refactor all implementations to use contracts

1 files changed, 67 insertions, 39 deletions

diff --git a/stlc-ref.rkt b/stlc-ref.rkt
index d0afec0..4f5ac91 100644
--- a/stlc-ref.rkt
+++ b/stlc-ref.rkt
@@ -1,92 +1,120 @@
 #lang racket
 (require "lib.rkt")
 (require "base.rkt")
-(provide interpret check infer)
+(provide (all-defined-out))
 
 ;; The Simply-Typed Lambda Calculus with references
 
-;;      (interpret Expr Table[Sym, Expr] Table[Sym, Expr]): Value
+; 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 (strip (desugar expr)) #hash() (make-hash)))
-(define (interpret-core expr Γ Σ)
+  (interpret/core (desugar expr) #hash() (make-hash)))
+(define/contract (interpret/core expr Γ Σ)
+  (-> stlc-ref/expr? dict? dict? stlc-ref/value?)
   (match expr
-    ['sole 'sole]
-    [n #:when (natural? n) n]
     [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)])
+      (let ([r (interpret/core e Γ Σ)])
       (if (dict-has-key? Σ r)
-        (interpret-core (dict-ref Σ r) Γ Σ)
+        (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))
+      (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 ,e) `(λ ,x ,e ,Γ)]
-    [`(λ ,x ,e ,env) `(λ ,x ,e ,env)] ; ???
+    [`(λ (,x : ,t) ,e) `(λ ,x ,e ,Γ)]
     [`(,e1 ,e2)
-      (match (interpret-core e1 Γ Σ)
+      (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))])]
-
-    [e (err (format "attempting to interpret unknown expression ~a" e))]))
+          (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))])]))
 
-;;      (check Expr Type Table[Sym, Type]): Bool
+;; Checks an expression against some type, in a given context.
 (define (check expr with)
-  (check-core (desugar expr) with #hash()))
-(define (check-core expr with Γ)
+  (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)]
+        [`(Ref ,t) (check/core e t Γ)]
         [_ #f])]
-    [`(! ,e) (check-core e `(Ref ,with) Γ)]
+    [`(! ,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)))]
+          (and (equal? t1 t) (check/core e t2 (dict-set Γ x t1)))]
         [_ #f])]
 
-    [_ (equal? (infer-core expr Γ) with)]))
+    [_ (equal? (infer/core expr Γ) with)]))
 
-;;      (infer Expr Table[Sym, Type]): Type
+;; Infers a type from some expression, in a given context.
 (define (infer expr)
-  (infer-core (desugar expr) #hash()))
-(define (infer-core expr Γ)
+  (infer/core (desugar expr) #hash()))
+(define/contract (infer/core expr Γ)
+  (-> stlc-ref/expr? dict? stlc-ref/type?)
   (match expr
     ['sole 'Unit]
-    [n #:when (natural? n) 'Nat]
     [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))]
+    [`(new ,e) `(Ref ,(infer/core e Γ))]
     [`(! ,e)
-      (match (infer-core e Γ)
+      (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)
+      (match (infer/core e1 Γ)
         [`(Ref ,t)
-          (if (check-core e2 t Γ) 'Unit
+          (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))))]
+              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)))]
+      `(,t → ,(infer/core e (dict-set Γ x t)))]
     [`(,e1 ,e2)
-      (match (infer-core e1 Γ)
+      (match (infer/core e1 Γ)
         [`(,t1 → ,t2)
-          (if (check-core e2 t1 Γ) 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))])]
-
-    [e (err (format "attempting to infer an unknown expression ~a" e))]))
+        [t (err (format "expected → type on application body, got ~a" t))])]))