Variables VS Functions

I have pretty strict rules on when to define something as a variable or a function: it mostly comes down to complexity.

An Example

The following example use Swift, but this policy can be applied generally across frameworks and programming languages.

Let’s say we have a viewModel class that needs to expose some data we could do something like:

struct SessionViewModel {
    private var account: Account

    var accountId: Int {
        return account.id
    }

}

or like this:

struct SessionViewModel {
    private var account: Account

    func getAccountId() -> Int {
        return account.id //O(1)
    }

}

In both cases the result is the same. I prefer to use the first (variable) if the we are simply returning a value or doing a simple O(1) computation that we can assume is going to take very little time and not being computationally expensive (aka basically instant).

If there is any work behind it use a function:

struct SessionViewModel {
    private var accounts: [Account]

    func getAccountIds() -> [Int] {
        return accounts.map{$0.id} //O(n)
    }

}

Map needs to go through each element of the array to return the id which makes it a O(n) complexity. Using an instance variable, in this case, tricks the callee into making wrong assumptions about the kind of work being performed.

My Policy

Use variables for O(1) operations. Use functions in all other cases.