Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`
David Feuer
david.feuer at gmail.com
Sun Sep 27 22:16:35 UTC 2015
For the benefit of people trying out code from papers, tutorials, etc.,
would it be possible to craft a permanent custom error message for explicit
return definitions? I don't remember AMP having such for the new
constraint; if it doesn't, maybe it should.
On Sep 24, 2015 5:43 PM, "Herbert Valerio Riedel" <hvr at gnu.org> wrote:
> Hello *,
>
> Concluding AMP and MFP, We (David and I) proudly present you the final
> installment of the Monad trilogy:
>
>
> Monad of no `return` Proposal
> =============================
>
> TLDR: To complete the AMP, turn `Monad(return)` method into a
> top-level binding aliasing `Applicative(pure)`.
>
>
> Current Situation
> -----------------
>
> With the implementation of Functor-Applicative-Monad Proposal (AMP)[1] and
> (at some point) the MonadFail proposal (MFP)[2] the AMP class hierarchy
> becomes
>
>
> class Functor f where
> fmap :: (a -> b) -> f a -> f b
>
>
> class Functor f => Applicative f where
> pure :: a -> f a
> (<*>) :: f (a -> b) -> f a -> f b
>
> (*>) :: f a -> f b -> f b
> u *> v = …
>
> (<*) :: f a -> f b -> f a
> u <* v = …
>
>
> class Applicative m => Monad m where
> (>>=) :: m a -> (a -> m b) -> m b
>
> return :: a -> m a
> return = pure
>
> (>>) :: m a -> m b -> m b
> m >> k = …
>
>
> class Monad m => MonadFail m where
> fail :: String -> m a
>
>
> Consequently, the `Monad` class is left with a now redundant `return`
> method as a historic artifact, as there's no compelling reason to
> have `pure` and `return` implemented differently.
>
> Traditionally, `return` is often used where `pure` would suffice
> today, forcing a `Monad` constraint even if a weaker `Applicative`
> would have sufficed.
>
> As a result, language extensions like `ApplicativeDo`[3] have to
> rewrite `return` to weaken its `Monad m =>` constraint to
> `Applicative m =>` in order to benefit existing code at the cost
> of introducing magic behavior at the type level.
>
> Finally, this redundancy becomes even more significant when viewed in
> light of the renewed Haskell standardisation process[7]: The next
> Haskell Report will almost certainly incorporate the AMP (and MFP)
> changes, and there's no justification for the Report to retain
> `return` as a method of `Monad`. A good reason would have been to
> retain backward compatibility with Haskell 2010. However, as the AMP
> superclass hierarchy requires `Monad` instances to be accompanied by
> `Applicative` instances (which aren't part of Haskell 2010, c.f. [6]),
> backward compatibility with Haskell 2010 goes out the window when it
> comes to defining `Monad` instances (unless via use of `-XCPP` or
> similar). Consequently, meeting the high bar for a formal document
> such as the Haskell Report demands that `Monad` shall not carry a
> redundant `return` method that serves no purpose anymore. Moreover,
> getting `return` out of the way is desirable to facilitate
> standardising potential candidates such as the earlier mentioned
> `ApplicativeDo` in the future and avoids the technical debt incurred
> by keeping around this language wart.
>
>
> Proposed Change
> ---------------
>
> Remove `return` as a method from the `Monad` class and in its place
> define a top-level binding with the weaker `Applicative` typeclass
> constraint:
>
>
> -- | Legacy alias for 'pure'
> return :: Applicative f => a -> f a
> return = pure
>
>
> This allows existing code using `return` to benefit from a weaker
> typeclass constraint as well as cleaning the `Monad` class from a
> redundant method in the post-AMP world.
>
> A possible migration strategy is described further below.
>
>
> Compatibility Considerations
> ----------------------------
>
> Generalizing the type signature of a function from a `Monad`
> constraint to its superclass `Applicative` doesn't cause new
> type-errors in existing code.
>
> However, moving a method to a top-level binding obviously breaks code
> that assumes `return` to be a class method. Foremost, code that
> defines `Monad` instances it at risk:
>
> ### Instance Definitions
>
> Code defining `return` as part of an instance definition
> breaks. However, we had the foresight to provide a default
> implementation in `base-4.8` for `return` so that the following
> represents a proper minimal instance definition post-AMP:
>
>
> instance Functor Foo where
> fmap g foo = …
>
> instance Applicative Foo where
> pure x = …
> a1 <*> a2 = …
>
> instance Monad Foo where
> m >>= f = …
>
> -- NB: No mention of `return`
>
>
> Consequently, it is possible to write forward-compatible instances
> that are valid under this proposal starting with GHC 7.10/`base-4.8`.
>
> Heuristically `grep`ing through Hackage source-code reveals a
> non-negligible number of packages defining `Monad` instances with
> explicit `return` definitions[4]. This has a comparable impact to the
> AMP, and similarly will require a transition scheme aided by compiler
> warnings.
>
> ### Module Import/Export Specifications
>
> A second source of incompatibility may be due to
> `import`s. Specifically module import that assert `return` to be a
> method of `Monad`, e.g.:
>
> import Control.Monad (Monad ((>>=), return))
>
> or
>
> import Prelude hiding (Monad(..))
> import Control.Monad (Monad(..)) as Monad
>
> f = Monad.return ()
>
> The dual situation can occur when re-exporting `return` via module
> export specifications.
>
> However, given that `return` is exported by `Prelude` and the examples
> above are rather artificial, we don't expect this to be a major source
> of breakage in the case of `return`. In fact, a heuristic grep[5] over
> Hackage source-code revealed only 21 packages affected.
>
> ### Example for writing compatible code
>
>
> instance Functor Foo where
> fmap g foo = …
>
> instance Applicative Foo where
> pure x = …
> a1 <*> a2 = …
>
> instance Monad Foo where
> m >>= f = …
>
> #if !(MIN_VERSION_base(4,8,0))
> return = pure
> #endif
>
>
> Migration Strategy
> ------------------
>
> The migration strategy is straightforward:
>
> **Phase 1** *(GHC 8.0)*: Implement new warning in GHC which gets
> triggered when `Monad` instances explicitly override the
> default `return` method implementation.
>
> **Phase 2** *(GHC 8.2 or later)*: When we're confident that the
> majority of Hackage has reacted to the warning (with the help of
> Stackage actively pursuing maintainers to update their packages) we
> turn the `return` method into a top-level binding and remove the
> warning implemented in Phase 1 from GHC again.
>
>
> Discussion period
> -----------------
>
> A discussion period of three weeks (until 2015-10-15) should be enough
> to allow everyone to chime in as well as leave enough time to make the
> required preparations for GHC 8.0 should this proposal pass as we hope.
>
> ----
>
> [1]: https://wiki.haskell.org/Functor-Applicative-Monad_Proposal
> [2]: https://wiki.haskell.org/MonadFail_Proposal
> [3]: https://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo
> [4]: https://gist.github.com/hvr/b0e34463d85b58f169d9
> [5]: https://gist.github.com/hvr/afcd040783d980594883
> [6]: https://ghc.haskell.org/trac/ghc/ticket/9590
> [7]:
> https://mail.haskell.org/pipermail/haskell-prime/2015-September/003936.html
>
> --
>
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>
>
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