Export `liftA2` from `Prelude`

[andreasabel]

(Transfered from https://gitlab.haskell.org/ghc/ghc/-/issues/20945):
As alternative to <*> method liftA2 can be used to instantiate Applicative, but only the former is exported from Prelude. I propose to also export the latter, for reasons of symmetry and clarity of error messages.

Detailed, anecdotic motivation: Consider:

{-# LANGUAGE DeriveFunctor #-}

data Foo a = Foo a deriving (Functor)

instance Applicative (Foo) where
  pure = Foo

Since base shipped with GHC 8.2, this gives the following warning:

warning: [-Wmissing-methods]
    • No explicit implementation for
        either ‘<*>’ or ‘GHC.Base.liftA2’
    • In the instance declaration for ‘Applicative Foo’

Trying to follow up on this literally,

instance Applicative Foo where
  pure = Foo
  GHC.Base.liftA2 f (Foo a) (Foo b) = Foo $ f a b

I get other errors:

    ‘GHC.Base.liftA2’ is not a (visible) method of class ‘Applicative’
   |
10 |   GHC.Base.liftA2 f (Foo a) (Foo b) = Foo $ f a b
   |   ^^^^^^^^^^^^^^^

    Qualified name in binding position: GHC.Base.liftA2
   |
10 |   GHC.Base.liftA2 f (Foo a) (Foo b) = Foo $ f a b
   |   ^^^^^^^^^^^^^^^

GHC is giving me contradictory information here.
The workaround is import Control.Applicative, but it seems inconsistent that some methods of Applicative are exported by Prelude and some not, especially those that comprise a MINIMAL definition of an instance.

Proposal: export liftA2 also from Prelude.

Related, find some discussion there:

• https://gitlab.haskell.org/ghc/ghc/-/issues/13191, referenced in https://hackage.haskell.org/package/base-4.10.0.0/changelog
• MR https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6468 (Note: this one is on liftA2 and join)

[treeowl]

Well I'm certainly in favor!

[googleson78]

Thanks! 🙇

And please! I use liftA2 way more often than I use (<*>) too, regardless of symmetry and clarity of error messages.

[googleson78]

I also believe it's more beginner-friendly - it's always what I show first when teaching (instead of (<*>)). Explaining "lift this 2-argument function" is easier understood imo once you've seen a Functor which "lifts a 1-argument function" compared to the "scary" f (a -> b). And beginners are the most affected by what's available by default, imo.

[tomjaguarpaw]

I'm in favour of this.

[Bodigrim]

While I'm totally in favor, this is a breaking change, so we need an impact assessment and patches. @andreasabel may I ask you to prepare it?

https://hackage-search.serokell.io/?q=%5EliftA2%5Cb gives a rough list of affected packages, including hoopl, pandoc, parsley and shake. There are might be more packages, which define liftA2 locally, but I doubt it.

[andreasabel]

@Bodigrim wrote:

we need an impact assessment and patches. @andreasabel may I ask you to prepare it?

Are there instructions how to go about it? (Had a quick look, but didn't find such on this repo.)

[Bodigrim]

@andreasabel compile GHC 9.0 with liftA2exported from Prelude and try building https://github.com/Bodigrim/clc-stackage.

[andreasabel]

compile GHC 9.0 with liftA2exported from Prelude and try building Bodigrim/clc-stackage.

Ok, let me try to expand this instruction (WIP):

• Clone the GHC repo and switch to the ghc-9.0 branch:

  git clone git@gitlab.haskell.org:ghc/ghc.git 
  cd ghc
  git checkout ghc-9.0
  git submodule update --init
  

• Modify libraries/base/Prelude.hs
• Activate old enough ghc (e.g. 9.0.2)
• Downgrade happy to 1.19, e.g.:

  cabal install happy-1.19.12
  

• Build GHC-9.0

  ./boot
  ./configure
  make clean  # if you have leftovers from a previous compilation
  make
  

• Wait for three hours.
• Fail with a stupid error like

cd libraries && sh gen_contents_index --intree
haddock: can't find a package database at /Users/abel/.cabal/store/ghc-9.0.1/package.db
make[1]: *** [ghc.mk:804: libraries/dist-haddock/index.html] Error 1

Why ghc-9.0.1? No one ever mentioned "9.0.1"! We are on 9.0.2 now.

[Bodigrim]

@andreasabel I've updated https://github.com/Bodigrim/clc-stackage to work with GHC 9.2.2. Could you please give it another go?

• Build GHC-9.0

     ./boot
     ./configure
     make clean  # if you have leftovers from a previous compilation
     make
  

Why are you using make instead of hadrian?

[Bodigrim]

@andreasabel any chance you might have time to return to this?

[andreasabel]

@Bodigrim : Thanks for the nudge. Unfortunately, I got too much on my plate atm.
If I was a routine GHC builder, I could do it on the side...

I would be happy if someone took over.

[googleson78]

I'll try to see what breakage happens given your instructions.

[googleson78]

Just as an update, I've been struggling with this, due to (probably) not building ghc right. As an example, I can't satisfy a dependency on libffi. If anyone has some tips, I'm happy to hear them (either here, or via other channels, e.g. email).

[Bodigrim]

@googleson78 could you quote an exact error message? If you are on macOS, this incantation works for me.