#12797: Default Rules stop working when providing some constraints -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | 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: | -------------------------------------+------------------------------------- Description changed by danilo2: @@ -7,1 +7,0 @@ - {-# LANGUAGE OverloadedStrings #-} @@ -21,1 +20,1 @@ - liftIO $ print "tst" + liftIO $ print 6 @@ -25,1 +24,1 @@ - liftIO $ print "tst" + liftIO $ print 6 @@ -34,0 +33,1 @@ + @@ -46,1 +46,1 @@ - ...plus 7 instances involving out-of-scope types + ...plus six instances involving out-of-scope types @@ -48,3 +48,3 @@ - • In the second argument of ‘($)’, namely ‘print "tst"’ - In a stmt of a 'do' block: liftIO $ print "tst" - In the expression: do { liftIO $ print "tst" } + • In the second argument of ‘($)’, namely ‘print 6’ + In a stmt of a 'do' block: liftIO $ print 6 + In the expression: do { liftIO $ print 6 } @@ -53,2 +53,1 @@ - • Could not deduce (Data.String.IsString a0) - arising from the literal ‘"tst"’ + • Could not deduce (Num a0) arising from the literal ‘6’ @@ -61,3 +60,4 @@ - instance a ~ Char => Data.String.IsString [a] - -- Defined in ‘Data.String’ - ...plus one instance involving out-of-scope types + instance Num Integer -- Defined in ‘GHC.Num’ + instance Num Double -- Defined in ‘GHC.Float’ + instance Num Float -- Defined in ‘GHC.Float’ + ...plus two others @@ -65,3 +65,3 @@ - • In the first argument of ‘print’, namely ‘"tst"’ - In the second argument of ‘($)’, namely ‘print "tst"’ - In a stmt of a 'do' block: liftIO $ print "tst" + • In the first argument of ‘print’, namely ‘6’ + In the second argument of ‘($)’, namely ‘print 6’ + In a stmt of a 'do' block: liftIO $ print 6 @@ -71,1 +71,1 @@ - Giving explicit types to String literals fixes the problem. + Giving explicit types to literals fixes the problem. New description: I've just found a very strange behavior. Let's consider following program: {{{ {-# LANGUAGE NoMonomorphismRestriction #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ExtendedDefaultRules #-} module Main where import Prelude import Control.Monad.IO.Class type family FuncArg m where FuncArg ((->) t) = 'Just t FuncArg m = 'Nothing test1 :: (MonadIO m) => m () test1 = do liftIO $ print 6 test2 :: (MonadIO m, FuncArg m ~ 'Nothing) => m () test2 = do liftIO $ print 6 main :: IO () main = return () }}} The function `tst1` compiles fine, while `tst2` fails: {{{ exe/Main.hs:21:14: error: • Could not deduce (Show a0) arising from a use of ‘print’ from the context: (MonadIO m, FuncArg m ~ 'Nothing) bound by the type signature for: tst2 :: (MonadIO m, FuncArg m ~ 'Nothing) => m () at exe/Main.hs:19:1-49 The type variable ‘a0’ is ambiguous These potential instances exist: instance Show Ordering -- Defined in ‘GHC.Show’ instance Show Integer -- Defined in ‘GHC.Show’ instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’ ...plus 22 others ...plus six instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the second argument of ‘($)’, namely ‘print 6’ In a stmt of a 'do' block: liftIO $ print 6 In the expression: do { liftIO $ print 6 } exe/Main.hs:21:20: error: • Could not deduce (Num a0) arising from the literal ‘6’ from the context: (MonadIO m, FuncArg m ~ 'Nothing) bound by the type signature for: tst2 :: (MonadIO m, FuncArg m ~ 'Nothing) => m () at exe/Main.hs:19:1-49 The type variable ‘a0’ is ambiguous These potential instances exist: instance Num Integer -- Defined in ‘GHC.Num’ instance Num Double -- Defined in ‘GHC.Float’ instance Num Float -- Defined in ‘GHC.Float’ ...plus two others (use -fprint-potential-instances to see them all) • In the first argument of ‘print’, namely ‘6’ In the second argument of ‘($)’, namely ‘print 6’ In a stmt of a 'do' block: liftIO $ print 6 }}} Giving explicit types to literals fixes the problem. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12797#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler