Implementing let! ... and! ... in a custom Computation Expression

Question

I want to add support to my Computation Expression builder for this construct:

let f = 
  foo {
    let! x =
      foo {
        return 1
      }
    and! y =
      foo {
        return 2
      }

    return x + y
  }

The compiler says I must implement either Bind2 or MergeSource. However, I cannot find them in the docs.

What signatures should these have? Can you give a simple example?

Answer 1

Here's a simple builder that works with your example:

type Foo<'t> = MkFoo of 't

type FooBuilder() =
    member _.Bind(MkFoo x, f) = f x
    member _.Return(x) = MkFoo x
    member _.MergeSources(MkFoo x, MkFoo y) = MkFoo (x, y)

let foo = FooBuilder()

let f =
    foo {
        let! x = foo { return 1 }
        and! y = foo { return 2 }
        return x + y
    }
printfn "%A" f   // output: MkFoo 3

The purpose of MergeSources is to create a tuple of results that will be automatically de-tupled by the compiler. You might also find this example and this SO question useful.