Hi,
Two points:
1) What you want to do is a common operation in functional programming called
flatMap. Optional type already supports it. To get “x != nil ? f(x) : nil” you
say:
x.flatMap(f)
2) You don’t need multiple versions, because there is a subtype-supertype
relationship between functions: You need to specify the most general signature
in your single function. The most general form of the function is:
(XType) throws -> RetType?
To handle throwing case, you will need to define the return type of your own
function as:
fnil<...> (...) rethrows -> RetType?
Then it will accept any of the following signatures and it just works:
func f0(_ x: XType) -> RetType
func f1(_ x: XType) -> RetType?
func f2(_ x: XType?) -> RetType
func f3(_ x: XType?) -> RetType?
func f0t(_ x: XType) throws -> RetType
func f1t(_ x: XType) throws -> RetType?
func f2t(_ x: XType?) throws -> RetType
func f3t(_ x: XType?) throws -> RetType?
Hope this helps,
Hooman
> On Jan 11, 2018, at 5:55 PM, Kenny Leung via swift-users
> <swift-users@swift.org> wrote:
>
> Hi All.
>
> I’m trying to write a utility method that is kind of the opposite of “x ??
> y”, which means “x != nil ? x : y”. I want “x != nil ? f(x) : nil” This is
> not difficult to write, but if you have "f(x)->z" and "g(x)->z?", I seem to
> need two versions of the function in order to handle it. Is there any way to
> do this with only one function?
>
> public func fnil<XType,RetType> (
> _ x:XType?,
> _ f:(XType)->RetType
> )
> -> RetType?
> {
> if let x = x {
> return f(x)
> } else {
> return nil
> }
> }
>
> public func fnil<XType,RetType> (
> _ x:XType?,
> _ g:(XType)->RetType?
> )
> -> RetType?
> {
> if let x = x {
> return g(x)
> } else {
> return nil
> }
> }
>
> private func f(_ x:Int) -> Int {
> return x
> }
>
> private func g(_ x:Int) -> Int? {
> if x == 5 {
> return nil
> } else {
> return x
> }
> }
>
> func testFnil() {
> XCTAssertNil(fnil(nil, {f($0)}))
> XCTAssertEqual(7, fnil(7,{f($0)}))
>
> XCTAssertNil(fnil(nil, {g($0)}))
> XCTAssertEqual(7, fnil(7, {g($0)}))
> XCTAssertNil(fnil(5, {g($0)}))
> }
>
> Thanks!
>
> -Kenny
>
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> swift-users@swift.org
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