Initial sketch of a free applicative implementation by ethul · Pull Request #9 · ethul/purescript-freeap

ethul · 2016-11-30T23:50:05Z

The goal of this implementation is to improve performance
characteristics of the operations of the free applicative functor in
PureScript.

Reference #7

Reference #8

Add benchmarks
Extract and Comonad instances

The goal of this implementation is to improve performance characteristics of the operations of the free applicative functor in PureScript. Reference #7 Reference #8

ethul · 2016-12-02T03:21:35Z

foldFreeAp

retractFreeAp

paf31 · 2016-12-02T03:27:59Z

Wow 😄

ethul · 2016-12-02T03:30:58Z

:) I'd still like to work on #8 but might not have a chance until this weekend. It's on the list to do though!

ethul · 2016-12-02T03:54:25Z

(Updated to improve variance)

foldFreeAp (small)

retractFreeAp (small)

paf31 · 2016-12-02T04:03:15Z

@ethul Here you go. Sorry the names are a bit crap, I did this using typed holes for the most part. I think it's right though, it should correspond to the version which uses Day:

instance extendFreeAp :: Extend f => Extend (FreeAp f) where
  extend f x@(Pure a) = Pure (f x)
  extend f (Ap x) = Ap (runExists (\(ApF h j) ->
    mkExists (ApF h (\u -> extend (\g a1 -> f (map (_ $ a1) g)) (j u)))) x)

instance comonadFreeAp :: Comonad f => Comonad (FreeAp f) where
  extract (Pure a) = a
  extract (Ap x) = runExists (\(ApF h j) ->
    let a1 = extract (h unit)
        a2 = extract (j unit)
    in a2 a1) x

paf31 · 2016-12-02T04:05:15Z

Oh, I did it on the wrong branch 😆

Oh well, I can try it on the new branch at some point, unless you beat me to it.

ethul · 2016-12-02T04:11:16Z

Thanks for giving the instances a go! I will probably take a look this weekend, but I am open to any suggestions on the implementations you may have. Thanks!

paf31 · 2016-12-02T04:14:54Z

The only suggestion I can give is to think in terms of FreeAp as a free monoid object, generated in terms of coproducts and Day convolution - that representation made it simpler for me to see what the Comonad instance was doing in terms of reannotating some sort of tree.

In fact, you might like to consider the representation in terms of Day for this library - I dare say it could be pretty efficient, although maybe not as efficient as what you have above, which is pretty impressive 😄

ethul · 2016-12-02T13:51:10Z

Thanks for the insight. I will definitely dig more into this. I am interested by a representation in terms of Day. If I can get one together, I am curious to stack it against the two we have. Thanks again!

safareli · 2016-12-02T15:10:18Z

btw is there any resource to understand the "day convolution" which does not require too much category theory? (can't understand ncatlab)

garyb · 2016-12-02T15:12:32Z

Phil wrote a bit about them a while back: http://blog.functorial.com/posts/2016-08-08-Comonad-And-Day-Convolution.html but since they are very much a category theory based construct there's not going to be a whole lot of other material out there, I suspect!

safareli · 2016-12-02T17:57:19Z

@garyb I know that article, but didn't quite understood it, thanks tho!

ethul · 2016-12-04T22:17:52Z

I've added an implementation based on Day in #10 as suggested by @paf31. I am kinda liking the results in #10 over #9, but I am wondering if anyone has thoughts on one implementation versus the other.

Another question is concerning thunks. @garyb you asked about this earlier. The current v2.0.0 implementation thunks the f i and FreeAp f (i -> a). In #9 and #10, I've removed the thunks. Not sure if we need this. Originally, it was to follow the implementation in scalaz's Ap. I am open to suggestions here though.

safareli · 2016-12-09T13:13:33Z

@ethul I have made 1 to 1 port of https://www.eyrie.org/~zednenem/2013/05/27/freeapp to js it's pritty fast, but if the tree is large (for example 4000) stack overflows (on 3000 it works fine)

const buildTree = n => {
  let res = FreeAp.of(function f(a) {
    return a == n - 1 ? a : f
  })
  for (let i = 0; i < n; i++) {
    res = FreeAp.lift(i).ap(res)
  }
  return res
}
buildTree(4000).foldPar(Identity.of, Identity).toString()

Can you try run benchmark with largar input (more than 2000) as i think it would overflow.

ethul · 2016-12-09T13:41:07Z

@safareli You're right. I have tried this before and it does overflow. Certainly something I'd like to look into further. Thanks for bringing this up!

safareli · 2016-12-09T14:59:16Z

if we have large chain of function composition (f . g . h ...) and execute it we would get stack overflow, and in this implementation we have only functions :d, so i think issue is with function composition (f . g). I will investigate it further, but it's a bit criptic for me (so many functions are passed around).

hypothetical solution would be to have a heterogenous list of functions, which could be instance of Category and it's composition would just build up the List (or just make it a Functor and fmap will do the same, as . and fmap are same for function)

data FunList a b where
  One  :: (a -> b) -> FunList a b
  Cons :: (z -> b) -> FunList a z -> FunList a b
  
instance Functor (FunList z) where
  fmap :: (a -> b) -> FunList z a -> FunList z b
  fmap g f = Cons g f
  -- or just `fmap = Cons`

Then we could have some function to actually execute it in a stack safe way using for or something

run :: FunList a b -> a -> b

for example in js:

// list of functions 
const fs = Array(100000).fill().map((_,idx) => x => x + idx)
const compose = (f,g) => x => f(g(x))
// if we compose them and then run it 
fs.reduce(compose)(1) // we get stack overflow
// but if this is stack safe
fs.reduceRight((arg,f) => f(arg),1)

so if we have wrapper for function which during composition builds up a list then it could be executed without stack issues.

i think we could add more variants to the FunList type and make it a monad if needed

ethul · 2016-12-09T18:07:17Z

@safareli I like your idea. Thanks for the example. I will have to dig into this a bit more this weekend, but I like the direction of this so far.

paf31 · 2016-12-09T18:10:47Z

I bet you could write retractFreeAp as a tail recursive function pretty easily using the Day representation, although I haven't tried it yet.

ethul · 2016-12-09T18:12:26Z

Good call!

…

On Fri, Dec 9, 2016 at 13:10 Phil Freeman ***@***.***> wrote: I bet you could write retractFreeAp as a tail recursive function pretty easily using the Day representation, although I haven't tried it yet. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#9 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAVYy_bmnQOn5QkEl77M7mu2wOIgUg3fks5rGZmngaJpZM4LA1bj> .

safareli · 2016-12-10T00:22:15Z

btw just wrote about the stack safe function composition

ethul · 2016-12-10T00:27:44Z

Nice! Thanks for the link

…

On Fri, Dec 9, 2016 at 19:22 Irakli Safareli ***@***.***> wrote: btw just wrote about the stack safe function composition ***@***.***/stack-safe-function-composition-85d61feee37e#.876z5dsdg> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#9 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAVYy8-AOEd0OATBGSwHHrT3EJo9P-MMks5rGfC3gaJpZM4LA1bj> .

safareli · 2016-12-17T20:53:17Z

continuation of that first article https://www.eyrie.org/~zednenem/2013/06/freeapp-2

ethul · 2016-12-17T23:05:05Z

Nice! Thanks for the link

…

On Sat, Dec 17, 2016 at 15:53 Irakli Safareli ***@***.***> wrote: continuation of that first article https://www.eyrie.org/~zednenem/2013/06/freeapp-2 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#9 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAVYyxp80ULuKtgWdWwHo5tciOJDQlBOks5rJEu9gaJpZM4LA1bj> .

safareli · 2017-01-15T19:55:44Z

Hey recently i was thinking on how to make it stack safe and and came up with this implementation of FreeAp which has O(1) complexity on map and ap and O(n) on fold which is also stack safe if target applicative is stacksafe. I think it this is also simpler than previous versions:

//data Par f a where
//  Pure :: a -> Par f a
//  Lift :: f a -> Par f a
//  Ap :: Par f (a -> b) -> Par f a -> Par f b
const Par = union('Par', {
  Pure: ['x'],
  Lift: ['i'],
  Ap: ['i', 'x'],
})
const { Pure, Lift, Ap } = Par

Object.assign(Par, {
  // :: a -> Par f a
  of: Pure,
  // :: f a -> Par f a
  lift: Lift,
})

const foldArg = (node, f, T) => {
  if (node.tag == 'Pure') {
    return of(T, node.x)
  } else if (node.tag == 'Lift') {
    return f(node.f)
  }
  throw new TypeError('Invalid argument for foldArg')
}

Object.assign(Par.prototype, {
  // :: Par f a ~> (a -> b) -> Par f b
  map(f) {
    return ap(Par.of(f), this)
  },
  // :: Par f a ~> Par f (a -> b) -> Par f b
  ap(that) {
    return Ap(that, this)
  },
  // :: Applicative g => Par f a ~> (Ɐ x. f x -> g x, TypeRep g) -> g a
  foldPar(f, T) {
  // # this is non stack safe version:
  // return this.cata({
  //   Pure: (x) => of(T, x),
  //   Lift: (i) => f(i),
  //   Ap: (i, x) => ap(i.foldPar(f,T), x.foldPar(f,T)),
  // })
  // # this is stack safe version:
    const args = [this]
    const fns = []
    while (true) {
      let arg = args.pop()
      if(arg.tag === 'Ap') {
        while (arg.tag == 'Ap') {
          args.push(arg.x)
          arg = arg.f
        }
        fns.push(foldArg(arg, f, T))
      } else {
        const resArg = foldArg(arg, f, T)
        if (fns.length == 0) {
          return resArg
        }
        let resFn = ap(fns.pop(), resArg)
        if (args.length > 0) {
          fns.push(resFn)
        } else if (fns.length > 0) {
          fns.push(resFn)
          return fns.reduce((fn, arg) => ap(fn, arg))
        } else {
          return resFn
        }
      }
    }
  },
  // :: Par f a ~> (Ɐ x. f x -> g x) -> Par g a
  hoistPar(f) {
    return this.foldPar(compose(Par.lift)(f), Par)
  },
  // :: (Applicative f) => Par f a ~> TypeRep f -> f a
  retractPar(m) {
    return this.foldPar(id, m)
  },
  // :: Par f a ~> (Ɐ x. f x -> Par g x) -> Par g a
  graftPar(f) {
    return this.foldPar(f, Par)
  },
})

I used this snippet to test stack safety:

const apTimes = (n) => {
    const args = []
    const f = (a) => {
      args.push(a)
      if (args.length == n) {
        return args.length
      }
      return f
    }
    let res = Par.lift(f)
    for (let i = 0; i < n; i ++ ) {
      res = ap(res, Par.lift(i))
    }
    return res
  }

  const testN = MAX_STACK

  t.equals(apTimes(testN).hoistPar(id).foldPar(Identity.of, Identity).get(), testN, 'is Par stack safe')
  t.equals(apTimes(testN).foldPar(Identity.of, Identity).get(), testN, 'Works with Identity')

Will soon update the PR safareli/free#31 I was working on.

ethul · 2017-01-15T20:15:30Z

Nice! Thanks for the code snippet. I will have to dig into this PR again.

…

On Sun, Jan 15, 2017 at 14:55 Irakli Safareli ***@***.***> wrote: Hey recently i made a quick implementation of FreeAp which has O(1) complexity on map and ap and O(n) on fold which is also stack safe if target applicative is stacksafe: //data Par f a where // Pure :: a -> Par f a // Lift :: f a -> Par f a // Ap :: Par f (a -> b) -> Par f a -> Par f b const Par = union('Par', { Pure: ['x'], Lift: ['i'], Ap: ['i', 'x'], }) const { Pure, Lift, Ap } = Par Object.assign(Par, patch({ // :: a -> Par f a of: Pure, // :: f a -> Par f a lift: Lift, })) const foldArg = (node, f, T) => { if (node.tag == 'Pure') { return of(T, node.x) } else if (node.tag == 'Lift') { return f(node.f) } throw new TypeError('Invalid argument for foldArg') } Object.assign(Par.prototype, { // :: Par f a ~> (a -> b) -> Par f b map(f) { return ap(Par.of(f), this) }, // :: Par f a ~> Par f (a -> b) -> Par f b ap(that) { return Ap(that, this) }, // :: Applicative g => Par f a ~> (Ɐ x. f x -> g x, TypeRep g) -> g a foldPar(f, T) { // # this is non stack safe version: // return this.cata({ // Pure: (x) => of(T, x), // Lift: (i) => f(i), // Ap: (i, x) => ap(i.foldPar(f,T), x.foldPar(f,T)), // }) // # this is stack safe version: const args = [this] const fns = [] while (true) { let arg = args.pop() if(arg.tag === 'Ap') { while (arg.tag == 'Ap') { args.push(arg.x) arg = arg.f } fns.push(foldArg(arg, f, T)) } else { const resArg = foldArg(arg, f, T) if (fns.length == 0) { return resArg } let resFn = ap(fns.pop(), resArg) if (args.length > 0) { fns.push(resFn) } else if (fns.length > 0) { fns.push(resFn) return fns.reduce((fn, arg) => ap(fn, arg)) } else { return resFn } } } }, // :: Par f a ~> (Ɐ x. f x -> g x) -> Par g a hoistPar(f) { return this.foldPar(compose(Par.lift)(f), Par) }, // :: (Applicative f) => Par f a ~> TypeRep f -> f a retractPar(m) { return this.foldPar(id, m) }, // :: Par f a ~> (Ɐ x. f x -> Par g x) -> Par g a graftPar(f) { return this.foldPar(f, Par) }, }) I used this snippet to test stack safety: const apTimes = (n) => { const args = [] const f = (a) => { args.push(a) if (args.length == n) { return args.length } return f } let res = Par.lift(f) for (let i = 0; i < n; i ++ ) { res = ap(res, Par.lift(i)) } return res } const testN = MAX_STACK t.equals(apTimes(testN).hoistPar(id).foldPar(Identity.of, Identity).get(), testN, 'is Par stack safe') t.equals(apTimes(testN).foldPar(Identity.of, Identity).get(), testN, 'Works with Identity') Will soon update the @safereli/Free PR I was working. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#9 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAVYy3EfcLnXywJP6vHzLI9p-zBGF6diks5rSnnAgaJpZM4LA1bj> .

safareli · 2017-01-18T22:27:34Z

The implementation of fold, I had written in last comment, was incorrect and this one is correct (it needs a bit of refactoring tho).

ethul · 2018-01-27T14:41:26Z

Superseded by #13

ethul added 2 commits November 30, 2016 18:49

Initial sketch of a free applicative implementation

31c8826

The goal of this implementation is to improve performance characteristics of the operations of the free applicative functor in PureScript. Reference #7 Reference #8

Remove documentation link

71acf67

ethul mentioned this pull request Dec 2, 2016

Question: what are compexity of map, ap and retract? #7

Closed

xuwei-k mentioned this pull request Dec 4, 2016

consider another implementation of FreeAp scalaz/scalaz#1301

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ethul mentioned this pull request Dec 4, 2016

Initial work implenting with day #10

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safareli mentioned this pull request Dec 7, 2016

Make FreeApplicative stack safe typelevel/cats#1151

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safareli mentioned this pull request Jul 5, 2017

Alternative formulation of Free Applicative ekmett/free#158

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ethul mentioned this pull request Jan 13, 2018

Stack safe version #13

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ethul closed this Jan 27, 2018

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foldFreeAp

retractFreeAp

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foldFreeAp (small)

retractFreeAp (small)

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