ResultBuilder

A simple computational expression builder to work with Result<'a, 'b>.

Working with values

You can use it to construct a Result from simple values (though that might not make much sense ;)

let value : Result<int, unit> = result {
    return 0
} // Ok 0

It can help, when working with the inner values of Result – for instance:

let value : Result<int, unit> = result { 
    let! a = Ok 1
    let! b = Ok 2

    return a + b
} // Ok 3

The benefit is the handling of Error values:

let value : Result<int, string> = result { 
    let! a = Ok 1
    let! b = Error "oh no :("

    return a + b
} // Error "oh no :("

Working with collections

While the above examples can also be done with pattern matching, it gets trickier when working with collections. The semantic of the ResultBuilder is, when processing a list of Result values, either there are only Ok values, and you get an Ok of a list of all values. If there is at least one Error value, you get an Error of a list of all encountered errors. For example:

let cs : Result<int, string> list = [
    Ok 1
    Ok 2
    Ok 3
    Error "4"
    Ok 5
    Error "6"
]

let a : Result<int list, string list> = result {
    for (c : int) in cs do
        yield c + 1
} // Error ["4"; "6"]

let a' = result {
    for c in cs |> List.take 3 do
        yield c + 1
} // Ok [2; 3; 4]

for constructs do not have to iterate over a value of type Result:

let b = result {
    for (i, c) in cs |> List.indexed do
        let! c = c // c is now Result<int, string> and must be unwrapped
        yield c + i
}

Usage

Just copy the file into your Project, or use Pakets GitHub dependency feature and open FSharp.Control.Results.