Data.Class.Closed

Closed type classes are defined by adding a private field with a default implementation that requires a Closed constraint. For example:

{-# LANGUAGE DefaultSignatures, FlexibleContexts #-}

module Foo (Foo) where

class Foo a where
  default _private :: Closed Foo => Proxy a
  _private :: Proxy a
  _private = Proxy

instance Foo Int where
  _private = Proxy

Since _private is not exported, any new instance defined outside the Foo module will be forced to use the default implementation, which has the unsatisfiable Closed Foo constraint.

module Bar where

import Foo

instance Foo Char -- • User defined Foo instances are not allowed.

Thus new instances can only be defined in the same module as Foo, where _private is visible. Note that Closed itself is a closed type class, implemented using the same trick!

To automatically generate the boilerplate, see Data.Class.Closed.TH.

Documentation

class Closed (a :: k)

Closed constraints are unsatisfiable. Attempting to resolve a Closed <class> constraint will result in

• User defined <class> instances are not allowed.

Instances

(TypeError (('Text "User defined " :<>: 'ShowType a) :<>: 'Text " instances are not allowed.") :: Constraint) => Closed (a :: k)

Defined in Data.Class.Closed

Methods

_private :: Proxy a