#14322: Simplifying an instance context makes a rewrite rule no longer typecheck -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This code (taken from the `reducers` package) compiles: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} import Prelude (Applicative(..), Functor(..), (.)) class Semigroup m where (<>) :: m -> m -> m class Semigroup m => Reducer c m where snoc :: m -> c -> m newtype Traversal f = Traversal { getTraversal :: f () } instance Applicative f => Semigroup (Traversal f) where Traversal a <> Traversal b = Traversal (a *> b) instance Applicative f => Reducer (f a) (Traversal f) where Traversal a `snoc` b = Traversal (() <$ (a *> b)) snocTraversal :: Reducer (f ()) (Traversal f) => Traversal f -> f () -> Traversal f snocTraversal a = (<>) a . Traversal {-# RULES "snocTraversal" snoc = snocTraversal #-} }}} But on GHC 8.2.1 and later, it gives this warning: {{{ GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:21:18: warning: [-Wsimplifiable-class-constraints] • The constraint ‘Reducer (f ()) (Traversal f)’ matches an instance declaration instance Applicative f => Reducer (f a) (Traversal f) -- Defined at Bug.hs:18:10 This makes type inference for inner bindings fragile; either use MonoLocalBinds, or simplify it using the instance • In the type signature: snocTraversal :: Reducer (f ()) (Traversal f) => Traversal f -> f () -> Traversal f | 21 | snocTraversal :: Reducer (f ()) (Traversal f) => Traversal f -> f () -> Traversal f | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} I decided to follow GHC's orders and reduce the `Reducer (f ()) (Traversal f)` context to just `Applicative f`: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} import Prelude (Applicative(..), Functor(..), (.)) class Semigroup m where (<>) :: m -> m -> m class Semigroup m => Reducer c m where snoc :: m -> c -> m newtype Traversal f = Traversal { getTraversal :: f () } instance Applicative f => Semigroup (Traversal f) where Traversal a <> Traversal b = Traversal (a *> b) instance Applicative f => Reducer (f a) (Traversal f) where Traversal a `snoc` b = Traversal (() <$ (a *> b)) snocTraversal :: Applicative f => Traversal f -> f () -> Traversal f snocTraversal a = (<>) a . Traversal {-# RULES "snocTraversal" snoc = snocTraversal #-} }}} But after doing so, the file no longer typechecks! {{{ GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:23:34: error: • Could not deduce (Applicative f) arising from a use of ‘snocTraversal’ from the context: Reducer (f ()) (Traversal f) bound by the RULE "snocTraversal" at Bug.hs:23:11-46 Possible fix: add (Applicative f) to the context of the RULE "snocTraversal" • In the expression: snocTraversal When checking the transformation rule "snocTraversal" | 23 | {-# RULES "snocTraversal" snoc = snocTraversal #-} | ^^^^^^^^^^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14322 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler