[GHC] #14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- 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: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This program doesn't typecheck due to the monomorphism restriction: {{{#!hs f = (==) }}} In GHC 8.0.2, the error message was quite helpful in informing you of this fact: {{{ GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:1:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘==’ prevents the constraint ‘(Eq a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Eq Ordering -- Defined in ‘GHC.Classes’ instance Eq Integer -- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’ instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’ ...plus 22 others ...plus 7 instances involving out-of-scope types (use -fprint-potential-instances to see them all) • When instantiating ‘f’, initially inferred to have this overly-general type: forall a. Eq a => a -> a -> Bool NB: This instantiation can be caused by the monomorphism restriction. }}} Notice the `NB: This instantiation can be caused by the monomorphism restriction` part. But in GHC 8.2.1, this advice has mysteriously vanished! {{{ 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:1:5: error: • Ambiguous type variable ‘a0’ arising from a use of ‘==’ prevents the constraint ‘(Eq a0)’ from being solved. Relevant bindings include f :: a0 -> a0 -> Bool (bound at Bug.hs:1:1) Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Eq Ordering -- Defined in ‘GHC.Classes’ instance Eq Integer -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’ instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’ ...plus 22 others ...plus 9 instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the expression: (==) In an equation for ‘f’: f = (==) | 1 | f = (==) | ^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: simonpj (added) Comment: This regression occurred in commit 3f5673f34a2f761423027bf46f64f7499708725f (`A collection of type-inference refactorings.`). Simon, do you recall why you changed this error message? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): It was an ad-hoc special case before. This is tricky to report well. For example {{{ f = Just (==) }}} does not give that hint. And {{{ f = (==) foo = (f 'c', f True) }}} gives {{{ • Couldn't match expected type ‘Char’ with actual type ‘Bool’ • In the first argument of ‘f’, namely ‘True’ }}} Ironically, we do generate a warning {{{ Foo.hs:7:1: warning: The Monomorphism Restriction applies to the binding for ‘f’ Consider giving it a type signature }}} but since it's a warning it is suppressed when there is an error. Maybe errors should not suppress warnings? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I'm OK with not being able to report every possible occurrences of the monomorphism restriction. But as you've demonstrated, GHC clearly //can// detect that the monorphism restriction is kicking in here (in a warning)—can't we just copy that message in the error? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
can't we just copy that message in the error
But ''which'' errors? The two sites are quite separate. E.g. the Char/Bool error is much later. I'm sure there are improvements to be had here; just not sure exactly what and how. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14368: GHC 8.2.1 doesn't inform you when the monomorphism restriction kicks in anymore -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => invalid Comment: Oh, I see what you mean. This program: {{{#!hs f = (==) foo = (f 'c', f True) }}} //only// emits an error about `foo`, and doesn't separately error for `f`. Blast. In that case, I agree that this is probably not worth the effort to implement. I'll close as invalid (but feel free to reopen if you can think of a feasible way to do this.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14368#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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