switch vs match

Station 1.10 11 min Interchange: switch & match & match (true) → switch

PHP has two of these. switch compares loosely and falls through unless you remember break. match, from PHP 8, compares strictly, never falls through, and returns a value. Go's switch sits between them, a lot nearer to match.

No breaks to forget

switch line {
case "Central":
	colour = "red"
case "Jubilee":
	colour = "grey"
default:
	colour = "black"
}

Each case ends where the next one starts. There's no break, because there's no fallthrough to stop.

A few more things that work like match:

• Several values share a case with commas: case 301, 302, 307, 308:.
• Comparisons are strict. Both sides must be the same type, so "1" never matches 1, and the compiler stops you trying.
• Cases are tried top to bottom, and the first match wins. default can go anywhere, but it only runs if nothing else matches.
• Cases can be any expression of the right type, not only literals: case maxDelay: or case limit + 1:.

If you really do want PHP's fallthrough, Go has a keyword for it: end a case with fallthrough, and the next case's body runs too, without checking its value. You'll almost never see it.

The difference shows up the moment a case matches:

%%{init: {"flowchart": {"nodeSpacing": 18}}}%%
flowchart TB
  accTitle: What happens after a case matches, in PHP and in Go
  accDescr: In PHP, when case 301 matches, its empty body runs and then execution falls into case 302's body, and on through every case below until it reaches a break. In Go, when case 301 matches, only its own body runs, which is empty, and then the switch ends.
  subgraph go ["Go"]
    direction TB
    g1["301 matches"] --> g2["empty body runs"] --> g3["switch ends"]
  end
  subgraph php ["PHP"]
    direction TB
    p1["301 matches"] --> p2["empty body runs"] --> p3["falls into 302"] --> p4["…to a break"]
  end

Where it's not match

Go's switch is a statement, not an expression. There's no $label = match (…) { … }, so you assign inside each case, or put the switch in a small function and return from each case. That second option reads almost exactly like a match.

And when nothing matches and there's no default, nothing happens. PHP's match would throw an UnhandledMatchError. Go just carries on after the switch, so if a missing case would be a bug, add a default that says so.

The tagless switch

Leave out the value after switch, and each case becomes a condition of its own:

switch {
case delay <= 0:
	band = "On time"
case delay < 15:
	band = "Late"
default:
	band = "Very late"
}

The first case that's true runs. This is PHP's match (true) trick, built in, and it's the usual Go replacement for a long if / else if chain. You can't mix the two styles: a switch code compares every case with code, so a condition like case code >= 500: in one is a compile error.

Like if (1.8), a switch can run a short statement first, switch s := status(); s { … }, with s scoped to the switch.

break in a switch

break works inside a switch, though you rarely need it. The catch is that it leaves the innermost for or switch. Inside a loop, a break in a case leaves the switch, and the loop carries on. To leave the loop, label it and use break label, as in 1.9.

(There's one more kind, the type switch, which picks a case based on a value's type. It needs interfaces, so it waits for lesson 3.6.)