Computation Expression Stub

open System

// Replace with your type
type MyType<'a> = Async<'a>
type Internal<'a> = MyType<'a>

// Replace with types that feel similar and can be converted to `MyType`
type External1<'a> = System.Threading.Tasks.Task<'a>
type External2<'a> = System.Threading.Tasks.ValueTask<'a>

/// https://fsharpforfunandprofit.com/posts/computation-expressions-builder-part3/
type Delayed<'a> = unit -> MyType<'a>

// What is InlineIfLambda? This allows generation of high performance code which Computation Expressions have had in the past.
// See https://github.com/fsharp/fslang-design/blob/main/FSharp-6.0/FS-1098-inline-if-lambda.md
// We also mark everything an inline to try to squeeze as much performance out of the implementation as possible.


/// https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/computation-expressions#creating-a-new-type-of-computation-expression
type StubBuilder() =

  /// Called for let! and do! in computation expressions.
  member inline this.Bind(input: Internal<'T>, [<InlineIfLambda>] f: ('T -> Internal<'U>)) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for efficient let! and and! in computation expressions without merging inputs.
  member inline this.Bind2
    (
      input: Internal<'T1>,
      input2: Internal<'T2>,
      [<InlineIfLambda>] f: ('T1 * 'T2 -> Internal<'U>)
    ) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for efficient let! and and! in computation expressions without merging inputs.
  member inline this.Bind3
    (
      input: Internal<'T1>,
      input2: Internal<'T2>,
      input3: Internal<'T3>,
      [<InlineIfLambda>] f: ('T1 * 'T2 * 'T3 -> Internal<'U>)
    ) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for return in computation expressions.
  member inline this.Return(input: 'T) : Internal<'T> = raise <| NotImplementedException()

  /// Called for return! in computation expressions.
  member inline this.ReturnFrom(input: Internal<'T>) : Internal<'T> = input

  /// Called for an efficient let! ... return in computation expressions.
  member inline this.BindReturn(input: Internal<'T>, [<InlineIfLambda>] f: ('T -> 'U)) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for efficient let! ... and! ... return in computation expressions without merging inputs.
  member inline this.Bind2Return
    (
      input: Internal<'T1>,
      input2: Internal<'T2>,
      [<InlineIfLambda>] f: ('T1 * 'T2 -> 'U)
    ) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for efficient let! ... and! ... return in computation expressions without merging inputs.
  member inline this.Bind3Return
    (
      input: Internal<'T1>,
      input2: Internal<'T2>,
      input3: Internal<'T3>,
      [<InlineIfLambda>] f: ('T1 * 'T2 * 'T3 -> 'U)
    ) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for and! in computation expressions.
  member inline this.MergeSources(input: Internal<'T1>, input2: Internal<'T2>) : Internal<'T1 * 'T2> =
    raise <| NotImplementedException()

  /// Called for and! in computation expressions, but improves efficiency by reducing the number of tupling nodes.
  member inline this.MergeSources3
    (
      input: Internal<'T1>,
      input2: Internal<'T2>,
      input3: Internal<'T3>
    ) : Internal<'T1 * 'T2 * 'T3> =
    raise <| NotImplementedException()

  /// Wraps a computation expression as a function. Delayed<'T> can be any type, commonly Internal<'T> or unit -> Internal<'T> are used.
  /// Many functions use the result of Delay as an argument: Run, While, TryWith, TryFinally, and Combine
  member inline this.Delay([<InlineIfLambda>] f: unit -> Internal<'T>) : Delayed<'T> =
    raise <| NotImplementedException()

  /// Executes a computation expression.
  member inline this.Run([<InlineIfLambda>] f: Delayed<'T>) : Internal<'T> = raise <| NotImplementedException()

  /// Called for sequencing in computation expressions.
  member inline this.Combine(input: Internal<'T>, [<InlineIfLambda>] f: Delayed<'T>) : Internal<'T> =
    raise <| NotImplementedException()

  /// Called for sequencing in computation expressions.
  member inline this.Combine(input: Internal<unit>, f: Internal<'T>) : Internal<'T> = raise <| NotImplementedException()

  /// Called for while...do expressions in computation expressions.
  member inline this.While
    (
      [<InlineIfLambda>] guard: unit -> bool,
      [<InlineIfLambda>] body: Delayed<'T>
    ) : Internal<'T> =
    raise <| NotImplementedException()

  /// Called for while...do expressions in computation expressions.
  member inline this.While
    (
      [<InlineIfLambda>] guard: unit -> bool,
      [<InlineIfLambda>] body: Delayed<unit>
    ) : Internal<unit> =
    raise <| NotImplementedException()

  /// Called for for...do expressions in computation expressions.
  member inline this.For(xs: #seq<'T>, [<InlineIfLambda>] f: 'T -> Internal<'U>) : Internal<'U> =
    raise <| NotImplementedException()

  /// Called for for...do expressions in computation expressions.
  // Only need one of these For implementations
  // member inline this.For(xs : #seq<'T>, [<InlineIfLambda>] f : 'T -> Internal<'U> ) : seq<Internal<'U>> =
  //     raise <| NotImplementedException()

  /// Called for try...finally expressions in computation expressions.
  member inline this.TryFinally
    (
      [<InlineIfLambda>] body: Delayed<'T>,
      [<InlineIfLambda>] final: unit -> unit
    ) : Internal<'T> =
    raise <| NotImplementedException()

  /// Called for try...finally expressions in computation expressions.
  member inline this.TryWith
    (
      [<InlineIfLambda>] body: Delayed<'T>,
      [<InlineIfLambda>] failure: exn -> Internal<'T>
    ) : Internal<'T> =
    raise <| NotImplementedException()

  /// Called for use bindings in computation expressions.
  member inline this.Using(resource: 'T when 'T :> IAsyncDisposable, [<InlineIfLambda>] f: 'T -> Internal<'U>) : Internal<'U> =
    // This may not need to be implemented for non async-like CEs and can be replaced by the Using in the Extensions module
    raise <| NotImplementedException()

  /// Called for empty else branches of if...then expressions in computation expressions.
  member inline _.Zero() : Internal<'a> = raise <| NotImplementedException()

  /// Called to convert an External to Internal type before any others calls in computation expression.
  /// This is the identity function for the internal type
  member inline _.Source(identity: Internal<'a>) : Internal<'a> = identity

  /// Called to convert an External to Internal type before any others calls in computation expression.
  /// This is the identity function for for loops.
  member inline _.Source(identity: #seq<'a>) : #seq<'a> = identity

  /// Called to convert an External to Internal type before any others calls in computation expression.
  member inline _.Source(other: External1<Option<'a>>) : Internal<'a> = raise <| NotImplementedException()

  /// Called to convert an External to Internal type before any others calls in computation expression.
  member inline _.Source(other: External2<'a>) : Internal<'a> = raise <| NotImplementedException()

/// F#'s overloading resolution prioritizes extension methods lower
/// so if you have a more Derived type like `Async<Option<'T>>` vs `Async<'T>`, it won't know
/// which one to choose. You'll want to have the more specific in the Builder itself
/// and the less specific in an extension method.
///
/// See: https://github.com/fsharp/fslang-suggestions/issues/905
[<AutoOpen>]
module StubBuilderExtension =

  let stub = StubBuilder()

  type StubBuilder with

    /// Called to convert an External to Internal type before any others calls in computation expression.
    member inline _.Source(other: External1<'a>) : Internal<'a> = raise <| NotImplementedException()

    /// Called for use bindings in computation expressions.
    member inline this.Using(resource: 'T :> IDisposable, [<InlineIfLambda>] f: 'T -> Internal<'U>) : Internal<'U> =
      // In extensions because of the priority resolution discussed above
      // Otherwise you get a "Duplicate method. The method 'Using' has the same name and signature as another method in type 'StubBuilder'"
      // Reason is, .NET cannot have overloaded methods based on constraints alone
      // This can be moved to main builder if IAsyncDisposable is not required
      raise <| NotImplementedException()


module Example =
  open System.Threading.Tasks

  let bindExamples () =
    stub {
      let! result = async { return "lol" } // string -> Used Normal Bind
      let! result = task { return Some "lol" } // string -> Used Source member External1<Option<'a>>
      let! result = task { return "lol" } // string -> Used Source member External1<'a> that is an extension member for lower binding resolution.
      and! result4 = ValueTask<string>("lol") // string -> Used Source member Extenrnal2<'a>
      return "lol"
    }

  type DisposableExample() =

    interface IDisposable with
      member this.Dispose() : unit = printfn "disposed"

  let disposeExample () =
    stub {
      use d = new DisposableExample()
      return "lol"
    }

  type AsyncDisposableExample() =
    interface IAsyncDisposable with
      member this.DisposeAsync() : ValueTask =
        printfn "disposed"
        ValueTask()

  let asyncDisposeExample () =
    stub {
      use d = new AsyncDisposableExample()
      return "lol"
    }