13.1 if/let

The if/let statement is a concise way to treat the case of an enum value matching one pattern only.

Let us define a tagged union:

:clear

enum Speed {
  Miles(f32), // Miles per hour
  Km(f32),    // Km/h
  Unknown,
}

And an array of values:

let speeds = [Speed::Unknown, Speed::Km(60.5), Speed::Miles(40.2)];

In the following example the code block of the if/let statement is used twice in order to process the two cases Miles and Km:

for x in &speeds {
  if let Speed::Miles(speed) = x {
    println!("Speed is {speed} miles/h.");
  }
  else if let Speed::Km(speed) = x {
    println!("Speed is {speed} km/h.");
  }
}

Speed is 60.5 km/h.

Speed is 40.2 miles/h.

13.1.1Returned value¶

With the if/let/else expression we may return values, the restriction being that all returned values must be of the same type. Let us use a if/let/else expression to get a speed in km/h all the time:

for x in &speeds {
  let speed = if let Speed::Miles(s) = x { s / 0.62 }
              else if let Speed::Km(s) = x { *s }
              else { 0.0 };
  println!("Speed is {speed} km/h.");
}

Speed is 0 km/h.

Speed is 60.5 km/h.

Speed is 64.83871 km/h.

13.1.2let/else¶

The let/else control flow works the other way, letting us match a value and run a code block when it does not match:

for x in &speeds {
  let Speed::Miles(speed) = x else {
    continue;
  };
  println!("Speed is {speed} miles/h.");
}

Speed is 40.2 miles/h.