Answer by Thomas M. DuBuisson for Haskell expression data type

You could define a newtype wrapper Var or just use String. First the newtype wrapper example:

newtype Var = V String-- ^ "Var" here is a type declaration. "V" is declaring a data constructor.data Exp = Add Exp Exp     | Var Var          -- The first "Var" here is declaring a data constructor. The second is referring to the type.     | Let Var Exp Exp  -- "Var" here is referring to the type     | Int Int

Or just with strings:

data Exp = Add Exp Exp     | Var String      | Let String Exp Exp     | Int Int 

EDIT: The point my comments were trying to make is probably unclear. In your use of Var, such as Let (Var String) Exp Exp, you tried to use a constructor named Var (and it's field, String) in a location that requires a type. Each data constructor can be used to build a value of the type (Exp in this case) and can not be further distinguished by the type system. In lieu of an additional data declaration you can't distinguish between a value of type Exp that is a Var from one that is an Add, Let, or Int.