[GHC] #15800: Overlapping instances error with single instance
#15800: Overlapping instances error with single instance -------------------------------------+------------------------------------- Reporter: roland | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 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: -------------------------------------+------------------------------------- {{{ {-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, FlexibleInstances #-} module Bug where class C a b instance C a Int x :: () x = undefined :: C a Int => () }}} {{{ ghc -c Bug.hs Bug.hs:10:18: error: • Overlapping instances for C a0 Int Matching givens (or their superclasses): C a Int bound by an expression type signature: forall a. C a Int => () at Bug.hs:10:18-30 Matching instances: instance C a Int -- Defined at Bug.hs:7:10 (The choice depends on the instantiation of ‘a0’) • In the ambiguity check for an expression type signature To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In an expression type signature: C a Int => () In the expression: undefined :: C a Int => () | 10 | x = undefined :: C a Int => () | ^^^^^^^^^^^^^ }}} The "matching instances" bit of the error messages only lists a single instance. Doesn't it take at least two instances for something to overlap? Also, following the algorithm laid out in the user guide (section "Overlapping Instances"), it appears this program should be accepted. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15800 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15800: Overlapping instances error with single instance
-------------------------------------+-------------------------------------
Reporter: roland | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.6.1
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: |
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Comment (by Simon Peyton Jones
#15800: Overlapping instances error with single instance -------------------------------------+------------------------------------- Reporter: roland | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 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: | -------------------------------------+------------------------------------- Comment (by simonpj): I've tried to improve the docs. The key is that GHC looks at ''both'' top level instances ''and'' in-scope constraints, not just the former. In the error messages they are listed as "Matching instances" and "Matching givens" respectively. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15800#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15800: Overlapping instances error with single instance -------------------------------------+------------------------------------- Reporter: roland | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 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: | -------------------------------------+------------------------------------- Comment (by roland): Thanks, this indeed clarifies things! I was trying to trigger the search procedure with `C a Int` as the target constraint. It seems that for the ambiguity check it ended up not only as the target, but also as one of the givens, at which point the error is to be expected. Replacing `()` with `a` (thus making the type non-ambiguous) or adding the AllowAmbiguousTypes pragma both make the program type-check. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15800#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15800: Overlapping instances error with single instance -------------------------------------+------------------------------------- Reporter: roland | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Resolution: fixed | 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: | -------------------------------------+------------------------------------- Changes (by roland): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15800#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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