IDisposable disposed prematurely in monad transformers

Description

Monad transformers don’t work nicely with “use” IDisposable (e.g. SqlCommand). Disposable object gets disposed prematurely.

Repro steps

Minimal code to reproduce below, reproduces with other monad transformers e.g. ResultT

type SomethingDisposiable() =
    interface IDisposable with 
        member __.Dispose() =
            printfn "I'm disposed!"
    
    member __.AsyncSomeOption() : Async<int option> =
        async { 
            printfn "I'm doing something async..."
            return Some 1
            }

let reproducePrematureDisposal : Async<int option> =
    OptionT.run <|
    monad {
        use somethingDisposable = new SomethingDisposiable()
        let! (result : int) = OptionT <| somethingDisposable.AsyncSomeOption()
        
        printfn "Unpacked async option: %A" result
        return result
    }

Expected behavior

expected output:

I’m doing something async… I’m disposed! Unpacked async option: 1

Actual behavior

actual output:

I’m disposed! I’m doing something async… Unpacked async option: 1

i.e. somethingDisposable already disposed by the time when AsyncSomeOption invoked.

Known workarounds

don’t use monad transformers, use async builder, write explicit control flow for nested monadic results (Option<>, Result<>).

Related information

• nuget package version: 1.1.0-CI00252
• .NET Framework 4.7.2
• Operating system: Windows 10

[gusty]

@DunetsNM I’ve applied the fix and used your repro as test.

It would be nice to add more tests for cexp-monad-trans so feel free to submit more repros here and we can add them, or if you feel like creating a PR with more tests that’s always welcome.

[gusty]

Thanks for the clarification. Of course, there’s not doubt that it should dispose at the end. I think that’s the issue itself.

I’m adding all the missing methods to the transformers which fixes the issue, at least testing with this kind of samples pass, so I will add some tests similar to your repro.

The idea is to produce a nuget afterwards, since there is plenty of new functionality, so you’ll be able to use this very soon.