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  2. (How) can I make this monadic bind tail-recursive?

(How) can I make this monadic bind tail-recursive?

由 余生长醉 提交于 2019-12-03 06:21:45

It seems to me your monad is a State with error handling.

It's basically ErrorT< State<'s,Either<'e,'a>>> but the error branch binds again which is not very clear to me why.

Anyway I was able to reproduce your Stack Overflow with a basic State monad:

type State<'S,'A> = State of ('S->('A * 'S))

module State =
    let run (State x) = x :'s->_
    let get() = State (fun s -> (s , s))  :State<'s,_>
    let put x = State (fun _ -> ((), x))  :State<'s,_>
    let result a = State(fun s -> (a, s))
    let bind (State m) k = State(fun s -> 
                                    let (a, s') = m s
                                    let (State u) = (k a) 
                                    u s')     :State<'s,'b>

    type StateBuilder() =
        member this.Return op = result op
        member this.Bind (m, cont) = bind m cont

    let state = StateBuilder()

    let rec loop (i: 'i) (next: 'i -> 'i) (pred: 'i -> 's -> bool) (m: 'i -> State<'s, unit>) =
        state {
            let! s = get()
            do! if pred i s then
                    state {
                        do! m i
                        let i = next i
                        do! loop i next pred m }
                else result () }

    let during (pred : 's -> bool) (m : State<'s, unit>) =
        loop () id (fun _ -> pred) (fun _ -> m)

// test
open State
ignore <| run (state { do! during (fun c -> true) (result ()) })  () // boom

As stated in the comments one way to solve this is to use a StateT<'s,Cont<'r,'a>>.

Here's an example of the solution. At the end there is a test with the zipIndex function which blows the stack as well when defined with a normal State monad.

Note you don't need to use the Monad Transformers from FsControl, I use them because it's easier for me since I write less code but you can always create your transformed monad by hand.

标签
f#
monads
tail-recursion
computation-expression
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