#lang racket
(require "lib.rkt")

;; The Simply-Typed Lambda Calculus, with let sugar and some base types

;;      (interpret Expr Table[Sym, Expr]): Value
(define (interpret expr [Γ #hash()])
  (interpret- (strip (desugar expr)) Γ))
(define (interpret- expr Γ)
  (match expr
    ['sole 'sole]
    [n #:when (natural? n) n]
    [x #:when (dict-has-key? Γ x) (dict-ref Γ x)]
    [`(λ ,x ,e) `(λ ,x ,e ,Γ)]
    [`(,e1 ,e2)
      (match (interpret- e1 Γ)
        [`(λ ,x ,e1 ,env) (interpret- e1 (dict-set env x (interpret- e2 Γ)))]
        [e1 `(,e1 ,(interpret- e2 Γ))])]
    [e e]))

;;      (check Expr Type Table[Sym, Type]): Bool
(define (check expr with [Γ #hash()])
  (check- (desugar expr) with Γ))
(define (check- expr 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)]
    [(`(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)
  (match expr
    ['sole 'Unit]
    [n #:when (natural? n) 'Nat]
    [`(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))]))

(provide interpret check infer)

(require rackunit)
(check-equal? (interpret '(λ x x)) '(λ x x #hash()))
(check-equal? (interpret '((λ a a) (x y))) '(x y))
(check-equal? (interpret '((λ a (x y)) (λ z z))) '(x y))
(check-equal? (interpret '((λ a (a y)) x)) '(x y))