#r "nuget: FSharpPlus"

open FSharpPlus
open FSharpPlus.Data

type DeriverBuilder<'``monad<'t>``> () =
        inherit MonadFxBuilder<'``monad<'t>``> ()
        member inline _.BindReturn (x : '``Monad<'T>``, f: 'T -> 'U) : '``Monad<'U>`` = FSharpPlus.Control.Map.Invoke f x

let monad<'``monad<'t>``> = new DeriverBuilder<'``monad<'t>``> ()

let lst1: StateT<string,_> = StateT.lift [1;2]
let lst2: StateT<string,_> = StateT.lift [4;5]

let m = monad { 
    let! x =  lst1
    let! y =  lst2
    do! modify String.toUpper
    let! st = gets String.length
    return (x, y +  st) }

apcrvfcj..fsx(21,5): error FS0071: Type constraint mismatch when applying the default type 'obj' for a type inference variable. No overloads match for method 'Return'.

Known return type: (int -> obj)

Known type parameters: < obj , Control.Return >

Available overloads:
 - static member Control.Return.Return: 'a array * Control.Return -> ('a -> 'a array) // Argument at index 1 doesn't match
 - static member Control.Return.Return: 'a list * Control.Return -> ('a -> 'a list) // Argument at index 1 doesn't match
 - static member Control.Return.Return: 'a option * Control.Return -> ('a -> 'a option) // Argument at index 1 doesn't match
 - static member Control.Return.Return: 'a seq * Internals.Default2 -> ('a -> 'a seq) // Argument at index 1 doesn't match
 - static member Control.Return.Return: 'a voption * Control.Return -> ('a -> 'a voption) // Argument at index 1 doesn't match
 - static member Control.Return.Return: ('r -> 'a) * Control.Return -> ('a -> 'r -> 'a) // Argument at index 1 doesn't match
 - static member Control.Return.Return: (^m * 'a) * Control.Return -> ('a -> ^m * 'a) when (Control.Zero or ^m) : (static member Zero: ^m * Control.Zero -> ^m) // Argument at index 1 doesn't match
 - static member Control.Return.Return: Async<'a> * Control.Return -> ('a -> Async<'a>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: Choice<'a,'e> * Control.Return -> ('a -> Choice<'a,'e>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: Internals.Set2<'a> * Control.Return -> ('a -> Internals.Set2<'a>) when 'a: comparison // Argument at index 1 doesn't match
 - static member Control.Return.Return: NonEmptySeq<'a> * Internals.Default2 -> ('a -> NonEmptySeq<'a>) // Argument at index 1 doesn't match

 - static member Control.Return.Return: Quotations.Expr<'a> * Control.Return -> ('a -> Quotations.Expr<'a>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: ResizeArray<'a> * Control.Return -> ('a -> ResizeArray<'a>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: Result<'a,'e> * Control.Return -> ('a -> Result<'a,'e>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: Set<'a> * Control.Return -> ('a -> Set<'a>) when 'a: comparison // Argument at index 1 doesn't match
 - static member Control.Return.Return: System.Collections.Generic.IEnumerator<'a> * Internals.Default2 -> ('a -> System.Collections.Generic.IEnumerator<'a>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: System.Lazy<'a> * Control.Return -> ('a -> System.Lazy<'a>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: System.Text.StringBuilder * Control.Return -> (char -> System.Text.StringBuilder) // Argument at index 1 doesn't match
 - static member Control.Return.Return: System.Threading.Tasks.Task<'T> * Control.Return -> ('T -> System.Threading.Tasks.Task<'T>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: System.Threading.Tasks.ValueTask<'T> * Control.Return -> ('T -> System.Threading.Tasks.ValueTask<'T>) // Argument at index 1 doesn't match
 - static member Control.Return.Return: ^R * Internals.Default1 -> ('T -> ^R) when ^R: (static member Return: 'T -> ^R) // Argument at index 1 doesn't match
 - static member Control.Return.Return: string * Control.Return -> (char -> string) // Argument at index 1 doesn't match
 - static member Control.Return.Return: struct (^m * 'a) * Control.Return -> ('a -> struct (^m * 'a)) when (Control.Zero or ^m) : (static member Zero: ^m * Control.Zero -> ^m) // Argument at index 1 doesn't match Consider adding further type constraints