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I wrote the following in Swift to perform a sum:

let total:Double = session.courses.reduce(0) { $0 + $1.totalPrice() }

It's concise, it does the job but it does not seems very readable to me. Wouldn't a "classic" for loop be more explicit/readable/maintainable?

var total:Double = 0
for dish in session.courses {
    total += dish.totalPrice()
}

Should closures always be the way to go in Swift?

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Both solutions are correct. Some advantages of the "functional" approach with reduce():

  • Concise code.
  • Returns a constant. It is clear to the reader that total is assigned to exactly once, and the compiler may use this information for optimizing code.
  • The type annotation on the result value is not needed:

    let total = session.courses.reduce(0) { $0 + $1.totalPrice() }
    

    because that can be inferred from the compiler automatically. This will continue to compile without changes if the type of totalPrice() is changed, e.g. from Double to Float.

To increase legibility, you can use named closure parameters instead of the $n shortcuts:

let total = session.courses.reduce(0) { (partialSum, dish) in
    partialSum + dish.totalPrice()
}
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  • 1
    \$\begingroup\$ +1 for legibility! I find the $n shortcuts incredibly hard to read, and they offer no information on what the values represent. \$\endgroup\$ – user1118321 Jan 17 '18 at 3:18
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Another alternatives:

let sum = session.courses.map({$0.totalPrice()}).reduce(0, +)

Another note, I think it's more elegant to implement totalPrice as computed variable instead of function.

Another way is to use forEach :

var sum = 0
session.courses.forEach {sum+=$0.totalPrice()}

as @Martin R mentioned the functional approach has some advantages especially that the result is constant.

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