (format "expected → type on application body, got ~a" t))])]))
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