TypeSynonymInstance Error

Example

module TypeSynonymInstanceError where
import Prelude

type Mass = Number

instance showMass :: Show Mass where
  show mass = "Mass: " <> show mass <> "kg"

Cause

Type synonyms are merely aliases (usually created to avoid verbose type annotations) and do not declare a new unique type. It is still possible to use the synonym and its equivalent type interchangeably.

Since every value that structually matches the type synonym could be considered of the same type, it is not allowed to implement type classes based on type synonyms.

Fix

A newtype declaration generates a brand new type instead of introducing a synonym for an existing type. Newtypes can have type class instances declared for them:

newtype Mass = Mass Number

instance showMass :: Show Mass where
  show (Mass mass) = "Mass: " <> show mass <> "kg"

Notes

The same error occurs when the aliased type is a record:

type Point = { x :: Number, y :: Number }

instance showPoint :: Show Point where
  show p = "Point " <> show p.x <> ", " <> show p.y