The essence of the iterator pattern

[Chen Harry]

I am reading the paper(https://www.cs.ox.ac.uk/jeremy.gibbons/publications/iterator.pdf) these days. to better understanding it, I try to translate it into Scala. it looks like no-brain translation until I try to implement AppAdapter (page 13). Because Scala doesn't support rank-2 type directly, I use this encoding(https://gist.github.com/chenharryhua/0140ba492554fd81540082fb4b95579a#file-essence_of_iterator_pattern-scala-L37). 

it works but looks not very correct to me since the AppAdapter defined in Haskell is:

 data AppAdapter m where

    AppAdapter::Applicative (g m) ⇒

    (∀a. m a → g m a) → (∀a. g m a → m a) → AppAdapter m

which takes one type parameter. mine unfortunately has two. :(

I guess that in Haskell, the g is not an existential type(where a is)? 

Or the AppAdapter in Haskell might be defined like this?

 data AppAdapter g m where

    AppAdapter::Applicative (g m) ⇒

    (∀a. m a → g m a) → (∀a. g m a → m a) → AppAdapter m

I appreciate if someone shed a light on it.

Cheers,

Harry

 

[Tony Morris]

Hi Chen,
You might not know that Eric Torreborre has translated this paper to Scala:
https://etorreborre.blogspot.com.au/2011/06/essence-of-iterator-pattern.html

To answer your question, to represent (∀a. m a → g m a) you write something like:

trait HaveToThinkUpAName[M[_], G[_[_], _] {
  def apply[A](names: M[A]): G[M, A]
} // not type-checked, straight from head

The position of the forall quantifier (∀) informs where the type variables go in the translation.

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[Chen Harry]

Hi, Tony, thank you very much. just quote from Eric: :)

Eric said...

Wow, coming from you, that makes me both happy and proud!

7:39 PM

the HaveToThinkUpAName trait looks isomorphic to the encoding below. I am pretty sure it is type-checked, at least in Scala. :)

trait AppAdapter[G[_[_], _], M[_]] {
	def insert[A]: M[A] => G[M, A]
	def retrieve[A]: G[M, A] => M[A]
}

By the way, I didn't find the AppAdpater in Eric's post which looks great.

Cheers,

Harry

[Chen Harry]

Eric's post is very good, especially for Scala programmer. 

the insert method in AppAdapter is isomorphic to the apply method in HaveToThinkUpAName..