#13117: Derived functor instance for void types handles errors badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: 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 andreas.abel): What should the implementation of `fmap` be here? {{{#!hs fmap _ x = x `seq` error "Void fmap" }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13117#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13117: Derived functor instance for void types handles errors badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #7401 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #7401 Comment: The discussion is at #7401, which proposed an overhaul of the way derived instances for void types work. Ultimately, a consensus was never reached as to how derived `Eq` instances should work. See https://ghc.haskell.org/trac/ghc/ticket/7401#comment:46. If someone would be willing revive that discussion and advocate for a position, I'd be very appreciative, as I'm hesitant to poke that sleeping dragon again :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13117#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13117: Derived functor instance for void types handles errors badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #7401 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by David Feuer ): In [changeset:"69f070d8e4d6043937e3405675ac911448bfcb44/ghc" 69f070d/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="69f070d8e4d6043937e3405675ac911448bfcb44" Deriving for phantom and empty types Make `Functor`, `Foldable`, and `Traversable` take advantage of the case where the type parameter is phantom. In this case, * `fmap _ = coerce` * `foldMap _ _ = mempty` * `traverse _ x = pure (coerce x)` For the sake of consistency and especially simplicity, make other types with no data constructors behave the same: * `fmap _ x = case x of` * `foldMap _ _ = mempty` * `traverse _ x = pure (case x of)` Similarly, for `Generic`, * `to x = case x of` * `from x = case x of` Give all derived methods for types without constructors appropriate arities. For example, ``` compare _ _ = error ... ``` rather than ``` compare = error ... ``` Fixes #13117 and #13328 Reviewers: austin, bgamari, RyanGlScott Reviewed By: RyanGlScott Subscribers: ekmett, RyanGlScott, rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D3374 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13117#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13117: Derived functor instance for void types handles errors badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #7401, #10577 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: #7401 => #7401, #10577 Comment: Indeed. However, this ticket specifically concerns `Functor`, right? It might be best to tackle the other classes in #10577. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13117#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13117: Derived functor instance for void types handles errors badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature request | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #7401, #10577 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13117#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler