We are using coroutines to achieve monad bind sy

Question

< elizarov> We are using coroutines to achieve monad bind syntax in Kategory generalized to all monads and it looks like this for example with Option ``` Option monad binding val a = Option 1 bind val b = Option a + 1 bind yields b Option 2 ``` `binding` is a Continuation and `bind` is implemented as a `COROUTINE SUSPENDED` function that unwinds `flatMap` calls until the entire binding is completed as shown here For this to work we are resorting to a dirty hack which is making the private `label` and `completion` properties visible so we can mutate them accordingly Do you know if there is a better way of doing this Thanks in advance for any pointers in the right direction slightly smiling face

elizarov · Answer

That s a puzzler I don t quite grasp the whole idea on why you need to save restore them Can you point to some example code that will cease to work without this saving restoring of labels What if I just comment restroing the labels out What breaks

raulraja · Answer

<https github com Kotlin kotlin coroutines issues 72> our impl was based on that idea From that link gt A Continuation lt T gt implemented by a state machine modifies its label so that we cannot run one continuation several times from the same label The workaround I used back up the label through reflection before the resume call and restore it once the call returns

elizarov · Answer

Can you just use `for x in m ` instead of `val x = bind m ` The former avoids the need to save load state altogether as the compiler becomes responsible for it

raulraja · Answer

< elizarov> does `for in ` support multiple binds or are there plans to do that Something along the lines of ``` for x in listOf 1 y in listOf x + 1 yield y List 2 ```

raulraja · Answer

If `for` is just looking for `iterator ` I can potentially implement monadic bind in terms of `hasNext ` and `next ` in monads but if a single bind is supported it would result in a nested chain of for loops and I have no way to return the resulting values since `for` is not an expression but a control structure

raulraja · Answer

Have there been discussion regarding supporting `flatMap f A gt F lt B gt ` in the same way it supports `iterator ` so any structural evidence of having `flatMap` on a type allows for comprehensions enabling List generators etc

elizarov · Answer

Multiple binds with `for` are done like this `for x in listOf 1 for y in listOf x + 1 yield y `

elizarov · Answer

And it does not have to be an `Iterator` interface It works by convention It can be any interface that has `operator fun iterator SomeResult` where `SomeResult` has `operator fun hasNext Boolean` and `operator fun next T`

elizarov · Answer

That is how iteration over channels work You can do `for x in channel ` where `channel` is neither `Collection` neither `Sequence` nor `Iterable` at all

elizarov · Answer

It is Kotlin s approach to list comprehension amp do notation

elizarov · Answer

raulraja · Answer

< elizarov> awesome trying to come up with a prototype Thansk for the info

pakoito · Answer

< elizarov> is for an expression

raulraja · Answer

< pakoito> seems like with `yield` it becomes one

raulraja · Answer

< elizarov> assuming this ``` operator fun lt A gt Option lt A gt iterator OptionIterator lt A gt = OptionIterator this class OptionIterator lt A gt val fa Option lt A gt operator fun hasNext Boolean = fa fold false true operator fun next A = fa getOrElse throw IllegalStateException next invoked on empty Option ``` How can I implement this function using `for` ``` fun compute Option lt Int gt = for x in Option 1 for y in Option x + 1 yield y compute Option 2 ```

raulraja · Answer

Aside from the fact that `yield` requires to be invoked in a coroutine Do I need to resort to mutation to be able to return something or does `yield` lifts values back into the context of `Option` in this case

elizarov · Answer

You ll have to wrap it all in option builder like option Does not have to be a coroutine since there is only one element

raulraja · Answer

ok would `yield y ` in that case match the return type expected by the option builder If say the option builder expects back `Option lt Int gt ` for example would `yield` satisfy that or do builders always need to return `Unit`

raulraja · Answer

nevermind I see yield would change the state in the continuation builder and then I can return myself whatever I want from it

raulraja · Answer

thanks for the info < elizarov>