F# Kung Fu #2: Custom Numeric Literals.

All of you probably know that F# has a set of predefined numerical literals that allow you to clarify meaning of numbers. For example, number 32 will be interpreted as int by default. If you add ‘L’ suffix – 32L, you will get int64 number. If you add ‘I’ suffix- 32I, you will get System.Numerics.BigInteger number.

But F# has an extensible point here: you can define custom interpretation for suffixes Q, R, Z, I, N, G. The trick is that you need to declare an F# module with a special name and define converters functions. (For example NumericLiteralZ for Z literal).

module NumericLiteralZ =
    let FromZero() = 0
    let FromOne() = 1
    let FromInt32 n =
        let rec sumDigits n acc =
            if (n=0) then acc
            else sumDigits (n/10) (acc+n%10)
        sumDigits n 0
    //let FromInt64 n = ...
    //let FromString s = ...

let x = 11111Z
//val x : int = 5
let y = 123Z
//val y : int = 6

Have fun, but be careful.

Update: Note that you cannot use constants integer literals with suffixes Q, R, Z, I, N, G in pattern matching.