#11174: Traversable can't be derived for datatypes with unboxed arguments -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: RyanGlScott Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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: | -------------------------------------+------------------------------------- Description changed by RyanGlScott: Old description:

Unlike `Functor` and `Foldable`, `Traversable` cannot be derived for datatypes that contain arguments with unboxed types. A simple example:

{{{#!hs {-# LANGUAGE DeriveTraversable, MagicHash #-}

import GHC.Prim (Int#)

data IntHash a = IntHash Int# deriving (Functor, Foldable, Traversable) }}}

The generated `Traversable` instance reveals the issue:

{{{#!hs instance Traversable IntHash where traverse f (IntHash a1) = fmap IntHash (pure a1) }}}

{{{ Couldn't match kind `*' with `#' When matching types a0 :: * Int# :: # Expected type: a0 -> IntHash b Actual type: Int# -> IntHash b In the first argument of `fmap', namely `IntHash' In the expression: fmap IntHash (pure a1) When typechecking the code for `traverse' in a derived instance for `Traversable IntHash': To see the code I am typechecking, use -ddump-deriv }}}

We have to avoid calling `pure` on `a1`, since `pure` expects an argument with a `*`-kinded type, not a `#`-kinded one.

One way to fix this would be restructuring the derived `traverse` implementation such that unboxed arguments are moved to the function initially lifted with `pure`, and doing nothing with them later. To better articulate what I mean, envision something like this:

{{{#!hs data IntHash2 a = IntHash2 Int# a (IntHash2 a) Int# deriving (Functor, Foldable) }}}

Then a derived `Traversable` instance that would typecheck would be:

{{{#!hs instance Traversable IntHash2 where traverse f (IntHash2 a1 a2 a3 a4) = pure (\x2 x3 -> IntHash2 a1 x2 x3 a4) <*> f a2 <*> traverse f a3 }}}

Conceptually, this doesn't sound hard to implement. The tricky part is figuring out how much of the existing `Functor`/`Foldable`/`Traversable` deriving machinery would need to be tweaked to make this work.

New description: Unlike `Functor` and `Foldable`, `Traversable` cannot be derived for datatypes that contain arguments with unboxed types. A simple example: {{{#!hs {-# LANGUAGE DeriveTraversable, MagicHash #-} import GHC.Prim (Int#) data IntHash a = IntHash Int# deriving (Functor, Foldable, Traversable) }}} The generated `Traversable` instance reveals the issue: {{{#!hs instance Traversable IntHash where traverse f (IntHash a1) = fmap IntHash (pure a1) }}} {{{ Couldn't match kind `*' with `#' When matching types a0 :: * Int# :: # Expected type: a0 -> IntHash b Actual type: Int# -> IntHash b In the first argument of `fmap', namely `IntHash' In the expression: fmap IntHash (pure a1) When typechecking the code for `traverse' in a derived instance for `Traversable IntHash': To see the code I am typechecking, use -ddump-deriv }}} We have to avoid calling `pure` on `a1`, since `pure` expects an argument with a `*`-kinded type, not a `#`-kinded one. One way to fix this would be restructuring the derived `traverse` implementation such that arguments which do not mention the last type parameter are moved to the function initially lifted with `pure`, and doing nothing with them later. To better articulate what I mean, envision something like this: {{{#!hs data IntHash2 a = IntHash2 Int# a (IntHash2 a) Int deriving (Functor, Foldable) }}} Then a derived `Traversable` instance that would type-check (and kind- check) would be: {{{#!hs instance Traversable IntHash2 where traverse f (IntHash2 a1 a2 a3 a4) = pure (\x2 x3 -> IntHash2 a1 x2 x3 a4) <*> f a2 <*> traverse f a3 }}} Conceptually, this doesn't sound hard to implement. The tricky part is figuring out how much of the existing `Functor`/`Foldable`/`Traversable` deriving machinery would need to be tweaked to make this work. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11174#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11174: Traversable can't be derived for datatypes with unboxed arguments -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: RyanGlScott Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1908 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D1908 Comment: I've uploaded a Phab:D1908, based on the design philosophy outlined in [https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFunctor#Prop... this wiki page]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11174#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11174: Traversable can't be derived for datatypes with unboxed arguments -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: RyanGlScott Type: bug | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: Generics Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1908 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"a82956df5b34175410e0feb9e2febe7d39b60b49/ghc" a82956d/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="a82956df5b34175410e0feb9e2febe7d39b60b49" Remove superfluous code when deriving Foldable/Traversable Currently, `-XDeriveFoldable` and `-XDeriveTraversable` generate unnecessary `mempty` and `pure` expressions when it traverses of an argument of a constructor whose type does not mention the last type parameter. Not only is this inefficient, but it prevents `Traversable` from being derivable for datatypes with unlifted arguments (see Trac #11174). The solution to this problem is to adopt a slight change to the algorithms for `-XDeriveFoldable` and `-XDeriveTraversable`, which is described in [this wiki page](https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFu nctor#Proposal:alternativestrategyforderivingFoldableandTraversable). The wiki page also describes why we don't apply the same changes to the algorithm for `-XDeriveFunctor`. This is techincally a breaking change for users of `-XDeriveFoldable` and `-XDeriveTraversable`, since if someone was using a law-breaking `Monoid` instance with a derived `Foldable` instance (i.e., one where `x <> mempty` does not equal `x`) or a law-breaking `Applicative` instance with a derived `Traversable` instance, then the new generated code could result in different behavior. I suspect the number of scenarios like this is very small, and the onus really should be on those users to fix up their `Monoid`/`Applicative` instances. Fixes #11174. Test Plan: ./validate Reviewers: hvr, simonpj, austin, bgamari Reviewed By: simonpj, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1908 GHC Trac Issues: #11174 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11174#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11174: Traversable can't be derived for datatypes with unboxed arguments -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: RyanGlScott Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1908 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: Generics => deriving -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11174#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler