typelevel/cats

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Soren

@srnb_gitlab

Which one do you recommend? @SystemFw

Fabio Labella

@SystemFw

as for the more general point, have you tried writing that instance manually if haskell can't derive it?

Soren

@srnb_gitlab

Yeah

Fabio Labella

@SystemFw

well, it depends, I haven't used streamly(I have looked at it in some depth though) but it looks nice (but that's because I like fs2 I guess). pipes and conduit are both nice, and if you just need simple streaming I've heard good things about streaming as well

Soren

@srnb_gitlab

I'm thinking about how Lexer m a = Stream m Char -> Stream m a and Parser t m a = Stream m t -> Stream m a, and I want to build streaming operations around that notion

Fabio Labella

@SystemFw

on that MonadTrans instance specifically, take this with a grain of salt because it's just a hunch and I don't have time to investigate, but I smell trouble since SerialT appears in both negative and positive position, and general MonadTrans lift cannot lift that sort of non algebraic operation (and you need more complex machinery)

so streaming parsing?

Soren

@srnb_gitlab

Yeah, I want to have streaming lexing into streaming parsing, and I'm not comfortable with the way megaparsec does it

Fabio Labella

@SystemFw

so streaming parsing?

if that's the case, I'd recommend pipes-parse or streaming, streamly is more for concurrency/control flow things

Soren

@srnb_gitlab

I'll give pipes/pipes-parse a shot

I think the reason I didn't go with pipes initially is because its Producer a m r which is a little weird but I guess that's like fs2's Pull

Fabio Labella

@SystemFw

generally libraries in the pipes ecosystem have a beautiful Tutorial module that you can read on hackage

yeah, most streaming abstraction in haskell have a monad instance on the returned value, rather than the emitted one

which is great for pure streaming

for control flow, a monad instance on the emitted value is much more convenient: this is what streamly has, but then streamly suffers in the other use case

one under appreciated design decision of fs2 is to separate the two into Stream and Pull (which are related to each other ofc)

where Pull is a monad in the returned type, and can be used for stateful streaming, and Stream is a monad in the emitted type, and can be used for control flow

and conceptually Stream f a = Pull f a ()

when I started with fs2 the existence of both Stream and Pull seemed like a wart (although at the time it was obscured by a bunch of other stuff like Handle), but in truth it's one of the best design decisions of the whole library

Adam Rosien

@arosien

i still don't know anything about Pull. where to start?

Fabio Labella

@SystemFw

my writings have a bunch of long-ish explanations (for lack of a more structured resource, sorry) http://systemfw.org/writings

the tl;dr is that it's a fancy iterator, just built as an algebra rather than side-effecty interface

Adam Rosien

@arosien

thanks

oh, that's interesting, i didn't realize that a lot of the links point back to gitter. great idea!

Gavin Bisesi

@Daenyth

https://gist.io/@Daenyth/024c5584da01acabe7a435c8a53c4f3c

I also wrote up a thing about Pull

that I think helps drive home the "iterator" aspect

Fabio Labella

@SystemFw

oh, that's interesting, i didn't realize that a lot of the links point back to gitter. great idea!

Eventually I will have to go and do a backup somewhere else, but I dread that time

Adam Rosien

@arosien

ThreadApp, please unroll (this gitter link)

Gavin Bisesi

@Daenyth

fabio needs to hire a gitter assistant to take his conversations and translate to blogs :)

Fabio Labella

@SystemFw

tbh for some things I quite like the conversational format (although others would be better served by a proper exposition)

I worry more about gitter getting shutdown at brief notice

Gavin Bisesi

@Daenyth

I learn and internalize the things I teach better, in conversation, than trying to lecture

At least gitter is indexed on google

Some option to grab archives from there

Fabio Labella

@SystemFw

oh yeah still miles better than the proposed alternatives (which is why I opposed migrating to Discord or Slack)

Nathaniel Fischer

@kag0

https://typelevel.org/cats/datatypes/either.html#solution-2-adts-all-the-way-down talks about error ADTs, what's SOP for common errors that you want to share across modules?

Nick Telford

@nicktelford

Is there anything stopping cats from defining typeclass instances for SeqMap? It's useful for preserving the ordering of the mapping.

Jasper Moeys

@Jasper-M

Is there some well known name for this method foo?

def foo[A, B, C](f: A => B)(g: (A, B) => C): A => C =
  a => g(a, f(a))

Fabio Labella

@SystemFw

@Jasper-M that seems equivalent of Applicative.ap for F = A =>, with uncurrying and tuple == parameter lists

but it's not worth writing it differently than what you have I don't think

you can notice by changing (A, B) => C into A => B => C

then factorising the whole signature into (A => B) => (A => (B => C)) => (A => C)

then abstracting A => into F[_]

F[B] => F[B => C] => F[C]

and then alfa-rename and switch order into

F[A => B] => F[A] => F[B], which is the key signature of the Apply typeclass

Fabio Labella

@SystemFw

for F[E] = E => *

(renamed some type variables for clarity)

Jasper Moeys

@Jasper-M

Thanks! I felt like there had to be some cats concept for this

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