CODEWITHSUNDEEP
haskelltypes
Never Mind
Never Mind

Reputation: 297

Haskell expression data type

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?

Upvotes: 0

Views: 570

Answers (1)

Thomas M. DuBuisson
Thomas M. DuBuisson

Reputation: 64740

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.

Upvotes: 4

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