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I want to write a function that works on any Scala type with a total ordering (i.e. I can use '<' on it). What's the syntax for that? The best I've come up with is
def lessThan[T <: Ordered[T]](x: T, Y: T) = x < y
That doesn't work, though, when I try using it from the REPL:
scala> lessThan(1, 2)
<console>:8: error: inferred type arguments [Int] do not conform to method lessThan's type parameter bounds [T <: Ordered[T]]
lessThan(1, 2)
^
scala> import runtime._
import runtime._
scala> lessThan(new RichInt(1), new RichInt(2))
<console>:8: error: inferred type arguments [scala.runtime.RichInt] do not conform to method lessThan's type parameter bounds [T <: Ordered[T]]
lessThan(new RichInt(1), new RichInt(2))
Essentially, I believe I want the equivalent of this Haskell code:
lessThan :: (Ord a) => a -> a -> Bool
lessThan x y = x < y
I'm using scala 2.7.3 on a Debian system.
What am I missing, and where?
405
Syntax. def functionName ([list of parameters]) : [return type] = { function body return [expr] } Here, return type could be any valid Scala data type and list of parameters will be a list of variables separated by comma and list of parameters and return type are optional.
=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class. For example, the type Int => String , is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String .
To define a function in Scala, you need to use the keyword def. Then add the name of your function which in our case will be favoriteDonut followed by an empty pair of parenthesis (). To specify the return type you need to use the colon : followed by the return type which in our case is a String.
Scala functions are first-class values. Scala functions are first-class values. You must mention the return type of parameters while defining the function and the return type of a function is optional. If you don't specify the return type of a function, the default return type is Unit.
The equivalent of Haskell's type classes in Scala is done via implicits. There are two ways to do what you want
The first is with view bounds
scala> def lessThan[T <% Ordered[T]](x : T, y : T) = x < y
lessThan: [T](T,T)(implicit (T) => Ordered[T])Boolean
scala> lessThan(1,2)
res0: Boolean = true
The second is with an implicit parameter
scala> def lessThan[T](x : T, y : T)(implicit f : T => Ordered[T]) = x < y
lessThan: [T](T,T)(implicit (T) => Ordered[T])Boolean
scala> lessThan(4,3)
res1: Boolean = false
The former is syntax sugar for the later. The later allows more flexibility.
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