In-place map for MutableCollection

Evolution Pitches
78

One case where the difference matters is when you have a collection-of-collections. To avoid confusion, let’s say an Array of Data:

var x = [Data(repeating: 10, count: 2),
         Data(repeating: 11, count: 3)]

// Element-mutating version:
x.modifyAll{ data in
  data[0] = 15
}

// Element-replacing version
x.updateAll{ data in
  var newData = data
  newData[1] = 14
  return newData
}

print(x.map{Array($0)})

Even if the compiler is smart enough to produce equivalent code for the two implementations (can we get confirmation of that?) there still remains a massive readability difference at the call-site.

Although…I just stumbled on a weird case where the compiler requires type annotation (and has an unhelpful diagnostic): 
var z = [[2, 2], [3, 3, 3]]
z.modifyAll{ array in
  array.modifyAll{ n in     // error: passing value of type 'Int' to an inout parameter requires explicit '&'
    n += 10
  }
}

The actual fix is to replace “n in” with “(n: inout Int) in”, OR to replace “n += 10” with “n = n + 10”. And I don’t understand why.

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