Type-safe Transducers in Clojure, Scala, and Haskell(blog.podsnap.com)

74 pointsby luuvor 12 Jahren11 comments

blog.podsnap.com

Type-safe Transducers in Clojure, Scala, and Haskell

http://blog.podsnap.com/ducers2.html

2 comments

kentosivor 12 Jahren

This line made me laugh out loud: "... whenever something is even slightly ugly in Scala, you introduce an implicit to make it confusing instead."

frowaway001vor 12 Jahren

Odersky wrote a simple implementation, and it looks a lot nicer. It also doesn't use any casts or implicit conversaions: https://gist.github.com/odersky/6b7c0eb4731058803dfd

podsnapvor 12 Jahren

True. In retrospect, trying to make the Scala transducer type look like Haskell and then hacking into working order was didn't do justice to either language. As I sort of note in the text, the attempt to do it this way was at least partially meant to be "amusing."

brandonbloomvor 12 Jahren

This is interesting, but it's still missing some very important features of transducers. As you alluded, stateful operations [1] won't work at all in your Haskell model. Also, you've missed early termination [2], which Rich's talk [3] covered well.

[1] https://github.com/clojure/clojure/blob/b01adb859c66322c5cff...

[2] https://github.com/clojure/clojure/blob/b01adb859c66322c5cff...

[3] https://www.youtube.com/watch?v=6mTbuzafcII

wyagervor 12 Jahren

>As you alluded, stateful operations [1] won't work at all in your Haskell model.

Couldn't you just parametrize over IO/ST/etc.? E.g. something like

    IO r -> a -> IO r

I don't have a great mental model of transducers yet, so I'm not confident about that.

telvor 12 Jahren

You can just include local state.

    data Fold i o = forall st . Fold
      { merge :: st -> i -> st
      , this  :: st
      , view  :: st -> o
      }

    type (i ~> o)  = forall r . Fold o r      -> Fold i r
    -- compare
    type Trans i o = forall r . (r -> o -> r) -> (r -> i -> r)

Now each "step" has local pure state and no other step can break abstraction barriers and view it. This also gives you all of the effects of the indexed state passing Rich called untypeable.

Fold is equivalent to an infinite state Moore machine, so a stack of composed transducers applied to a base Moore machine which produces whatever result you want can be compiled a really efficient form.

Early termination can be done by changing `merge` to `merge :: st -> i -> Either o st`.

julesvor 12 Jahren

You can do that, but it's more elegant if you define a functional stateful transducer analogous to scanl for lists.

[deleted]vor 12 Jahren

davdarvor 12 Jahren

That's like saying "your program is more elegant if it has no monads". It's an incorrect statement. The monadic version is perfectly elegant. Even better: it's the right one.

julesvor 12 Jahren

By the same logic scanl should just have the type:

    (b -> IO a) -> [b] -> IO [a]

instead of

    (a -> b -> a) -> a -> [b] -> [a]

Just putting the whole thing in the IO/ST monad isn't the right solution when you need a very specific form of local state.

podsnapvor 12 Jahren

There's more missing than that, but, in the spirit of early termination, I had to stop somewhere! Specifically to your points: 1. To preempt the faithful: Stateful transducers don't work (for a broad definition of work) in Clojure or Scala either. It's just that there's no tradition of acknowledging state via type in those languages. 2. Early termination is weird. In Clojure it's a property of the accumulated reduction value, i.e. not the transducer, the reducing function or the reducer/collection. I'm not sure yet whether to try modeling this via type or to try sticking it somewhere else. 3. Rich's talk is great. Also, it's been transcribed: https://github.com/matthiasn/talk-transcripts/blob/master/Hi...

telvor 12 Jahren

See here for points (1) and (2): https://news.ycombinator.com/item?id=8532710