[Haskell-cafe] Was simplified subsumption worth it for industry Haskell programmers?

[Richard O'Keefe]

This sounds interesting, but I don't have time to wade through
reddit.  Aut mailing list aut nihil.

On Sat, 7 May 2022 at 07:48, Ruben Astudillo <ruben.astud at gmail.com> wrote:

> I have nothing to say about the topic you propose on here. But to avoid
> having a fractured discussion, let's us all agree to answer on that reddit
> thread instead of this mail thread. Otherwise some good arguments will be
> repeated.
>
> --
> Rubén. (pgp: 1E88 3AC4 89EB FA22)
>
> On 06-05-22 15:34, pareto optimal via Haskell-Cafe wrote:
> > I originally posted on Reddit and the thread contains much debate and
> discussion.
> >
> > I'm concerned the views of the mailing list and likely ghc devs might
> not be as represented there or discourse, so i'm also copying it here.
> >
> > Copy-paste of post:
> >
> > Warning: Long post
> >
> > tl;dr
> >
> > - simplified subsumption seems to make common code I write in industry
> clunky for no good reason in lots of places
> >
> > - Are others concerned with motivating what seem like "pointless
> lambdas" to new hires or students for simple tasks?
> >
> > - Are there more real world advantages that make these frequent
> annoyances worth it?
> >
> > - How is quicklook impredicativity useful in industry?
> >
> > The biggest advantage seems to be that laziness is more predictable.
> >
> > However looking at [commits fixing simplified subsumption errors on
> github](
> https://github.com/search?q=simplified+subsumption+language%3Ahaskell&type=commits)
> I see very common patterns in industry Haskell now need an explicit lambda
> for "reasons" such as:
> >
> >       readFreqSumFile :: (MonadSafe m) => FilePath -> m
> (FreqSumHeader, Producer FreqSumEntry m ())
> >     - readFreqSumFile file = readFreqSumProd $ withFile file ReadMode
> PB.fromHandle
> >     + readFreqSumFile file = readFreqSumProd $ withFile file ReadMode
> (\h -> PB.fromHandle h)
> >
> > and:
> >
> >     - toOrders <- asks _pdfConfToOrder
> >     + toOrders <- asks (\r -> _pdfConfToOrder r)
> >
> > And this typical use of id is no longer valid:
> >
> >     instance MonadOrvilleControl IO where
> >     -    liftWithConnection = id
> >     -    liftFinally = id
> >     +   liftWithConnection ioWithConn = ioWithConn
> >     +   liftFinally ioFinally = ioFinally
> >
> > On my $work codebase that means hundreds of changes that make our code
> worse with seemingly no benefit.
> >
> > This case is addressed in the proposal, but seems to handwave this as:
> >
> > > The benefit, in terms of programming convenience, is small.
> >
> > From my perspective while updating my codebase, it certainly doesn't
> feel that way.
> >
> > From the persective of onboarding new Haskell hires, it doesn't feel
> simpler. I envision a future teaching session like:
> >
> > > student: This code looks correct but I get an error that means
> nothing to me of
> >
> >      error:
> >          • Couldn't match type: b0 -> b0 with: forall q. q -> q
> >               Expected: p -> forall q. q -> q
> >               Actual: p -> b0 -> b0
> >          • In the first argument of ‘g’, namely ‘f’ In the expression: g
> f In an equation for ‘h’: h = g f | | h = g f | ^
> >
> > > me: Ah, that's because of something called simplified subsumption
> which we'll cover much later.
> > > me: For now, just know putting it in an explicit lambda fixes it
> when you notice a compile error like that.
> > > me: Now lets try to move past that and get back to basic file
> reading and writing
> > > student: oookkkay? (feeling unsure, disillusioned about Haskell
> requiring pointless ceremony and being overly complex for no seeming
> benefit)
> >
> > Being a fan of and proponent of Haskell I think: If this complication is
> being added, surely something is made possible in return that gives more
> value.
> >
> > This led me to [the proposal](
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst)
> and I found with simplified subsumption:
> >
> > - Laziness characteristics and semantics of programs will be changed
> less, which I infer will lead to more predictable performance
> > - I assume that simplifying a compiler step like this will speed up
> compile times and reduce space usage
> > - Quick look impredicativity seems to be the real driving reason behind
> simplified subsumption and somehow makes dealing with very polymorphic code
> easier
> >
> > At this point my thought is:
> >
> > > Making highly polymorphic code simpler to write that isn't as
> typical in industry Haskell code in ways I can't determine without great
> effort was valued over "small incoveniences" that I'll run into daily
> >
> > But, still wanting to give the benefit of the doubt I dive face first
> into [The proposal for Quicklook impredicativity](
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0274-quick-look-impredicativity.rst
> ).
> >
> > Reading the whole thing, I still cannot ground this concept in real
> world terms that may effect me or that I could take advantage of.
> >
> > So, I go to the paper [A quick look at impredicativity](
> https://www.microsoft.com/en-us/research/publication/a-quick-look-at-impredicativity/)
> and start reading many things I don't fully understand.
> >
> > Running out of energy, I start skimming and finally find some examples
> in section 10 APPLICATIONS.
> >
> > I see an example with gZipWithM that I still don't understand. Further
> down I see reference to pieces of code updated in Streamly that take
> advantage of quick look polymorphism and wonder why the real world example
> wasn't included and explained.
> >
> > So, i'm left frustrated with "simplified" subsumption and posting here
> for help answering:
> >
> > - Are others in the same boat?
> > - Are there advantages i'm not seeing?
> > - Can we use my reflection to improve industry/academic communication?
> >
> > And finally, any revant commentary surrounding this I may be oblivious
> to.
> >
> > Thanks!
> >
> >
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