#lang racket
(require "lib.rkt")

;; The Simply-Typed Lambda Calculus with higher-order *predicative* references

(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)
  ; (print (format "check: ~a" (fmt expr)))
  (match* (expr with)
    [(n 'Nat) #:when (natural? n) #t]       ; ↝ Γ ⊢ n: Nat
    [('⟨⟩ 'Unit) #t]                         ; ↝ Γ ⊢ ⟨⟩: Unit
    [(x _) #:when (dict-has-key? Γ x)        ; x: τ ∈ Γ → Γ ⊢ x: τ
      (equal? (dict-ref Γ x) with)]

    [(`(new ,e) `(Ref ,τ)) (check- e τ Γ)]  ; Γ ⊢ e: τ → Γ ⊢ new e: Ref τ
    [(`(! ,e) τ) (check- e `(Ref ,τ) Γ)]    ; Γ ⊢ e: Ref τ → Γ ⊢ !e: τ
    [(`(set ,e1 ,e2) 'Unit)                 ; ↝ Γ ⊢ e1 := e2: Unit
      (match (infer- e1 Γ)
        [`(Ref ,τ) (check- e2 τ Γ)]         ; Γ ⊢ e1: Ref τ, Γ ⊢ e2: τ
        [type (err (format "attempting to update non-reference ~a: ~a" e1 type))])]

    [(`(λ ,x (: ,τ) ,e) `(→ ,k ,τ1 ,τ2))    ; ↝ Γ ⊢ λx: τ1.e: τ1 →k τ2
      (and
        (equal? τ τ1)
        (>= k (max-level e (dict-set Γ x τ1) τ1 τ2))  ; k ≥ max-level(Γ, τ1, τ2) (KNOB)
        (check- e τ2 (dict-set Γ x τ1)))]             ; Γ, x: τ1 ⊢ e: τ2

    [(`(,e1 ,e2) τ)                                   ; ↝ Γ ⊢ (e1 e2): τ2
      (match (infer- e1 Γ)
        [`(→ ,k ,τ1 ,τ2)                              ; Γ ⊢ e1: τ1 →k τ2
          (and (equal? τ2 τ)                          ; Γ ⊢ e2: τ1
               (equal? τ1 (infer- e2 Γ)))]
        [type (err (format "expected → type on application body, got ~a" type))])]

    [(`(λ ,id (: ,type) ,body) `(→ ,a ,b)) ; error handling
      (err "you forgot to add a level annotation dummy")]
    [(expr _) (err (format "checking an unknown expression ~a with type ~a" expr with))]))

;;      (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
    ['⟨⟩ 'Unit]                          ; ↝ Γ ⊢ ⟨⟩: Unit
    [n #:when (natural? n) 'Nat]        ; ↝ Γ ⊢ n: Nat
    [x #:when (dict-has-key? Γ x)       ; x: τ ∈ Γ
      (dict-ref Γ x)]                   ; ↝ Γ ⊢ x: τ

    [`(new ,e) `(Ref ,(infer- e Γ))]    ; Γ ⊢ e: τ → Γ ⊢ new e: Ref τ
    [`(! ,e)
      (match (infer- e Γ)
        [`(Ref ,type) type]             ; Γ ⊢ e: Ref τ → Γ ⊢ !e: τ
        [_ (err "attempting to deref term not of Ref type!")])]
    [`(set ,e1 ,e2)
      (match (infer- e1 Γ)
        [`(Ref ,τ)                       ; Γ ⊢ e1: Ref τ, Γ ⊢ e2: τ
          (if (check- e2 τ Γ) 'Unit      ; ↝ Γ ⊢ e1 := e2: Unit
            (err (format "attempting to update ~a: ~a with term ~a: ~a of differing type"
              e1 τ e2 (infer- e2 Γ))))]
        [type (err (format "attempting to update non-reference ~a: ~a" e1 type))])]

    [`(λ ,x (: ,τ1) ,e)
      (let ([τ2 (infer- e (dict-set Γ x τ1))])            ; Γ, x: τ1 ⊢ e: τ2
        (let ([k (max-level e (dict-set Γ x τ1) τ1 τ2)])  ; k ≥ max-level(Γ, τ1, τ2) (KNOB)
          `(→ ,k ,τ1 ,τ2)))]                              ; ↝ Γ ⊢ λx: τ1.e: τ1 →k τ2

    [`(,e1 ,e2)
      (match (infer- e1 Γ)
        [`(→ ,k ,τ1 ,τ2)                                  ; Γ ⊢ e1: τ1 →k τ2
          (if (check- e2 τ1 Γ)                            ; Γ ⊢ e2: τ1
            τ2                                            ; ↝ Γ ⊢ (e1 e2): τ2
            (err (format "inferred argument type ~a does not match arg ~a" τ1 e2)))]
        [type (err (format "expected → type on application body, got ~a" type))])]

    [`(λ ,x ,e) ; error handling
      (err "unable to infer type from λ lacking parameter annotation")]
    [expr (err (format "inferring an unknown expression ~a" expr))]))

;;      (max-level Table[Sym, Type] Expr Type Type) → Natural
(define (max-level e Γ τ1 τ2)
  (max
    (level-type τ1)
    (level-type τ2)
    (level-body e Γ)))

;;      (level-type Type) → Natural
(define (level-type τ)
  (match τ
    ['Unit 0]
    ['Nat 0]
    [`(→ ,k ,τ1 ,τ2)
      (if (or (< k (level-type τ1)) (< k (level-type τ2)))
        (err (format "annotated level ~a is less than inferred levels of ~a and ~a!"
          k τ1 τ2))
        k)]
    [`(Ref ,τ) (+ 1 (level-type τ))] ; (KNOB)
    [τ (err (format "inferring the level of unknown type ~a" τ))]))

;;      (level-body Expr Table[Sym, Type]) → Natural
(define (level-body e Γ)
  (match e
    ['⟨⟩ 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 (: ,τ) ,e) (level-body e (dict-set Γ x τ))] ; todo: should be 0?
    [`(,e1 ,e2) (max (level-body e1 Γ) (level-body e2 Γ))]))

; simple diverging program in STLC-ref
; https://www.youtube.com/watch?v=XNgE8kBfSz8
#;
(interpret '
  (let id (: (→ 0 Nat Nat)) (λ x x)
    (let r (: (Ref (→ 0 Nat Nat))) (ref id)
      (let f (: (→ 1 Nat Nat)) (λ x ((deref r) x))
        (set r f
          (f 0))))))
#;
(print (fmt '
  (let id (: (→ 0 Nat Nat)) (λ x x)
    (let r (: (Ref (→ 0 Nat Nat))) (ref id)
      (let f (: (→ 1 Nat Nat)) (λ x ((deref r) x))
        (set r f
          (f 0)))))))

(require rackunit)
(check-exn
  exn:fail?
  (λ () (infer '
    (let id (: (→ 0 Nat Nat)) (λ x x)
      (let r (: (Ref (→ 0 Nat Nat))) (ref id)
        (let f (: (→ 1 Nat Nat)) (λ x ((deref r) x))
          (set r f
            (f 0))))))))

(check-eq?
  (infer '
    (let id (: (→ 0 Nat Nat)) (λ x x)
      (let r (: (Ref (→ 0 Nat Nat))) (ref id)
        (let f (: (→ 1 Nat Nat)) (λ x ((deref r) x))
          (f 0)))))
  'Nat)

(check-eq?
  (check '
    (let id (: (→ 0 Nat Nat)) (λ x x)
      (let r (: (Ref (→ 0 Nat Nat))) (ref id)
        (let f (: (→ 1 Nat Nat)) (λ x ((deref r) x))
          (f 0))))
    'Nat)
  #t)