F# 4.5–The ‘match!’ keyword

F# 4.5 introduces the match! keyword which allows you to inline a call to another computation expression and pattern match on its result. Let’s have an example.

Here is the code I had to write before F# 4.5:

	let doThingsAsync (url: string) =
	async {
	let! r = callServiceAsync url
	match r with
	| Some data -> printfn "Hello F#"
	| None -> printfn "%s" s
	}

view raw doThingsAsync.fs hosted with ❤ by GitHub

Notice that I have to bind the result of callServiceAsync before I can pattern match on it. Now let’s see how we can simplify this using the ‘match!’ keyword:

	let doThingsAsync (url: string) =
	async {
	match! callServiceAsync url with
	| Some data -> printfn "Hello F#"
	| None -> printfn "%s" s
	}

view raw doThingsAsyncFixed.fs hosted with ❤ by GitHub

When calling a computation expression with match!, it will realize the result of the call like let!. This is often used when calling a computation expression where the result is an optional.