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diff --git a/stlc-rec.rkt b/stlc-rec.rkt
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+++ b/stlc-rec.rkt
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+#lang racket
+(require "lib.rkt")
+
+;; The Simply-Typed Lambda Calculus with iso-recursive types
+
+; Γ ⊢ e: [x ↦ μx.t] t
+; ------------------------
+; Γ ⊢ fold [μx.t] e: μx.t
+
+; Γ ⊢ e: μx.t
+; -----------------------------------
+; Γ ⊢ unfold [μx.t] e: [x ↦ μx.t] t
+
+;;      (interpret Expr Table[Sym, Expr]): Value
+(define (interpret expr [ctx #hash()])
+  (interpret- (strip (desugar expr)) ctx))
+(define (interpret- expr ctx)
+  (match expr
+    ['sole 'sole]
+    [n #:when (natural? n) n]
+    [x #:when (dict-has-key? ctx x) (dict-ref ctx x)]
+
+    [`(fold ,t ,e) `(fold ,t ,(interpret- e))]
+    [`(unfold ,t ,e) `(unfold ,t ,(interpret- e))]
+
+    [`(λ ,x ,e) `(λ ,x ,e ,ctx)]
+    [`(,e1 ,e2)
+      (match (interpret- e1 ctx)
+        [`(λ ,x ,e ,env)
+          (interpret- e (dict-set env x (interpret- e2 ctx)))]
+        [e (err (format "applying arg ~a to unknown expression ~a" e2 e))])]
+
+    [e (err (format "interpreting an unknown expression ~a" e))]))
+
+;;      (check Expr Type Table[Sym, Type]): Bool
+(define (check expr with [Γ #hash()])
+  (check- (desugar expr) with Γ))
+(define (check- expr with Γ)
+  ; (print (format "check: ~a" (fmt expr)))
+  (match* (expr with)
+    [('sole 'Unit) #t]
+    [(n 'Nat) #:when (natural? n) #t]
+    [(x _) #:when (dict-has-key? Γ x)
+      (equal? (dict-ref Γ x) with)]
+
+    [(`(fold (μ ,x ,t) ,e) `(μ ,x ,t))
+      (check- e t (dict-set Γ x `(μ ,x ,t)))]
+    [(`(unfold (μ ,x ,t) ,e) with)
+      (and (check- e `(μ ,x ,t) Γ)
+        (equiv? with t #hash() #hash((x . `(μ ,x ,t)))))]
+
+    [(`(λ ,x (: ,t) ,e) `(→ ,t1 ,t2))
+      (and (equal? t t1) (check- e t2 (dict-set Γ x t1)))]
+
+    [(`(,e1 ,e2) t)
+      (match (infer- e1 Γ)
+        [`(→ ,t1 ,t2)
+          (and (equal? t2 t) (equal? t1 (infer- e2 Γ)))]
+        [t #f])]
+
+    [(e t) #f]))
+
+;;      (infer Expr Table[Sym, Type]): Type
+(define (infer expr [Γ #hash()])
+  (infer- (desugar expr) Γ))
+(define (infer- expr Γ)
+  ; (print (format "infer: ~a" (fmt expr)))
+  (match expr
+    ['sole 'Unit]
+    [n #:when (natural? n) 'Nat]
+    [b #:when (boolean? b) 'Bool]
+    [x #:when (dict-has-key? Γ x)
+      (dict-ref Γ x)]
+
+    [`(fold (μ ,x ,t) ,e)
+      (if (check- e t (dict-set Γ x `(μ ,x ,t))) `(μ ,x ,t)
+        (err (format ("expected ~a to be of type ~a, got ~a"
+          e t (infer e (dict-set Γ x `(μ ,x ,t)))))))]
+    [`(unfold (μ ,x ,t) ,e)
+      (if (check- e `(μ ,x ,t)) (α-convert t #hash((x . `(μ ,x ,t))))
+        (err (format ("expected ~a to be of type ~a, got ~a"
+          e `(μ ,x ,t) (infer- e Γ)))))]
+
+    [`(λ ,x (: ,t) ,e)
+      `(→ ,t ,(infer- e (dict-set Γ x t)))]
+
+    [`(,e1 ,e2)
+      (match (infer- e1 Γ)
+        [`(→ ,t1 ,t2)
+          (if (check- 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))]))