[hofstee]

  data Exp = Num Int
           | Bool Bool
           | Var Var
           | If Exp Exp Exp
           | Lambda Var Exp
           | App Exp Exp

An Expression is either:

- a Number literal, with an Integer value

- a Boolean literal, with a Boolean value

- a Variable declaration, with some identifier

- an If expression (probably), with a condition, taken, and not-taken Expressions

- a Lambda, which substitutes a Variable into an Expression

- an Apply, which is how we chain Expressions and apply the results of one expression onto the next

  data Type = TyInt | TyBool | TyArrow Type Type

Our Types are TypeInt, TypeBool, or a Function (TyArrow) which goes from an input Type to an output Type