let compatiblePreferences = rawValues.filter { defaults[$0.key] != nil }
rawValues = defaults.merging(compatiblePreferences) { (_, new) in new }

// My understanding is that Swift lambdas can have anonymous parameters:

rawValues.filter { // no named lambda parameter here
  defaults[$0.key] != nil
}

// or named parameters

rawValues.filter { value in // `$0` is named value and `in` is used to finish statement
  defaults[value.key] != nil
}

// That helps us understand the second line:

defaults.merging(compatiblePreferences) { (_, new) in new }

// It can be reformatted as:

defaults.merging(compatiblePreferences) { (_, new) in // 2 names parameters for the conflicting values of both lists; note that `_` tells Swift to ignore the first parameter; we don't need it so we don't name it
  new // implicit `return` of new -> we always pick new value on conflicting key
}