#lang plait
(define-type FLANG
  [Num  (val : Number)]
  [Add  (l : FLANG) (r : FLANG)]
  [Sub  (l : FLANG) (r : FLANG)]
  [Mul  (l : FLANG) (r : FLANG)]
  [Div  (l : FLANG) (r : FLANG)]
  [Id   (name : Symbol)]
  [Let1 (id : Symbol) (named-expr : FLANG) (bound-body : FLANG)]
  [Lam  (param : Symbol) (body : FLANG)]
  [Call (fun : FLANG) (val : FLANG)]) ; first type!
;; a type for substitution caches:
(define-type Binding
  	     [bind (name : Symbol) (val : FLANG)])

(define-type-alias SubstCache (Listof Binding))

(define empty-subst empty)

(define (extend id expr sc)
  (cons (bind id expr) sc))