CODEWITHSUNDEEP
swift
Dasem
Dasem

Reputation: 440

Why do numbers outside of variable assignments compile?

Why in Swift can I type numbers without assigning them to var or let, and the code will compile fine? What is this good for?

import Foundation

55

25

23+8

print("Hello, World!")

4

11

-45

Upvotes: 1

Views: 107

Answers (4)

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36401

Like in any language you can pronounce sentences of no interest... 55 is such a swift sentence. You may think it could be easy to eliminate unuseful sentences, but it is impossible, so why not just eliminate (at compile-time) the obvious ones? Because, in general people don't try to use them or just to exercise, and it will not worth the effort. It would be easy to forbid 55, but what about n=n? Easy? Or n=2*n-n(it is much more tricky because you would need to implement mathematical properties of expressions inside the compiler, and optimisation tricks for them).

Upvotes: 0

matt
matt

Reputation: 535202

What is this good for?

It isn't good for anything, in the sense that it doesn't cause anything to happen with regard to the flow of the program. It's just something you're doing for fun, if you see what I mean. The numeric value is not being captured, so in effect it is immediately thrown away, like a virtual particle that flashes into existence one moment and back out of existence the next.

The reason it is legal is that a Swift statement is an evaluatable expression. A numeric literal is an evaluatable expression, so it's legal — though useless — as an independent statement.

You can see the same thing in many other ways. This is legal:

let n = 23
n

n is an evaluatable expression, so it's legal as a separate statement. But it is useless.

EDIT In answer to your comment below: I see no reason why a useless statement should prevent a program from compiling. But in a case like this, I would agree that it might be helpful if the compiler would warn that you're doing something useless, and in fact, for at least one case of this sort of thing, I have filed a bug report requesting this.

Upvotes: 2

sleblanc
sleblanc

Reputation: 3921

Swift is a language that has side effects, meaning that some operations can result in a mutation of the global state of the program. This has the implications that the compiler cannot simply eliminate a stand-alone statement without making extra sure that it can do so without affecting the execution of the program. Due to the complexity of state in a program, this is generally not a trivial problem, therefore in order not to penalize users who wish to invoke functions that have side effects, a line must be drawn; Swift has chosen to let users put any kind of stand-alone statement, even ones that are obviously free of side effects (constants or constant expressions), rather than spend a lot of effort trying to differentiate among various possibilities.

There could be a compile-time warning that shows lines of code that have no effect. I don't know much about Swift so I can't tell you, but if there is, you should be able to find it in the documentation.

Upvotes: 1

ryantxr
ryantxr

Reputation: 4219

It has to do with the fact that a number or an expression is a valid statement. So it can exist on its own. Some other languages have the same or similar feature.

Upvotes: 0

Related Questions