Extensible Pattern Matching

tolerance.fsx

//Hi Julie, I liked your article and couldn't resist offering some possible improvements. I don't know
//what all the formulae mean but F#'s pattern matching is pretty powerful but also extensible and can
//be used to greatly improve readability and your chances of getting the code right in the first go.
//
//Below you can see the definition of two Active Pattern to extend F#'s pattern matching with a way
//to extract information from simple values, in this case floats.

let (|GreaterThan|_|) num = function
    | value when value > num -> Some ()
    | _ -> None

let (|LesserThan|_|) num = function
    | value when value < num -> Some ()
    | _ -> None

//You can then apply it in pattern matching position to express your tolerance formula this way:

let tolerance2 = function
    | Some (GreaterThan 1.94 | LesserThan 1.) -> 1.
    | _ -> 0.979

//You can see pattern matching can be nested inside other pattern matches, also supporting alternation
//constructs and alleviating all needs for type annotation. The handy 'function' shortcut makes for a nice
//substitute in case of the form ``let f x = match x with ...``.

//I had little doubts when I implemented my pattern matching but decided to import FsCheck and make sure it
//behaved just like your original function:

#r @"C:\Projects\Lib\FsCheck.dll"

FsCheck.Check.Quick("Tolerance", fun x -> tolerance x = tolerance2 x)

//Gave the following result: Tolerance-Ok, passed 100 tests.