ListBuilder.fsx

// Computation expression builder that uses a custom bind operation
// to allow binding a value to a name and yielding it as part of a 
// list in one go.
type ListBuilder() =
    member this.Bind(x, f) = x::f(x)
    member this.Delay(f) = f()
    member this.Combine(x, xs) = x @ xs
    member this.Yield(x) = [x]
 
let list = ListBuilder()
 
// Produces [2, 4, 8, 16]
list {
    let! x = 2
    let! x2 = x * x
    let! x3 = x2 * x
    yield x3 * x
}
 
// Equivalent list comprehension
[
    let x = 2
    yield x
    let x2 = x * x
    yield x2
    let x3 = x2 * x
    yield x3
    yield x3 * x
]

// It would be kind of cool if this "let!" behavior was in the built-in list comprehensions and 
// sequence expressions. Another thing that would be nice is if I could do "let! x4 = x3 * x" at
// the end of the first snippet for a uniform syntax. It doesn't really make sense to end an 
// expression with a normal let binding, but a custom one might make sense since it can have a 
// different behavior.