switch 文で Optional Binding を使いたかった話 ref: http://qiita.com/lovee/items/7bd9aee01d7ccbf72d2b

file0.swift

let dict = [1: 2, 3: 4]
let key = 1

if let value = dict[key] {
    print(value)
}

file1.swift

// このコードは間違いです。
let dict = [1: 2, 3: 4]
let key = 1

switch key {
case let value = dict[key]:
    print(value)

default:
    break
}

file2.swift

let dict = [1: 2, 3: 4]
let key = 1

switch dict[key] {
case .Some(let value):
    print(value)

default:
    break
}

file3.swift

let dict = [1: 2, 3: 4]
let key = 1

switch (key, dict[key]) {
case (let key, _) where key < 0:
    print("Invalid")

case (_, let value?):
    print(value)

default:
    print("nil")
}

file4.swift

let dict1 = [1: 2, 3: 4]
let dict2 = [5: 6, 7: 8]
let key = 5

var value = 0

switch key {
case let key where {() -> Int? in if let res = dict1[key] { value = res }; return dict1[key]}() != nil:
    print(value)

case let key where {() -> Int? in if let res = dict2[key] { value = res }; return dict2[key]}() != nil:
    print(value)

default:
    print("nil")

}

file5.swift

let dict1 = [1: 2, 3: 4]
let dict2 = [5: 6, 7: 8]
let key = 5

var value = 0
let getDictValueFor = {(key: Int, fromDict dict: [Int: Int], inout andWriteTo value: Int) -> Int? in
    if let result = dict[key] {
        value = result
    }
    return dict[key]
}

switch key {
case let key where getDictValueFor(key, fromDict: dict1, andWriteTo: &value) != nil:
    print(value)

case let key where getDictValueFor(key, fromDict: dict2, andWriteTo: &value) != nil:
    print(value)

default:
    print("nil")

}

file5.txt

let dict1 = [1: 2, 3: 4]
let dict2 = [5: 6, 7: 8]
let key = 5

var value = 0
let getDictValueFor = {(key: Int, fromDict dict: [Int: Int], inout andWriteTo value: Int) -> Int? in
    if let result = dict[key] {
        value = result
    }
    return dict[key]
}

switch key {
case let key where getDictValueFor(key, fromDict: dict1, andWriteTo: &value) != nil:
    print(value)

case let key where getDictValueFor(key, fromDict: dict2, andWriteTo: &value) != nil:
    print(value)

default:
    print("nil")

}