Difference between `data` and `newtype` in Haskell

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臣服心动
臣服心动 2020-11-27 08:49

What is the difference when I write this?

data Book = Book Int Int

versus

newtype Book = Book (Int, Int) -- \"Book Int Int\         


        
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  • 2020-11-27 09:44

    Great question!

    There are several key differences.

    Representation

    • A newtype guarantees that your data will have exactly the same representation at runtime, as the type that you wrap.
    • While data declares a brand new data structure at runtime.

    So the key point here is that the construct for the newtype is guaranteed to be erased at compile time.

    Examples:

    • data Book = Book Int Int

    data

    • newtype Book = Book (Int, Int)

    newtype

    Note how it has exactly the same representation as a (Int,Int), since the Book constructor is erased.

    • data Book = Book (Int, Int)

    data tuple

    Has an additional Book constructor not present in the newtype.

    • data Book = Book {-# UNPACK #-}!Int {-# UNPACK #-}!Int

    enter image description here

    No pointers! The two Int fields are unboxed word-sized fields in the Book constructor.

    Algebraic data types

    Because of this need to erase the constructor, a newtype only works when wrapping a data type with a single constructor. There's no notion of "algebraic" newtypes. That is, you can't write a newtype equivalent of, say,

    data Maybe a = Nothing
                 | Just a
    

    since it has more than one constructor. Nor can you write

    newtype Book = Book Int Int
    

    Strictness

    The fact that the constructor is erased leads to some very subtle differences in strictness between data and newtype. In particular, data introduces a type that is "lifted", meaning, essentially, that it has an additional way to evaluate to a bottom value. Since there's no additional constructor at runtime with newtype, this property doesn't hold.

    That extra pointer in the Book to (,) constructor allows us to put a bottom value in.

    As a result, newtype and data have slightly different strictness properties, as explained in the Haskell wiki article.

    Unboxing

    It doesn't make sense to unbox the components of a newtype, since there's no constructor. While it is perfectly reasonable to write:

    data T = T {-# UNPACK #-}!Int
    

    yielding a runtime object with a T constructor, and an Int# component. You just get a bare Int with newtype.


    References:

    • "Newtype" on the Haskell wiki
    • Norman Ramsey's answer about the strictness properties
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