type Meter = Double

This simple approach has serious drawbacks for unit handling as every other type that is a Double will be compatible with it:

type Second = Double
var length: Meter = 3
val duration: Second = 1
length = duration
length = 0d

All of the above compiles, so in this case units can only be used for marking input/output types for the readers of the code (only the intent).

case class Meter(meters: Double) extends AnyVal
case class Gram(grams: Double) extends AnyVal

Value classes provide a type-safe way to encode units, even if they require a bit more characters to use them:

var length = Meter(3)
var weight = Gram(4)
//length = weight //type mismatch; found : Gram required: Meter

By extending AnyVals, there is no runtime penalty for using them, on the JVM level, those are regular primitive types (Doubles in this case).

In case you want to automatically generate other units (like Velocity aka MeterPerSecond), this approach is not the best, though there are libraries that can be used in those cases too:

• Squants
• units
• ScalaQuantity