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#lang racket
(require "lib.rkt")
(require "base.rkt")
(provide interpret check infer expand equiv-type equiv-term)
;; The Simply-Typed Lambda Calculus, with simple extensions
;; Unit/String/Natural/Boolean, pairs, sums, lists, ascryption
;; (interpret Expr Table[Sym, Expr]): Value
(define (interpret expr)
(interpret-core (strip (desugar expr)) #hash()))
(define (interpret-core expr Γ)
(match expr
['sole 'sole]
[s #:when (string? s) s]
[n #:when (natural? n) n]
[b #:when (boolean? b) b]
[x #:when (dict-has-key? Γ x) (dict-ref Γ x)]
[`(inc ,e)
(match (interpret-core e Γ)
[n #:when (natural? n) (+ n 1)]
[e (format "incrementing an unknown value ~a" e)])]
[`(if ,c ,e1 ,e2)
(match (interpret-core c Γ)
['#t (interpret-core e1 Γ)]
['#f (interpret-core e2 Γ)]
[e (err (format "calling if on unknown expression ~a" e))])]
[`(pair ,e1 ,e2)
`(pair ,(interpret-core e1 Γ) ,(interpret-core e2 Γ))]
[`(car ,e)
(match (interpret-core e Γ)
[`(pair ,e1 ,e2) e1]
[e (err (format "calling car on unknown expression ~a" e))])]
[`(cdr ,e)
(match (interpret-core e Γ)
[`(pair ,e1 ,e2) e2]
[e (err (format "calling cdr on unknown expression ~a" e))])]
[`(inl ,e) `(inl ,(interpret-core e Γ))]
[`(inr ,e) `(inr ,(interpret-core e Γ))]
[`(case ,e (,x1 ⇒ ,e1) (,x2 ⇒ ,e2))
(match (interpret-core e Γ)
[`(inl ,e) (interpret-core e1 (dict-set Γ x1 e))]
[`(inr ,e) (interpret-core e2 (dict-set Γ x2 e))]
[e (err (format "calling case on unknown expression ~a" e))])]
['nil 'nil]
[`(nil? ,e)
(match (interpret-core e Γ)
['nil '#t]
[`(cons ,e1 ,e2) '#f]
[e (err (format "calling isnil on unknown expression ~a" e))])]
[`(cons ,e1 ,e2)
`(cons ,(interpret-core e1 Γ) ,(interpret-core e2 Γ))]
[`(head ,e)
(match (interpret-core e Γ)
[`(cons ,e1 ,e2) (interpret-core e1 Γ)]
[e (err (format "calling head on unknown expression ~a" e))])]
[`(tail ,e)
(match (interpret-core e Γ)
[`(cons ,e1 ,e2) (interpret-core e2 Γ)]
[e (err (format "calling tail on unknown expression ~a" e))])]
[`(λ ,x ,e) `(λ ,x ,e ,Γ)]
[`(,e1 ,e2)
(match (interpret-core e1 Γ)
[`(λ ,x ,e ,env)
(interpret-core e (dict-set env x (interpret-core e2 Γ)))]
[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)
(check-core (desugar expr) with #hash()))
(define (check-core expr with Γ)
(match expr
[`(type ,t1 ,t2 ,in)
(check-core in with (dict-set Γ t1 t2))]
[`(if ,c ,e1 ,e2)
(and (check-core c 'Bool Γ)
(check-core e1 with Γ) (check-core e2 with Γ))]
[`(pair ,e1 ,e2)
(match with
[`(,t1 × ,t2) (and (check-core e1 t1 Γ) (check-core e2 t2 Γ))]
[_ #f])]
[`(inl ,e)
(match with
[`(,t1 ⊕ ,t2) (check-core e t1 Γ)]
[_ #f])]
[`(inr ,e)
(match with
[`(,t1 ⊕ ,t2) (check-core e t2 Γ)]
[_ #f])]
[`(case ,e (,x1 ⇒ ,e1) (,x2 ⇒ ,e2))
(match (infer-core e Γ)
[`(,a1 ⊕ ,a2)
(and (check-core e1 with (dict-set Γ x1 a1))
(check-core e2 with (dict-set Γ x2 a2)))]
[_ #f])]
['nil
(match with
[`(List ,t) #t]
[_ #f])]
[`(cons ,f1 ,f2)
(match with
[`(List ,t)
(and (check-core f1 t Γ)
(check-core f2 `(List ,t) Γ))]
[_ #f])]
[`(λ (,x : ,t) ,e)
(match with
[`(,t1 → ,t2)
(and (equiv-type t1 t Γ) (check-core e t2 (dict-set Γ x t1)))]
[_ #f])]
[_ (equiv-type (infer-core expr Γ) with Γ)]))
;; (infer Expr Table[Sym, Type]): Type
(define (infer expr)
(infer-core (desugar expr) #hash()))
(define (infer-core expr Γ)
(match expr
['sole 'Unit]
[s #:when (string? s) 'Str]
[n #:when (natural? n) 'Nat]
[b #:when (boolean? b) 'Bool]
[x #:when (dict-has-key? Γ x)
(expand (dict-ref Γ x) Γ)]
[`(type ,t1 ,t2 ,in)
(infer-core in (dict-set Γ t1 t2))]
[`(,e : ,t)
(if (check-core e (expand t Γ) Γ) (expand t Γ)
(err (format "annotated expression ~a is not of annotated type ~a" e t)))]
[`(inc ,e)
(if (check-core e 'Nat Γ) 'Nat
(err (format "calling inc on incorrect type ~a" (infer-core e Γ))))]
[`(if ,c ,e1 ,e2)
(if (check-core c 'Bool Γ)
(let ([t (infer-core e1 Γ)])
(if (check-core e2 t Γ) t
(err (format "condition has branches of differing types ~a and ~a"
t (infer-core e2 Γ)))))
(err (format "condition ~a has incorrect type ~a" c (infer-core c Γ))))]
[`(pair ,e1 ,e2)
`(,(infer-core e1 Γ) × ,(infer-core e2 Γ))]
[`(car ,e)
(match (infer-core e Γ)
[`(,t1 × ,t2) t1]
[t (err (format "calling car on incorrect type ~a" t))])]
[`(cdr ,e)
(match (infer-core e Γ)
[`(,t1 × ,t2) t2]
[t (err (format "calling cdr on incorrect type ~a" t))])]
[`(inl ,e) ; annotations necessary
(match (infer-core e Γ)
[`(,t1 ⊕ ,t2) `(,t1 ⊕ ,t2)]
[t (err (format "calling inl on incorrect type ~a" t))])]
[`(inr ,e) ; annotations necessary
(match (infer-core e Γ)
[`(,t1 ⊕ ,t2) `(,t1 ⊕ ,t2)]
[t (err (format "calling inr on incorrect type ~a" t))])]
[`(case ,e (,x1 ⇒ ,e1) (,x2 ⇒ ,e2))
(match (infer-core e Γ)
[`(,a1 ⊕ ,a2)
(let ([b1 (infer-core e1 (dict-set Γ x1 (expand a1 Γ)))]
[b2 (infer-core e2 (dict-set Γ x2 (expand a2 Γ)))])
(if (equiv-type b1 b2 Γ) b1
(err (format "case ~a is not of consistent type!" `(case (,a1 ⊕ ,a2) b1 b2)))))]
[t (err (format "calling case on incorrect type ~a" t))])]
['nil (err (format "unable to infer type of empty list!"))]
[`(cons ,e1 ,e2)
(let ([t (infer-core e1 Γ)])
(if (check-core e2 `(List ,t) Γ) `(List ,t)
(err (format "list ~a is not of consistent type!" `(cons ,e1 ,e2)))))]
[`(head ,e)
(match (infer-core e Γ)
[`(List ,t) t]
[t (err (format "calling head on incorrect type ~a" t))])]
[`(tail ,e)
(match (infer-core e Γ)
[`(List ,t) `(List ,t)]
[t (err (format "calling tail on incorrect type ~a" t))])]
[`(λ (,x : ,t) ,e)
`(,(expand t Γ) → ,(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))])]
[e (err (format "inferring an unknown expression ~a" e))]))
;; Expands a type alias into weak-head normal form, for literal matching.
;; (expand Type Table[Id, Expr ⊕ Type]): Type
(define (expand t Γ)
(if (dict-has-key? Γ `(type ,t))
(expand (dict-ref Γ `(type ,t)) Γ) t))
;; Checks if two types are equivalent up to α-conversion in context
;; (equiv-type Expr Expr Table[Sym Expr]): Bool
(define (equiv-type e1 e2 Γ)
(equiv-type-core e1 e2 Γ Γ))
(define (equiv-type-core e1 e2 Γ1 Γ2)
(match* (e1 e2)
; bound identifiers: if a key exists in the context, look it up
[(x1 x2) #:when (dict-has-key? Γ1 x1)
(equiv-type-core (dict-ref Γ1 x1) x2 Γ1 Γ2)]
[(x1 x2) #:when (dict-has-key? Γ2 x2)
(equiv-type-core x1 (dict-ref Γ2 x2) Γ1 Γ2)]
; check for syntactic equivalence on remaining forms
[(`(,l1 ...) `(,l2 ...))
(foldl (λ (x1 x2 acc) (if (equiv-type-core x1 x2 Γ1 Γ2) acc #f)) #t l1 l2)]
[(v1 v2) (equal? v1 v2)]))
;; Checks if two terms are equivalent up to α-conversion in context
;; (equiv-term Expr Expr Table[Sym Expr]): Bool
(define (equiv-term e1 e2 Γ)
(equiv-term-core e1 e2 Γ Γ))
(define (equiv-term-core e1 e2 Γ1 Γ2)
(match* (e1 e2)
; bound identifiers: if a key exists in the context, look it up
[(x1 x2) #:when (dict-has-key? Γ1 x1)
(equiv-term-core (dict-ref Γ1 x1) x2 Γ1 Γ2)]
[(x1 x2) #:when (dict-has-key? Γ2 x2)
(equiv-term-core x1 (dict-ref Γ2 x2) Γ1 Γ2)]
; function expressions: parameter names can be arbitrary
[(`(λ (,x1 : ,t1) ,e1) `(λ (,x2 : ,t2) ,e2))
(let ([name gensym])
(and (equiv-term-core e1 e2 (dict-set Γ1 x1 name) (dict-set Γ2 x2 name))
(equiv-term-core t1 t2 Γ1 Γ2)))]
[(`(λ ,x1 ,e1) `(λ ,x2 ,e2))
(let ([name gensym])
(equiv-term-core e1 e2 (dict-set Γ1 x1 name) (dict-set Γ2 x2 name)))]
; check for syntactic equivalence on remaining forms
[(`(,l1 ...) `(,l2 ...))
(foldl (λ (x1 x2 acc) (if (equiv-term-core x1 x2 Γ1 Γ2) acc #f)) #t l1 l2)]
[(v1 v2) (equal? v1 v2)]))
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