match in F#

An expression that must cover every case; the compiler audits.

// match is an expression: every arm yields a value, all cases covered.
let describe frames =
    match frames with
    | 0 -> "empty box"
    | 1 -> "a single frame"
    | n when n < 10 -> sprintf "%d frames, room to grow" n
    | _ -> "packed full"

[0; 1; 6; 14] |> List.iter (describe >> printfn "%s")

// Tuples destructure right in the pattern.
let advice =
    match ("spring", 12) with
    | ("spring", n) when n > 10 -> "add a super"
    | (season, _) -> sprintf "hold steady through %s" season
printfn "%s" advice

How it works

Arms try in order; when guards refine a pattern.
_ is the catch-all that completes the coverage.
Tuples destructure right in the pattern: ("spring", n).

Keywords and builtins used here

let
match
when
with

The run, in numbers

Lines: 16
Characters to type: 491
Tokens: 107
Three-star pace: 65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 99 seconds.

Type this snippet

Step 1 of 3 in Pattern matching, step 7 of 27 in Language basics.

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match in other languages

OCaml