InvalidNewtype Error

Example

newtype NotIdentity a = a

newtype NotTuple a b = NotTuple a b

Cause

Newtypes must define a single constructor with a single argument, so the two most common sources of this error are having multiple arguments (like a Tuple) or forgetting to provide a constructor.

Fix

• If you need multiple arguments, consider using a newtype around a record or Tuple:

newtype RecordTuple a b = RecordTuple {fst :: a, snd :: b}