Evaluation Strategy: Haskell vs Scala

Haskell is a non-strict language, and GHC uses a strategy called laziness which combines non-strictness and sharing for efficiency.

Thus, you can easily implement const which never uses the second argument.

const x y = x

With this definition, it is okay to pass undefined as the second argument of const because y is not never evaluated. But in Haskell, you can also make an argument strict using the BangPatterns GHC extension.

const x !y = x

Interestingly, the situation is reversed in Scala whose default evaluation strategy is strict.

def const(x: Int, y:Int) = x

You can make an argument non-strict by putting the => symbol between the variable name and the type.

def const(x: Int, y: => Int) = x