[GHC] #10016: UNPACK support for existentials
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.4 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Consider {{{ data Foo = forall a. Show a => Foo !a !a data Bar = Bar {-# UNPACK #-} !Foo }}} The UNPACK pragma should really be able to unpack the `Foo` field, so that (implementation-wise) it would be as if you'd declared `Bar` thus: {{{ data Bar = forall a. Show a => Bar !a !a }}} But currently we get {{{ Ignoring unusable UNPACK pragma on the first argument of ‘Bar’ }}} Suggested by Nicholas Clarke [https://www.haskell.org/pipermail/glasgow- haskell-users/2015-January/025590.html here]. Vaguely related tickets: #7647, #9214. But unlike those ones, this one looks quite feasible to me. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by vagarenko): * cc: vagarenko (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by thomie): * failure: None/Unknown => Runtime performance bug -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): See also #1965 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: DataCons Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => DataCons Old description:
Consider {{{ data Foo = forall a. Show a => Foo !a !a
data Bar = Bar {-# UNPACK #-} !Foo }}} The UNPACK pragma should really be able to unpack the `Foo` field, so that (implementation-wise) it would be as if you'd declared `Bar` thus: {{{ data Bar = forall a. Show a => Bar !a !a }}} But currently we get {{{ Ignoring unusable UNPACK pragma on the first argument of ‘Bar’ }}} Suggested by Nicholas Clarke [https://www.haskell.org/pipermail/glasgow- haskell-users/2015-January/025590.html here].
Vaguely related tickets: #7647, #9214. But unlike those ones, this one looks quite feasible to me.
New description: Consider {{{ data Foo = forall a. Show a => Foo !a !a data Bar = Bar {-# UNPACK #-} !Foo }}} The UNPACK pragma should really be able to unpack the `Foo` field, so that (implementation-wise) it would be as if you'd declared `Bar` thus: {{{ data Bar = forall a. Show a => Bar !a !a }}} But currently we get {{{ Ignoring unusable UNPACK pragma on the first argument of ‘Bar’ }}} Suggested by Nicholas Clarke [https://www.haskell.org/pipermail/glasgow- haskell-users/2015-January/025590.html here]. Vaguely related tickets: #7647, #9214. But unlike those ones, this one looks quite feasible to me. Fixing this would require quite a bit of new plumbing. E.g. the existential type variable of the worker and wrapper would differ, which is not true today. But morally it's the Right Thing. I'd be interested in actual use-cases. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: DataCons Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): As far as I can tell, the only time this feature would be ''essential'' would be a situation where you want to unpack an existential and don't have access to its constructor. That makes the "killer app" a bit tricky to find. On the other hand, it would certainly support the sum-of-records pattern: {{{#!hs data Foo = This | That !Bar data Bar = Bar { ... } }}} This lets people use named fields in sum types without introducing partial functions. As it is, it won't be efficient if `Bar` has any existentials. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10016: UNPACK support for existentials -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: DataCons Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by dfeuer): * cc: dfeuer (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10016#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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