SML coding:

Given: Define type 'a env as an abbreviation for a function type, and define exception NotFound, both as follows:

type 'a env = string -> 'a

exception NotFound of string

Define these functions

val emptyEnv : 'a env = (* ... *)

val bindVar : string * 'a * 'a env -> 'a env = (* ... *)

val lookup : string * 'a env -> 'a = (* ... *)

such that you can use 'a env for a type environment or a value environment. On an attempt to look up an identifier that doesnt exist, raise the exception NotFound. Dont worry about efficiency.

Lastly define a function

val isBound : string * 'a env -> bool

that tells if a name is bound in the environment.