[GHC] #8392: Suggest AllowAmbiguousTypes
#8392: Suggest AllowAmbiguousTypes -------------------------------------------+------------------------------- Reporter: rwbarton | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- The error message I get with GHC HEAD on the example program in #8390: {{{ tf.hs:38:10: Could not deduce (Fun g a b0) arising from the ambiguity check for an instance declaration from the context (Fun f b c, Fun g a b) bound by an instance declaration: (Fun f b c, Fun g a b) => Fun (Compose f g) a c at tf.hs:38:10-56 The type variable ‛b0’ is ambiguous In the ambiguity check for: forall f g a c b. (Fun f b c, Fun g a b) => Fun (Compose f g) a c In the instance declaration for ‛Fun (Compose f g) a c’ }}} could perhaps benefit from an appended line: {{{ (To defer this ambiguity check to use sites, enable AllowAmbiguousTypes) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8392 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8392: Suggest AllowAmbiguousTypes --------------------------------------------+------------------------------ Reporter: rwbarton | Owner: monoidal Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by monoidal): * owner: => monoidal -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8392#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8392: Suggest AllowAmbiguousTypes --------------------------------------------+------------------------------ Reporter: rwbarton | Owner: monoidal Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by simonpj): Actually I'm in the area so I can do this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8392#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8392: Suggest AllowAmbiguousTypes --------------------------------------------+------------------------------ Reporter: rwbarton | Owner: simonpj Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by monoidal): * owner: monoidal => simonpj Comment: Simon: I'm passing to you. It's trivial to edit the error message in TcValidity.checkAmbiguity. However, this program {{{ f :: Bool ~ Int => () f = () }}} gives then {{{ Couldn't match type ‛Bool’ with ‛Int’ Inaccessible code in the type signature for f :: Bool ~ Int => () To defer this ambiguity check to use sites, enable AllowAmbiguousTypes In the ambiguity check for: Bool ~ Int => () In the type signature for ‛f’: f :: Bool ~ Int => () }}} and enabling AllowAmbiguousTypes merely changes it to {{{ Couldn't match type ‛Bool’ with ‛Int’ Inaccessible code in the type signature for f :: Bool ~ Int => () }}} So ambiguity check is done before checking inaccessible code, and it might be better to change this order. I don't know if this is simple or has any side effects though. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8392#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8392: Suggest AllowAmbiguousTypes
--------------------------------------------+------------------------------
Reporter: rwbarton | Owner: simonpj
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
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Comment (by Simon Peyton Jones
#8392: Suggest AllowAmbiguousTypes
--------------------------------------------+------------------------------
Reporter: rwbarton | Owner: simonpj
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
--------------------------------------------+------------------------------
Comment (by Simon Peyton Jones
#8392: Suggest AllowAmbiguousTypes -------------------------------------------------+------------------------- Reporter: rwbarton | Owner: Type: feature request | simonpj Priority: normal | Status: Component: Compiler (Type checker) | closed Resolution: fixed | Milestone: Operating System: Unknown/Multiple | Version: 7.7 Type of failure: None/Unknown | Keywords: Test Case: | Architecture: typecheck/should_fail/T8392a, | Unknown/Multiple typecheck/should_compile/T8392 | Difficulty: Blocking: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Changes (by simonpj): * status: new => closed * testcase: => typecheck/should_fail/T8392a, typecheck/should_compile/T8392 * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8392#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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