-- Hoogle documentation, generated by Haddock -- See Hoogle, http://www.haskell.org/hoogle/ -- | Unsatisfiable type class -- -- This package provides a -- -- 
--   class Unsatisfiable (msg :: TypeError)
--
-- -- and -- -- 
--   unsatisfiable :: forall msg a. Unsatisfiable msg => a
--
-- -- combinator. -- -- There is also a plugin which -- -- -- -- The plugin also works with -fdefer-type-errors. @package unsatisfiable @version 0 -- | Unsatisfiable type-class. module Unsatisfiable.Class -- | Unsatisfiable type-class. -- -- This a common idiom, to have a type-class with a non-exported member. -- This class cannot be ever instantiated, and we can use it to "forbid" -- some instances from existence, by defining an instance with -- unsatisfiable constraint. -- -- The unsatisfiable acts as better undefined, using this -- type-class constraint. -- -- The behaviour of this class would be rather better defined than -- instantiating the polymorphic TypeError to kind -- Constraint, because it is clear when to report the custom -- error: instead of an unsolved constraint error, when the constraint -- solver fails to solve a wanted Unsatisfiable msg. -- -- The custom error reporting is done using plugin type-checker -- plugin. -- -- See discussion in GHC#18310. -- -- There are some examples of using this class and plugin at -- https://github.com/phadej/unsatisfiable/tree/master/unsatisfiable-examples/examples class Unsatisfiable (msg :: ErrorMessage) -- | See Unsatisfiable. -- -- If you need some other TYPE, you can use case trick -- by instantiating unsatisfiable to Void and using -- EmptyCase. -- -- 
--   case unsatisfiable @msg @Void of {}
--
unsatisfiable :: forall msg a. Unsatisfiable msg => a -- | A type-checker plugin for Unsatisfiable type class. module Unsatisfiable.Plugin -- | To use this plugin add -- -- 
--   {-# OPTIONS_GHC -fplugin=Unsatisfiable #-}
--
-- -- to your source file. -- -- This plugin does two things: -- -- Firstly, when there is wanted Unsatisfiable constraint, -- we pretty-pring its message. Unfortunately the actual No instance -- for (Unsatisfiable msg) error is also printed, as we don't solve -- it, except when -fdefer-type-errors is enabled. In that case -- unsatisfiable will throw an error with the rendered message. -- -- Secondly, when Unsatisfiable constraint is given, all other -- constraints are solved automatically using unsatisfiable as -- the evidence. This is useful -- -- 
--   class C a => D a
--   instance Unsatisfiable Msg => D Int  -- Note absence of C Int in the context
--
-- -- The motivation here is that if we use Unsatisfiable to ban an -- instance with a nice error message, we don't want to get extra error -- messages arising from failure to solve the superclass constraints, -- which we get if we are obliged to use -- -- 
--   instance (Unsatisfiable Msg, C Int) => D Int
--
-- -- When GHC looks for valid type hole fits this plugin might print some -- errors. You may disable them with -fno-show-valid-hole-fits -- ghc option. plugin :: Plugin -- | Unsatisfiable type class and plugin to empower it. module Unsatisfiable -- | Unsatisfiable type-class. -- -- This a common idiom, to have a type-class with a non-exported member. -- This class cannot be ever instantiated, and we can use it to "forbid" -- some instances from existence, by defining an instance with -- unsatisfiable constraint. -- -- The unsatisfiable acts as better undefined, using this -- type-class constraint. -- -- The behaviour of this class would be rather better defined than -- instantiating the polymorphic TypeError to kind -- Constraint, because it is clear when to report the custom -- error: instead of an unsolved constraint error, when the constraint -- solver fails to solve a wanted Unsatisfiable msg. -- -- The custom error reporting is done using plugin type-checker -- plugin. -- -- See discussion in GHC#18310. -- -- There are some examples of using this class and plugin at -- https://github.com/phadej/unsatisfiable/tree/master/unsatisfiable-examples/examples class Unsatisfiable (msg :: ErrorMessage) -- | See Unsatisfiable. -- -- If you need some other TYPE, you can use case trick -- by instantiating unsatisfiable to Void and using -- EmptyCase. -- -- 
--   case unsatisfiable @msg @Void of {}
--
unsatisfiable :: forall msg a. Unsatisfiable msg => a -- | To use this plugin add -- -- 
--   {-# OPTIONS_GHC -fplugin=Unsatisfiable #-}
--
-- -- to your source file. -- -- This plugin does two things: -- -- Firstly, when there is wanted Unsatisfiable constraint, -- we pretty-pring its message. Unfortunately the actual No instance -- for (Unsatisfiable msg) error is also printed, as we don't solve -- it, except when -fdefer-type-errors is enabled. In that case -- unsatisfiable will throw an error with the rendered message. -- -- Secondly, when Unsatisfiable constraint is given, all other -- constraints are solved automatically using unsatisfiable as -- the evidence. This is useful -- -- 
--   class C a => D a
--   instance Unsatisfiable Msg => D Int  -- Note absence of C Int in the context
--
-- -- The motivation here is that if we use Unsatisfiable to ban an -- instance with a nice error message, we don't want to get extra error -- messages arising from failure to solve the superclass constraints, -- which we get if we are obliged to use -- -- 
--   instance (Unsatisfiable Msg, C Int) => D Int
--
-- -- When GHC looks for valid type hole fits this plugin might print some -- errors. You may disable them with -fno-show-valid-hole-fits -- ghc option. plugin :: Plugin