Re: Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`

Yes, and I think there are still some more overly-specific things that come in here too, such as replicateM and filterM, which I don't think have been generalized yet. On Sep 24, 2015 10:40 PM, "Edward Kmett" wrote:

I'm a rather enthusiastic +1 on this, given a sufficiently long time table for the switch!

We ARE asking a lot of people to break code here.

It does drastically reduce the amount of "magic" involved in the ApplicativeDo story and avoids enshrining the current historical artifact in a future Haskell report that includes the AMP.

I'd also like to consider employing the exact same migration plan for (>>) at the same time. It is redundant with (*>), and the choice of which gets implemented in terms of the other introduces precisely the same set of issues. If we can do that then this would fix the remaining performance issues for mapM, letting us eventually remove that from Traversable as well, moving it to a top level alias in the same way -- maybe not until 8.8 or something, but it'd at least give us a roadmap to get there eventually.

-Edward

On Thu, Sep 24, 2015 at 5:43 PM, Herbert Valerio Riedel 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

--

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries