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#lang racket
(require "lib.rkt")
;; The Simply-Typed Lambda Calculus with higher-order *impredicative* references
; Γ, x: τ₁ ⊢ e: τ₂ k > max-level(Γ, τ₁, τ₂)
; ---------------------------------------------
; Γ ⊢ λx:τ₁.e : τ₁ →ᵏ τ₂
; Γ ⊢ e₁: τ₁ →ᵏ τ₂ Γ ⊢ e₂: τ₁
; --------------------------------
; Γ ⊢ (e₁ e₂): τ₂
; --------------------------
; Nat::Type₀, Unit::Type₀
; τ::Type₀
; ---------------
; Ref τ :: Type₀
; τ::Typeᵢ, i ≥ 0
; ---------------
; Ref τ :: Typeᵢ₊₁
; τ₁::Typeᵢ, τ₂::Typeⱼ, k > max-level(τ₁, τ₂)
; -----------------------------------------
; τ₁ →ᵏ τ₂ :: Typeₖ
(require (only-in "stlc-ref.rkt" interpret))
;; (check Expr Type Table[Sym, Type]): Bool
(define (check expr with [Γ #hash()])
(check- (desugar expr) with Γ))
(define (check- expr with Γ)
; (check- expr (dict-ref Γ with) Γ)
(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)]
[(`(new ,e) `(Ref ,t)) (check- e t Γ)]
[(`(! ,e) t) (check- e `(Ref ,t) Γ)]
[(`(set ,e1 ,e2) 'Unit)
(match (infer- e1 Γ)
[`(Ref ,t) (check- e2 t Γ)]
[t #f])]
[(`(λ ,x (: ,t) ,e) `(→ ,k ,t1 ,t2))
(and
(equal? t t1)
(> k (max-level e (dict-set Γ x t1) t1 t2)) ; KNOB
(check- e t2 (dict-set Γ x t1)))]
[(`(,e1 ,e2) t)
(match (infer- e1 Γ)
[`(→ ,k ,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]
[x #:when (dict-has-key? Γ x)
(dict-ref Γ x)]
[`(new ,e) `(Ref ,(infer- e Γ))]
[`(! ,e)
(match (infer- e Γ)
[`(Ref ,t) t]
[t (err "attempting to deref term not of Ref type!")])]
[`(set ,e1 ,e2)
(match (infer- e1 Γ)
[`(Ref ,t)
(if (check- e2 t Γ) 'Unit
(err (format "attempting to update ~a: ~a with term ~a: ~a of differing type"
e1 t e2 (infer- e2 Γ))))]
[t (err (format "attempting to update non-reference ~a: ~a" e1 t))])]
[`(λ ,x (: ,t1) ,e)
(let ([t2 (infer- e (dict-set Γ x t1))])
(let ([k (+ 1 (max-level e (dict-set Γ x t1) t1 t2))]) ; KNOB
`(→ ,k ,t1 ,t2)))]
[`(,e1 ,e2)
(match (infer- e1 Γ)
[`(→ ,k ,t1 ,t2)
(if (check- e2 t1 Γ) t2
(err (format "inferred argument type ~a does not match arg ~a of type ~a" t1 e2 (infer- e2 Γ))))]
[t (err (format "expected → type on application body, got ~a" t))])]
[e (err (format "attempting to infer an unknown expression ~a" e))]))
;; (max-level Table[Sym, Type] Expr Type Type): Natural
(define (max-level e Γ t1 t2)
(max
(level-type t1)
(level-type t2)
(level-body e Γ)))
;; (level-type Type): Natural
(define (level-type t)
(match t
['Unit 0]
['Nat 0]
[`(→ ,k ,t1 ,t2)
(if (or (< k (level-type t1)) (< k (level-type t2)))
(err (format "annotated level ~a is less than inferred levels of ~a and ~a!"
k t1 t2))
k)]
[`(Ref ,t)
(let ([k (level-type t)])
(if (zero? k) 0 (+ 1 k)))] ; KNOB
[t (err (format "attempting to infer the level of unknown type ~a" t))]))
;; (level-body Expr Table[Sym, Type]): Natural
(define (level-body e Γ)
(match e
['sole 0]
[n #:when (natural? n) 0]
[x #:when (dict-has-key? Γ x)
(level-type (dict-ref Γ x))]
[`(new ,e) (level-body e Γ)]
[`(! ,e) (level-body e Γ)]
[`(set ,e1 ,e2) (max (level-body e1 Γ) (level-body e2 Γ))]
[`(λ ,x (: ,t) ,e) (level-body e (dict-set Γ x t))] ; todo: should be 0?
[`(,e1 ,e2) (max (level-body e1 Γ) (level-body e2 Γ))]))
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