Tim Van Wassenhove

Passionate geek, interested in Technology. Proud father of two

30 Jan 2014

A parameterized active pattern to match the first elements of an array

Been writing code in F# for almost a year and never blogged about it. Time to change that. Earlier today someone asked the following on twitter:

Is it possible to pattern match the first part of an array in #FSharp? Something like | 1::2::3::tail but for arrays? #lazyweb

I accepted the challenge :)

let (|Array|_|) pattern toMatch =      
	let patternLength = Array.length pattern
	let toMatchLength = Array.length toMatch
	let tailLength = toMatchLength - patternLength

	if patternLength > toMatchLength then
		let firstElementsAreEqual = [ 0 .. (patternLength - 1) ] |> Seq.forall (fun i -> pattern.[i] = toMatch.[i])
		if firstElementsAreEqual then
			Some(Array.sub toMatch patternLength tailLength)
match [|1;2;3|] with
| Array \[|1|] tail -> sprintf "bingo %i" (tail |> Array.sum) // the tail is [|2;3|\]

Or as a gist: https://gist.github.com/timvw/8717796.