Multiple newtype constructors [GHC-05380]

A newtype constructor can only have a single constructor. If you want multiple constructors, you should use data instead of newtype.

The reason that you need exactly one constructor is that newtype constructors are removed at runtime. For example, if you define newtype Age = Age Int, the bits in memory that represent an Age are identical to those that would represent the Int in the constructor. The Age and Int types are distinguished only at compile time. If types defined with newtype could have multiple constructors, there would be no way to distinguish them during execution.

Examples

More than 1 constructor for a newtype

Error Message

Main.hs:1:1: error: [GHC-05380]
    A newtype must have exactly one constructor,
      but ‘Direction’ has two
    In the newtype declaration for ‘Direction’
  |
1 | newtype Direction = Up | Down
  |

Explanation

A newtype constructor can only have a single constructor. It is not allowed to have multiple constructors when defining a newtype.

More-than-1-constructor.hs

Before

newtype Direction = Up | Down

After

data Direction = Up | Down

Zero constructors for a newtype

Error Message

Main.hs:1:1: error: [GHC-05380]
    A newtype must have exactly one constructor, but ‘Void’ has none
    In the newtype declaration for ‘Void’
  |
1 | newtype Void
  |

Explanation

A newtype definition is required to have exactly one constructor, so constructorless newtypes are not allowed. Datatypes defined using data are allowed to be constructorless, so this problem can be fixed either by adding a constructor to the newtype or by changing the newtype keyword to a data keyword.

Zero-constructors.hs

Before

newtype Void

After

data Void