Drastic Prelude changes imminent

[Edward Kmett]

I happen to support the vast majority of my packages for 3 major GHC
releases, not just platforms, and for the current stackage build as well.

The idea that we can't personally benefit from things for ~3 years because
we happen to have packages with long support cycles leads to a fallacy by
which by induction no progress can ever be made to benefit the larger pool
of Haskell users we will have in the future because we can never pay a
price in the short term to benefit them.

Regarding Foldable, there is a chain of reasoning that begins with
accepting the Applicative-Monad Proposal:

Once we accept Applicative as a superclass of Monad then mapM is _never_
the right abstraction.

Why?

mapM is needlessly constrained to Monad, so you really start needing the
ability to traverse with Applicative.

mapM actively gets in the way of making code work with the reduced
Applicative constraints that are possible post AMP.

Dead End #1. We can't just generalize the signature of mapM in a way that
makes mapM work with Applicative without a whole host of other issues.
(mapM is a member of the Traversable class alongside traverse, there exists
a corner case where Traversable.mapM can have better termination behavior
on a weirdly designed container, so having Prelude mapM and
Traversable.mapM generalized differently would be bad.)

Dead End #2. To go the other way and put in a monomorphized traverse into
Prelude would break insane amounts of code across the entire ecosystem.

To expose traverse is to expose Traversable.

Exposing Traversable from the Prelude then really wants to drag in the
superclasses of Traversable lest you have a class that is exposed in the
Prelude with a superclass you can't define.

Dead End #3. We could have added Applicative as a superclass of Monad
without adding it to the Prelude, but it would have been a bad way to
proceed. Now users who want to either benefit from the change or define a
Monad, ever, would have to add an extra import Control.Applicative line,
what is the probem. This is the same argument against forcing Foldable into
hiding while possibly exposing Traversable, just mutatis mutandis for
Foldable/Applicative and Traversable/Monad.

Consequently, exposing the pieces you need to define classes that are
exposed through Prelude seems to be the direction that in the long term is
the most defensible in terms of how to explain the state of the world to
newcomers, once we get the hurdle of transition.

The current state of affairs was quite eloquently summarized by this
transcript from Nick Partridge about the process of describing the AMP to
his coworkers:


In a conversation with co-workers about these changes, I fired up a ghci
session to show the difference betweenmapM and traverse and what it would
mean to generalize the functions in Prelude, and how traverse was just a
more general mapM.

Here is how that went down:

> :i traverse

Top level:
    Not in scope: ‘traverse’
    Perhaps you meant ‘reverse’ (imported from Prelude)

Ah, of course. I forgot the import.

> import Data.Traversable
> :i traverse
class (Functor t, Data.Foldable.Foldable t) =>
      Traversable (t :: * -> *) where
  traverse ::
    Control.Applicative.Applicative f => (a -> f b) -> t a -> f (t b)
  ...
    -- Defined in ‘Data.Traversable’

Great. Now let's look at mapM:

> :i mapM

Top level:
    Ambiguous occurrence ‘mapM’
    It could refer to either ‘Data.Traversable.mapM’,
                             imported from ‘Data.Traversable’
                          or ‘Prelude.mapM’,
                             imported from ‘Prelude’ (and originally
defined in ‘Control.Monad’)

Oh. Okay we'll check out the one in Prelude by prefixing it.

> :i Prelude.mapM
Prelude.mapM :: Monad m => (a -> m b) -> [a] -> m [b]
    -- Defined in ‘Control.Monad’
> :t traverse
traverse
  :: (Traversable t, Control.Applicative.Applicative f) =>
     (a -> f b) -> t a -> f (t b)

Now let's specialise traverse for Monad and List, to show that it's
equivalent.

> :t (traverse :: Monad m => (a -> m b) -> [a] -> m [b])

<interactive>:1:2:
    Could not deduce (Control.Applicative.Applicative m1)
      arising from a use of ‘traverse’
    from the context (Monad m)
      bound by the inferred type of
               it :: Monad m => (a -> m b) -> [a] -> m [b]
      at Top level
    or from (Monad m1)
      bound by an expression type signature:
                 Monad m1 => (a1 -> m1 b1) -> [a1] -> m1 [b1]
      at <interactive>:1:2-50
    Possible fix:
      add (Control.Applicative.Applicative m1) to the context of
        an expression type signature:
          Monad m1 => (a1 -> m1 b1) -> [a1] -> m1 [b1]
        or the inferred type of it :: Monad m => (a -> m b) -> [a] -> m [b]
    In the expression:
      (traverse :: Monad m => (a -> m b) -> [a] -> m [b])

Curses! Looking forward to AMP. Insert explanation about how Applicative
should be a superclass of Monad. Pretend that it really is. Wave hands
about.

> :t (traverse :: Applicative m => (a -> m b) -> [a] -> m [b])

<interactive>:1:14:
    Not in scope: type constructor or class ‘Applicative’

Okay this is getting silly.

> import Control.Applicative
> :t (traverse :: Applicative m => (a -> m b) -> [a] -> m [b])
(traverse :: Applicative m => (a -> m b) -> [a] -> m [b])
  :: Applicative m => (a -> m b) -> [a] -> m [b]

This is *really* *really* common teaching/learning scenario using the
current state of the Prelude.

On Sat, Jan 31, 2015 at 12:59 PM, Johan Tibell <johan.tibell at gmail.com>
wrote:

> I re-read the goals* of the proposal:
> https://wiki.haskell.org/Foldable_Traversable_In_Prelude
>
> "One goal here is that people should be able to use methods from these
> modules without the need of a qualified import -- i.e. to prevent clash in
> the namespace, by resolving such in favor of the more generalized versions.
> Additionally, there are some new methods added to the Foldable class
> because it seems to be the "right" thing."
>
> Before FTP, I would have to write this to use Foldable/Traversable:
>
>     import qualified Data.Foldable as F
>
> The import is qualified as to not collide with the Prelude.
>
> If I have code that needs to be compatible with more than one GHC release
> (I typically need compatibility with the last 3 major releases), what do I
> have to write post-FTP? Since I cannot rely on the Prelude being
> generalized (because I might be compiling with a pre-FTP compiler) I need
> to either write:
>
>     #if MIN_VERSION_base(x,y,z)
>     -- Get Foldable etc from Prelude
>     #else
>     import Data.Foldable (...)
>     import Prelude hiding (...)  -- the same
>     #endif
>
> Which is terrible. Alternatively I can write
>
>     import qualified Data.Foldable as F
>
> but now nothing is gained over the pre-FTP state. Only after 3+ years (at
> the current GHC release phase) I can drop that one extra import. One out of
> perhaps 20. That seems quite a small gain given that we will then have made
> Data.List a very confusing module (it's essentially Data.Foldable under a
> different name), broken some code due to added type ambiguity, and also
> removed one of the simpler ways to remove that ambiguity, which would have
> been to import one of the monomorphic list functions that no longer exist.
>
> * People tell me that there are other goals, but they aren't really stated
> clearly anywhere.
>
> -- Johan
>
>
>
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