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How many parameters can a closure take?

Paul Hudson    @twostraws   

Updated for Xcode 14.2

Swift’s closures are like Swift’s functions, so they can take any number of parameters. However, you should exercise your power with restraint: I’ve previously said functions that accept many parameters can be hard to understand, and closures take that same problem further by adding their natural complexity on top.

So, in theory a closure can take as many parameters as you like, but in practice the overwhelming majority of examples you’ll see take zero to three parameters. This is likely helped by the fact that using $0, $1 and $2 is probably the most shorthand parameter names we can reasonably work with.

I’m not saying you can’t write closures that take four or more parameters, only that it’s extremely rare – one or two parameters covers the vast majority of cases, with three parameters being most of the remainder.

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