CODEWITHSUNDEEP
javascriptscalaimmutability
Uri Goren
Uri Goren

Reputation: 13700

Why does Scala need the def statement?

I am new to scala, but I have background in javascript.

While I see the need to separate between val and var (mutable and immutable), I can't see why the def statement should ever be needed.

If functions are truly first-class citizens, as in javascript, why should they be declared with def and not with val ?

Is that design decision based on JVM related constraints, or is there some underlying logic that I fail to comprehend ?

Upvotes: 6

Views: 148

Answers (3)

0__
0__

Reputation: 67290

One big limitation of functions is that they cannot be generic as a value. E.g.

def foo[A](bar: A): Unit

That cannot be expressed as a function value

val foo: A => Unit  // A is _not_ a type parameter

that requires a resolution of type parameter A. Other differences are: Methods carry a natural notion of this as the enclosing class; they can abort via return. They can have multiple parameter lists, most importantly an implicit parameter list. They have named and default arguments. (I'm trying to imagine how a function with named and default arguments would look, perhaps it could be designed)

It probably would not have been impossible to just have function values, but def seems like a better match for the OO aspects of Scala.

Upvotes: 9

Timothy Shields
Timothy Shields

Reputation: 79521

def differs from val and var in that every time the variable is referenced, its expression is reevaluated.

Upvotes: 4

Eugene Zhulenev
Eugene Zhulenev

Reputation: 9734

I think it's language decision made for easier migration from Java. It's quite a big shift in mind to watch on functions as on function-values.

Upvotes: 1

Related Questions