CODEWITHSUNDEEP
haskelltypesfunctional-programming
ceno980
ceno980

Reputation: 2011

Are all types declared with newtype phantom types

I am learning about phantom types in Haskell. I know that phantom types are parameterised types whose parameters do not appear on the right hand side of its definition. However, I would like to know whether all types declared with newtype are phantom types.

When I try to compile a program with the following code:

newtype SpecialInt Int = Special Int

I get an error stating:

 Unexpected type ‘Int’
    In the newtype declaration for ‘SpecialInt’
    A newtype declaration should have form
      newtype SpecialInt a = ...

Which leads me to believe that newtypes are phantom types if the type parameter on the left hand side doesn't appear on the right hand side. E.g. 

newtype SpecialInt a = Special a

Would not be a phantom type, but

newtype SpecialInt a = Special Int

would be a phantom type. Therefore not all types declared with newtype are phantom types. However, I am not sure if my reasoning is correct.

Upvotes: 1

Views: 232

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476574

Which leads me to believe that newtypes are phantom types if the type parameter on the left hand side doesn't appear on the right hand side. E.g.

Well this is a consequence of your quoted definitions. You say that:

I know that phantom types are parameterised types whose parameters do not appear on the right hand side of its definition.

Since newtype is a way to define a type, and since for parameterized types with parameters that do not appear at the right side are phantom types, types originating from a newtype clause with a type parameter that do not appear at the right side are phantom types as well.

I would like to know whether all types declared with newtype are phantom types.

No, since for example a newtype without any type parameter, or a newtype with type parameters where are type parameters are used at the right side are not phantom types.

When I try to compile a program with the following code:

newtype SpecialInt Int = Special Int

I get an error (..)

Type variables at the left side are, well, variables. Int is not a variable (it starts with an uppercase), so the above does not make much sense. If you want to define a SpecialInt, you should define this as:

newtype SpecialInt = Special Int

so without an Int at the left side of the = token.

Upvotes: 3

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