ConstraintPassing.hs

{-# LANGUAGE ConstraintKinds, DataKinds, FlexibleContexts, FlexibleInstances,
             MultiParamTypeClasses, OverlappingInstances, TypeFamilies,
             UndecidableInstances #-}
{- |
   Module      : Control.Monad.Levels.ConstraintPassing
   Description : Whether a transformer allows a constraint to pass through
   Copyright   : (c) Ivan Lazar Miljenovic
   License     : 3-Clause BSD-style
   Maintainer  : Ivan.Miljenovic@gmail.com

This module is defined separately as it's the only one that uses the
@OverlappingInstances@ extension.

 -}
module Control.Monad.Levels.ConstraintPassing where

import Control.Monad.Levels.Definitions

-- -----------------------------------------------------------------------------

-- | Indicates whether a specified constraint is allowed to pass
--   through a particular level.
--
--   (It may not be recognisable in Haddock documentation, but the @b@
--   parameter is of kind @'Bool'@ using the @DataKinds@ extension).
--
--   By default, for all monad levels this is set to the value of
--   'DefaultAllowConstraints' for all constraints, with the exception
--   of 'IsBaseMonad' for which it is set to 'True'.
--
--   Instances of this class can - and should when appropriate - be
--   overlapped\/overriden.
class (ValidConstraint c, MonadLevel m) => ConstraintPassThrough c m (b :: Bool)

instance (ValidConstraint c, MonadLevel m, DefaultAllowConstraints m ~ b)
          => ConstraintPassThrough c m b

instance (MonadLevel m) => ConstraintPassThrough IsBaseMonad m True