Andrea Andrea - 1 month ago 13
Scala Question

Side effects in Scala

I am learning Scala right in these days. I have a slight familiarity with Haskell, although I cannot claim to know it well.

Parenthetical remark for those who are not familiar with Haskell

One trait that I like in Haskell is that not only functions are first-class citizens, but side effects (let me call them actions) are. An action that, when executed, will endow you with a value of type

a
, belongs to a specific type
IO a
. You can pass these actions around pretty much like any other value, and combine them in interesting ways.

In fact, combining the side effects is the only way in Haskell to do something with them, as you cannot execute them. Rather, the program that will be executed, is the combined action which is returned by your
main
function. This is a neat trick that allows functions to be pure, while letting your program actually do something other than consuming power.

The main advantage of this approach is that the compiler is aware of the parts of the code where you perform side effects, so it can help you catch errors with them.

Actual question

Is there some way in Scala to have the compiler type check side effects for you, so that - for instance - you are guaranteed not to execute side effects inside a certain function?

Answer

No, this is not possible in principle in Scala, as the language does not enforce referential transparency -- the language semantics are oblivious to side effects. Your compiler will not track and enforce freedom from side effects for you.

You will be able to use the type system to tag some actions as being of IO type however, and with programmer discipline, get some of the compiler support, but without the compiler proof.