Haskell表达式数据类型

Question

I want to have the following data type in Haskell :

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

However, using (Var String) like that is not allowed. I could use Exp instead of (Var String), but I don't want an Int to be allowed there, so how can I solve this?

Answer 1

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 .