[GHC] #12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor
#12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple LevityPolymorphism | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When I say {{{ data X where MkX :: forall (a :: TYPE r). (a -> a) -> X }}} I get {{{ • A representation-polymorphic type is not allowed here: Type: a Kind: TYPE r • In the definition of data constructor ‘MkX’ In the data type declaration for ‘X’ }}} But that's silly, because the type is not levity-polymorphic! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12911 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by josef): I think I hit upon this restriction when trying to implement some of the code from the levity polymorphism paper. The section on levity polymorphic type classes explains how the dictionary from levity polymorphic Num is perfectly safe. {{{ data Num (a :: TYPE r) = MkNum {(+) :: a → a → a, abs :: a → a} }}} But sadly GHC rejects the data type declaration above. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12911#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes, that's over-restrictive; when Richard gets around to implementing the paper (which is very much on his radar I think) it'll be fixed! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12911#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12911: Levity polymorphism check eliminates non-levity-polymorphic data
constructor
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Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.0.1
Resolution: | Keywords:
| LevityPolymorphism
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by Ben Gamari
#12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_compile/T12911 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * testcase: => typecheck/should_compile/T12911 * status: new => closed * resolution: => fixed Comment: Thanks for merging, Ben! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12911#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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