Related
OK:
let em inp=sprintf"<em>%A</em>"inp
let bold inp=sprintf"<b>%A</b>"inp
printfn"%s"<|em"blabla"///<em>blabla</em>
Trying to define together (with compiling error):
let em2,bold2=
let tag a b=sprintf"<%s>%A</%s>"a b a
(fun inp->tag"em"inp),tag"b"
Error:
Value restriction. The value 'em2' has been inferred to have generic type
val em2 : ('_a -> string -> string)
Either make the arguments to 'em2' explicit or, if you do not intend for it to be generic, add a type annotation.F# Compiler(30)
I don't think this is going to work, because the F# compiler doesn't consider a tuple to be a "simple immutable value":
The compiler performs automatic generalization only on complete function definitions that have explicit arguments, and on simple immutable values.
This means that the compiler issues an error if you try to compile code that is not sufficiently constrained to be a specific type, but is also not generalizable. The error message for this problem refers to this restriction on automatic generalization for values as the value restriction.
Instead, I think you have to define them separately, like this:
let tag a b=sprintf"<%s>%A</%s>"a b a
let em2 inp=tag"em"inp
let bold2 b=tag"b"b
If your desire here is to hide the definition of tag, you can make it private.
I like the idea of centralizing the logic (here: HTML formatting) in a single factory function, to enforce the DRY principle.
Instead of fully hiding the tag factory function in a closure, we can hide it just from other modules, making it private which is usually enough encapsulation. After some renaming:
let private inside tag content = // 'a -> '-b -> string
$"<{tag}>{content}</{tag}>" // 👈 F# 5 interpolated string
Then, the usual way in F# to generate the specific function is through partial application. Since the current inside function is generic, we can't use the point free notation (meaning implicit parameter content) without loosing the generic type:
let em = inside "em" // ⚠️ obj -> string
We have 2 solutions:
Have explicit content parameter: let em content = inside "em" content but it's less elegant.
Change the signature of inside function and make all parameters of type string. In fact, the function inside does not care about the type of its parameters - it only cares about strings since it casts them to string implicitly using the ToString() method which can lead to bad surprises when calling this function.
let private inside tag content = // string -> string -> string
$"<%s{tag}>%s{content}</{tag}>" // 👈 %s to indicate parameters are strings
let em = inside "em" // string -> string
let strong = inside "strong"
From Apple's documentation:
The #dynamicCallable attribute lets you call named types like you call
functions using a simple syntactic sugar. The primary use case is
dynamic language interoperability.
Why would you want to use an #dynamicCallable instead of direct approch?
#dynamicCallable is a new feature of Swift 5. From Paul Hudson's article on "How to use #dynamicCallable in Swift" (emphasis mine):
SE-0216 adds a new #dynamicCallable attribute to Swift, which
brings with it the ability to mark a type as being directly callable.
It’s syntactic sugar rather than any sort of compiler magic,
effectively transforming this code:
let result = random(numberOfZeroes: 3)
Into this:
let result = random.dynamicallyCall(withKeywordArguments: ["numberOfZeroes": 3])
[...] #dynamicCallable is the natural extension of
#dynamicMemberLookup [SE-0195], and serves the same purpose: to
make it easier for Swift code to work alongside dynamic languages such
as Python and JavaScript. [...] #dynamicCallable is really flexible about which data
types its methods accept and return, allowing you to benefit from all
of Swift’s type safety while still having some wriggle room for
advanced usage.
Introduce user-defined dynamically "callable" types
Proposal: SE-0216
Authors: Chris Lattner, Dan Zheng
Review Manager: John McCall
Implementation: apple/swift#20305
Decision Notes: Rationale
Status: Implemented (Swift 5)
Introduction
This proposal is a follow-up to SE-0195 - Introduce User-defined "Dynamic Member
Lookup" Types,
which shipped in Swift 4.2. It introduces a new #dynamicCallable attribute, which marks
a type as being "callable" with normal syntax. It is simple syntactic sugar
which allows the user to write:
a = someValue(keyword1: 42, "foo", keyword2: 19)
and have it be rewritten by the compiler as:
a = someValue.dynamicallyCall(withKeywordArguments: [
"keyword1": 42, "": "foo", "keyword2": 19
])
Many other languages have analogous features (e.g. Python "callables", C++ operator(), and
functors in many other languages), but the
primary motivation of this proposal is to allow elegant and natural interoperation with
dynamic languages in Swift.
Swift-evolution threads:
- Pitch: Introduce user-defined dynamically "callable"
types.
- Pitch #2: Introduce user-defined dynamically “callable”
types.
- Current pitch thread: Pitch #3: Introduce user-defined dynamically “callable”
types
Motivation and context
Swift is exceptional at interworking with existing C and Objective-C APIs and
we would like to extend this interoperability to dynamic languages like Python,
JavaScript, Perl, and Ruby. We explored this overall goal in a long design
process wherein the Swift evolution community evaluated multiple different
implementation approaches. The conclusion was that the best approach was to put
most of the complexity into dynamic language specific bindings written as
pure-Swift libraries, but add small hooks in Swift to allow these bindings to
provide a natural experience to their clients.
SE-0195
was the first step in this process, which introduced a binding to naturally
express member lookup rules in dynamic languages.
What does interoperability with Python mean? Let's explain this by looking at
an example. Here's some simple Python code:
class Dog:
def __init__(self, name):
self.name = name
self.tricks = [] # creates a new empty list for each `Dog`
def add_trick(self, trick):
self.tricks.append(trick)
With the SE-0195 #dynamicMemberLookup feature
introduced in Swift 4.2, it is possible to implement a Python interoperability
layer
written in Swift. It interoperates with the Python runtime, and project all
Python values into a single PythonObject type. It allows us to call into the
Dog class like this:
// import DogModule.Dog as Dog
let Dog = Python.import.call(with: "DogModule.Dog")
// dog = Dog("Brianna")
let dog = Dog.call(with: "Brianna")
// dog.add_trick("Roll over")
dog.add_trick.call(with: "Roll over")
// dog2 = Dog("Kaylee").add_trick("snore")
let dog2 = Dog.call(with: "Kaylee").add_trick.call(with: "snore")
This also works with arbitrary other APIs as well. Here is an example working
with the Python pickle API and the builtin Python function open. Note that
we choose to put builtin Python functions like import and open into a
Python namespace to avoid polluting the global namespace, but other designs
are possible:
// import pickle
let pickle = Python.import.call(with: "pickle")
// file = open(filename)
let file = Python.open.call(with: filename)
// blob = file.read()
let blob = file.read.call()
// result = pickle.loads(blob)
let result = pickle.loads.call(with: blob)
This capability works well, but the syntactic burden of having to use
foo.call(with: bar, baz) instead of foo(bar, baz) is significant. Beyond
the syntactic weight, it directly harms code clarity by making code hard to
read and understand, cutting against a core value of Swift.
The proposed #dynamicCallable attribute directly solves this problem.
With it, these examples become more natural and clear, effectively matching the
original Python code in expressiveness:
// import DogModule.Dog as Dog
let Dog = Python.import("DogModule.Dog")
// dog = Dog("Brianna")
let dog = Dog("Brianna")
// dog.add_trick("Roll over")
dog.add_trick("Roll over")
// dog2 = Dog("Kaylee").add_trick("snore")
let dog2 = Dog("Kaylee").add_trick("snore")
Python builtins:
// import pickle
let pickle = Python.import("pickle")
// file = open(filename)
let file = Python.open(filename)
// blob = file.read()
let blob = file.read()
// result = pickle.loads(blob)
let result = pickle.loads(blob)
This proposal merely introduces a syntactic sugar - it does not add any new
semantic model to Swift. We believe that interoperability with scripting
languages is an important and rising need in the Swift community, particularly
as Swift makes inroads into the server development and machine learning
communities. This feature is also precedented in other languages (e.g. Scala's
Dynamic trait), and
can be used for other purposes besides language interoperability (e.g.
implementing dynamic proxy objects).
Proposed solution
We propose introducing a new #dynamicCallable attribute to the Swift language
which may be applied to structs, classes, enums, and protocols. This follows
the precedent of
SE-0195.
Before this proposal, values of these types are not valid in a call expression:
the only existing callable values in Swift are those with function types
(functions, methods, closures, etc) and metatypes (which are initializer
expressions like String(42)). Thus, it is always an error to "call" an
instance of a nominal type (like a struct, for instance).
With this proposal, types with the #dynamicCallable attribute on their
primary type declaration become "callable". They are required to implement at
least one of the following two methods for handling the call behavior:
func dynamicallyCall(withArguments: <#Arguments#>) -> <#R1#>
// `<#Arguments#>` can be any type that conforms to `ExpressibleByArrayLiteral`.
// `<#Arguments#>.ArrayLiteralElement` and the result type `<#R1#>` can be arbitrary.
func dynamicallyCall(withKeywordArguments: <#KeywordArguments#>) -> <#R2#>
// `<#KeywordArguments#>` can be any type that conforms to `ExpressibleByDictionaryLiteral`.
// `<#KeywordArguments#>.Key` must be a type that conforms to `ExpressibleByStringLiteral`.
// `<#KeywordArguments#>.Value` and the result type `<#R2#>` can be arbitrary.
// Note: in these type signatures, bracketed types like <#Arguments#> and <#KeywordArguments#>
// are not actual types, but rather any actual type that meets the specified conditions.
As stated above, <#Arguments#> and <#KeywordArguments#> can be any types
that conform to the
ExpressibleByArrayLiteral
and
ExpressibleByDictionaryLiteral
protocols, respectively. The latter is inclusive of
KeyValuePairs,
which supports duplicate keys, unlike Dictionary.
Thus, using KeyValuePairs is recommended to support duplicate keywords and
positional arguments (because positional arguments are desugared as keyword
arguments with the empty string "" as the key).
If a type implements the withKeywordArguments: method, it may be dynamically
called with both positional and keyword arguments: positional arguments have
the empty string "" as the key. If a type only implements the
withArguments: method but is called with keyword arguments, a compile-time
error is emitted.
Since dynamic calls are syntactic sugar for direct calls to dynamicallyCall
methods, additional behavior of the dynamicallyCall methods is directly
forwarded. For example, if a dynamicallyCall method is marked with throws
or #discardableResult, then the corresponding sugared dynamic call will
forward that behavior.
Ambiguity resolution: most specific match
Since there are two #dynamicCallable methods, there may be multiple ways to
handle some dynamic calls. What happens if a type specifies both the
withArguments: and withKeywordArguments: methods?
We propose that the type checker resolve this ambiguity towards the tightest
match based on syntactic form of the expression. The exact rules are:
If a #dynamicCallable type implements the withArguments: method and it is
called with no keyword arguments, use the withArguments: method.
In all other cases, attempt to use the withKeywordArguments: method.
This includes the case where a #dynamicCallable type implements the
withKeywordArguments: method and it is called with at least one keyword
argument.
This also includes the case where a #dynamicCallable type implements only
the withKeywordArguments: method (not the withArguments: method) and
it is called with no keyword arguments.
If #dynamicCallable type does not implement the withKeywordArguments:
method but the call site has keyword arguments, an error is emitted.
Here are some toy illustrative examples:
#dynamicCallable
struct Callable {
func dynamicallyCall(withArguments args: [Int]) -> Int { return args.count }
}
let c1 = Callable()
c1() // desugars to `c1.dynamicallyCall(withArguments: [])`
c1(1, 2) // desugars to `c1.dynamicallyCall(withArguments: [1, 2])`
c1(a: 1, 2) // error: `Callable` does not define the 'withKeywordArguments:' method
#dynamicCallable
struct KeywordCallable {
func dynamicallyCall(withKeywordArguments args: KeyValuePairs<String, Int>) -> Int {
return args.count
}
}
let c2 = KeywordCallable()
c2() // desugars to `c2.dynamicallyCall(withKeywordArguments: [:])`
c2(1, 2) // desugars to `c2.dynamicallyCall(withKeywordArguments: ["": 1, "": 2])`
c2(a: 1, 2) // desugars to `c2.dynamicallyCall(withKeywordArguments: ["a": 1, "": 2])`
#dynamicCallable
struct BothCallable {
func dynamicallyCall(withArguments args: [Int]) -> Int { return args.count }
func dynamicallyCall(withKeywordArguments args: KeyValuePairs<String, Int>) -> Int {
return args.count
}
}
let c3 = BothCallable()
c3() // desugars to `c3.dynamicallyCall(withArguments: [])`
c3(1, 2) // desugars to `c3.dynamicallyCall(withArguments: [1, 2])`
c3(a: 1, 2) // desugars to `c3.dynamicallyCall(withKeywordArguments: ["a": 1, "": 2])`
This ambiguity resolution rule works out naturally given the behavior of the
Swift type checker, because it only resolves call expressions when the type
of the base expression is known. At that point, it knows whether the base is a
function type, metatype, or a valid #dynamicCallable type, and it knows the
syntactic form of the call.
This proposal does not require massive or invasive changes to the constraint
solver. Please look at the implementation for more details.
Example usage
Here, we sketch some example bindings to show how this could be used in
practice. Note that there are lots of design decisions that are orthogonal to
this proposal (e.g. how to handle exceptions) that we aren't going into here.
This is just to show how this feature provides an underlying facility that
language bindings authors can use to achieve their desired result. These
examples also show #dynamicMemberLookup to illustrate how they work together,
but elides other implementation details.
JavaScript supports callable objects but does not have keyword arguments.
Here is a sample JavaScript binding:
#dynamicCallable #dynamicMemberLookup
struct JSValue {
// JavaScript doesn't have keyword arguments.
#discardableResult
func dynamicallyCall(withArguments: [JSValue]) -> JSValue { ... }
// This is a `#dynamicMemberLookup` requirement.
subscript(dynamicMember member: JSValue) -> JSValue {...}
// ... other stuff ...
}
On the other hand, a common JavaScript pattern is to take a dictionary of
values as a stand-in for argument labels (called like
example({first: 1, second: 2, third: 3}) in JavaScript). A JavaScript bridge
in Swift could choose to implement keyword argument support to allow this to be
called as example(first: 1, second: 2, third: 3) from Swift code (kudos to
Ben Rimmington for this
observation).
Python does support keyword arguments. While a Python binding could implement
only the withKeywordArguments: method, it is be better to implement both the
non-keyword and keyword forms to make the non-keyword case slightly more
efficient (avoid allocating temporary storage) and to make direct calls with
positional arguments nicer (x.dynamicallyCall(withArguments: 1, 2) instead of
x.dynamicallyCall(withKeywordArguments: ["": 1, "": 2])).
Here is a sample Python binding:
#dynamicCallable #dynamicMemberLookup
struct PythonObject {
// Python supports arbitrary mixes of keyword arguments and non-keyword
// arguments.
#discardableResult
func dynamicallyCall(
withKeywordArguments: KeyValuePairs<String, PythonObject>
) -> PythonObject { ... }
// An implementation of a Python binding could choose to implement this
// method as well, avoiding allocation of a temporary array.
#discardableResult
func dynamicallyCall(withArguments: [PythonObject]) -> PythonObject { ... }
// This is a `#dynamicMemberLookup` requirement.
subscript(dynamicMember member: String) -> PythonObject {...}
// ... other stuff ...
}
Limitations
Following the precedent of SE-0195, this attribute must be placed on the
primary definition of a type, not on an extension.
This proposal does not introduce the ability to provide dynamically callable
static/class members. We don't believe this is important given the goal of
supporting dynamic languages like Python, but it could be explored if a use
case is discovered in the future. Such future work should keep in mind that
call syntax on metatypes is already meaningful, and that ambiguity would have
to be resolved somehow (e.g. through the most specific rule).
This proposal supports direct calls of values and methods, but subsets out
support for currying methods in Smalltalk family languages. This is just an
implementation limitation given the current state of currying in the Swift
compiler. Support can be added in the future if there is a specific need.
Source compatibility
This is a strictly additive proposal with no source breaking changes.
Effect on ABI stability
This is a strictly additive proposal with no ABI breaking changes.
Effect on API resilience
This has no impact on API resilience which is not already captured by other
language features.
Future directions
Dynamic member calling (for Smalltalk family languages)
In addition to supporting languages like Python and JavaScript, we would also
like to grow to support Smalltalk derived languages like Ruby and Squeak. These
languages resolve method calls using both the base name as well as the
keyword arguments at the same time. For example, consider this Ruby code:
time = Time.zone.parse(user_time)
The Time.zone reference is a member lookup, but zone.parse(user_time) is a
method call, and needs to be handled differently than a lookup of zone.parse
followed by a direct function call.
This can be handled by adding a new #dynamicMemberCallable attribute, which
acts similarly to #dynamicCallable but enables dynamic member calls (instead
of dynamic calls of self).
#dynamicMemberCallable would have the following requirements:
func dynamicallyCallMethod(named: S1, withArguments: [T5]) -> T6
func dynamicallyCallMethod(named: S2, withKeywordArguments: [S3 : T7]) -> T8
Here is a sample Ruby binding:
#dynamicMemberCallable #dynamicMemberLookup
struct RubyObject {
#discardableResult
func dynamicallyCallMethod(
named: String, withKeywordArguments: KeyValuePairs<String, RubyObject>
) -> RubyObject { ... }
// This is a `#dynamicMemberLookup` requirement.
subscript(dynamicMember member: String) -> RubyObject {...}
// ... other stuff ...
}
General callable behavior
This proposal is mainly directed at dynamic language interoperability. For this
use case, it makes sense for the dynamicallyCall method to take a
variable-sized list of arguments where each argument has the same type.
However, it may be useful to support general callable behavior (akin to
operator() in C++) where the desugared "callable" method can have a fixed
number of arguments and arguments of different types.
For example, consider something like:
struct BinaryFunction<T1, T2, U> {
func call(_ argument1: T1, _ argument1: T2) -> U { ... }
}
It is not unreasonable to look ahead to a day where sugaring such things is
supported, particularly when/if Swift gets variadic
generics.
This could allow typesafe n-ary smart function pointer types.
We feel that the approach outlined in this proposal supports this direction.
When/if a motivating use case for general callable behavior comes up, we can
simply add a new form to represent it and enhance the type checker to prefer
that during ambiguity resolution. If this is a likely direction, then it may be
better to name the attribute #callable instead of #dynamicCallable in
anticipation of that future growth.
We believe that general callable behavior and #dynamicCallable are orthogonal
features and should be evaluated separately.
Alternatives considered
Many alternatives were considered and discussed. Most of them are captured in
the "Alternatives Considered" section of
SE-0195.
Here are a few points raised in the discussion:
It was suggested that we use subscripts to represent the call
implementations instead of a function call, aligning with
#dynamicMemberLookup. We think that functions are a better fit here: the
reason #dynamicMemberLookup uses subscripts is to allow the members to be
l-values, but call results are not l-values.
It was requested that we design and implement the 'static callable' version
of this proposal in conjunction with the dynamic version proposed here. In
the author's opinion, it is important to consider static callable support as
a likely future direction to make sure that the two features sit well next
to each other and have a consistent design (something we believe this
proposal has done) but it doesn't make sense to join the two proposals. So
far, there have been no strong motivating use case presented for the static
callable version, and Swift lacks certain generics features (e.g. variadics)
that would be necessary to make static callables general. We feel that static
callable should stand alone on its own merits.
I wrote a function that takes a non-optional String as a parameter.
I declared a variable property of type String, which is also not an optional.
When I try to call my function with this property as an argument, I get the following error.
Cannot invoke 'localesForCountryCode' with an argument list of type '(String)'
Notice that the error lists the type as '(String)' not 'String'. What do the parens signify? I thought they meant that the type was optional, but nothing is declared as an optional anywhere.
My Function (An extension of NSLocale):
func localesForCountryCode(countryCode: String) -> [NSLocale] {
let localeIdentifiers = localeIdentifiersForCountryCode(countryCode)
var locales = [NSLocale]()
for localeIdentifier in localeIdentifiers {
let localeForIdentifier = NSLocale(localeIdentifier: localeIdentifier)
locales.append(localeForIdentifier)
}
return locales
}
Code That Calls My Function
let currentCountryCode = "US"
var localesForCurrentCountry = [NSLocale]()
func updateWithNewLocation(newLocation: CLLocation) {
geoCoder.reverseGeocodeLocation(newLocation, completionHandler: { (placemarks, error) -> Void in
if placemarks.count > 0 {
let placemark = placemarks.first as! CLPlacemark
self.currentCountry = placemark.country
self.localesForCurrentCountry = NSLocale.localesForCountryCode(self.currentCountryCode)
}
})
}
Update 1
When I move my function code from the NSLocale extension to the view controller from which I am calling the function, the error goes away. Any ideas why this may be the case? Absolutely no changes to the function were made.
Update 2
I continue to run into this problem. The following is another example. Again, it seems to happen only when a function is called as a class method.
I was rereading your question and realized that your question is not really your question. Your problem has nothing to do with parentheses (see below about why). You're just calling the wrong method. NSDateComponentsFormatter is a class. stringFromTimeInterval() is an instance method. You have to crate an actual date formatter to work with. If you want a default one, you can do this:
return NSDateComponentsFormatter().stringFromTimeInterval(unwrappedTimespan)
Note the extra set of parentheses. Your probably don't want the default one, though. You probably want to configure one. See Date Formatters for an introduction to that topic. Note that date formatters can be pretty expensive to create, so you usually want to reuse them.
You're doing the same thing with localesForCountryCode. This is an instance method, not a class method. You have to create an NSLocale first.
This does open up an interesting topic, though. Why does NSDateComponentsFormatter.stringFromTimeInterval() act like a function that you're just passing the wrong arguments to? Why doesn't it say "hey, you're calling a class method?" It's because it is a function that you're just passing the wrong arguments to.
Every method is really just a curried function that takes the target object as the first parameter. See Ole Begemann's quick intro to the topic: Instance Methods are Curried Functions in Swift.
Some more on your explicit question about parentheses:
As others have noted, a (String) is a one-tuple of String. The important point is that in Swift, any type is trivially convertible to a one-tuple of that type, so the extra parentheses here are unimportant. There is no meaningful distinction between String and (String) in Swift.
All Swift functions technically take one value and return one value. So one can correctly think of:
func f(x: Int, y: Int) -> Int
as a function that takes a tuple (Int,y:Int) and returns an Int (or possibly (Int); I believe Swift actually does the former, though). This is subtly connected to how currying works in Swift. In a simpler case:
func f(x: Int)
This is a function that technically takes (Int) and returns (). That's why sometimes you will see (Type) show up in error messages. But it is not a meaningful distinction.
String - it's simple String type.
But (String) - it's a compound type called Tuple.
That means you passing to you function Tuple, not String.
A compound type is a type without a name, defined in the Swift language itself. There are two compound types: function types and tuple types. A compound type may contain named types and other compound types. For instance, the tuple type (Int, (Int, Int)) contains two elements: The first is the named type Int, and the second is another compound type (Int, Int).
In general the error message says (String) because that is the tuple / list of parameters. If you method would expect a String and afterwards an Int an error message might mention (String, paramName: Int)
So basically so far everything looks fine. You need to show us the code for us to be able to fix your exact problem. Because String and (String) normally should match in the given scenario.
Consider the following example:
func k (a:String, b:Int) {}
k(12, b:"123")
which will cause the error
Cannot invoke 'k' with an argument list of type (Int, b:String)
While
k("12", b:123)
does what you would expect.
I have a basic question about swift function calling syntax. I have read documentation but not able to figure it out. So decided to put a query over here.
A piece of code i wrote
var string1 = NSString().stringByAppendingString("First string")
var string2 = NSString.stringByAppendingString("Second string")
println(string1)
println(string2)
Both string have same function calling but return type is different. And only difference here is (). I got out put like
First string
(Function)
Question is why its not giving a warning/error. Is it that var string2 holds method body of stringByAppendingString? Whats going on, New swift developer like me can easily make a this type of typo mistake and not able to figure out.
Can you please explain why its not return value?
This happens because swift methods are curried functions (you can find detailed explanation in Ole Begemann's post).
So what you actually got in the following line:
var string2 = NSString.stringByAppendingString("Second string")
is a function that takes a string as parameter and returns the result of
"Second string".stringByAppendingString(parameter)
You can check that by calling string2 as an ordinary function:
string2("123")
// Prints: "Second string123"
func say(name:String, msg:String) {
println("\(name) say \(msg)")
}
say("Henry","Hi,Swift") <---- error because missing argument label 'msg' in call
I need to use
say("Henry",msg:"Hi,Swift")
Why ? If I put more than two var in func so that I need to write var name instead of first var when I call this func
It's really trouble, and I don't see any explain in iBook Swift tutorial.
One possible reason is that it is actually a method. Methods are very sneaky, they look just like regular functions, but they don't act the same way, let's look at this:
func funFunction(someArg: Int, someOtherArg: Int) {
println("funFunction: \(someArg) : \(someOtherArg)")
}
// No external parameter
funFunction(1, 4)
func externalParamFunction(externalOne internalOne: Int, externalTwo internalTwo: Int) {
println("externalParamFunction: \(internalOne) : \(internalTwo)")
}
// Requires external parameters
externalParamFunction(externalOne: 1, externalTwo: 4)
func externalInternalShared(#paramOne: Int, #paramTwo: Int) {
println("externalInternalShared: \(paramOne) : \(paramTwo)")
}
// The '#' basically says, you want your internal and external names to be the same
// Note that there's been an update in Swift 2 and the above function would have to be written as:
func externalInternalShared(paramOne paramOne: Int, #paramTwo: Int) {
print("externalInternalShared: \(paramOne) : \(paramTwo)")
}
externalInternalShared(paramOne: 1, paramTwo: 4)
Now here's the fun part, declare a function inside of a class and it's no longer a function ... it's a method
class SomeClass {
func someClassFunctionWithParamOne(paramOne: Int, paramTwo: Int) {
println("someClassFunction: \(paramOne) : \(paramTwo)")
}
}
var someInstance = SomeClass()
someInstance.someClassFunctionWithParamOne(1, paramTwo: 4)
This is part of the design of behavior for methods
Apple Docs:
Specifically, Swift gives the first parameter name in a method a local parameter name by default, and gives the second and subsequent parameter names both local and external parameter names by default. This convention matches the typical naming and calling convention you will be familiar with from writing Objective-C methods, and makes for expressive method calls without the need to qualify your parameter names.
Notice the autocomplete:
This is simply an influence of the Objective-C language. When calling a method, the first parameter of a method does not need to be explicitly labelled (as in Objective-C it is effectively 'labelled' by the name of the method). However all following parameters DO need a name to identify them. They may also take an (optional) local name for use inside the method itself (see Jiaaro's link in the comments above).
Simple:
Wrong call function syntax's( its not same in c/c++/java/c#)
Incorrect:
say("Henry")
Correct:
say(name:"Henry")
PS: You must always! add "name function parameter" before value.
Swift 3.0 update:
In swift 3.0, methods with one param name per inputs are required to have that param name as part of the function call. So if you define the function like this
func say(name:String, msg:String) {
print("\(name) say \(msg)")
}
Your function call will have to be like this
self.say(name: "Henry",msg: "Hi,Swift")
If you want to have English like readable function labels but do not want to change input param name, you can add the label in front of the parameter names, like this
func say(somethingBy name:String, whoIsActuallySaying msg:String) {
print("\(name) say \(msg)")
}
Then calling it like this
self.say(somethingBy: "Henry",whoIsActuallySaying: "Hi,Swift")
This is a quirk in the compiler. Functions (which are not members of a class) and class methods have different default behavior with regards to named parameters. This is consistent with the behavior of named parameters in objective-C (but makes no sense for someone new to swift with no experience with objective-C).
Here's what the language reference has to say about named parameters for functions (specifically parameters where an external name for the parameter is not given, and the parameter does not have a default value)
However, these parameter names are only used within the body of the
function itself, and cannot be used when calling the function. These
kinds of parameter names are known as local parameter names, because
they are only available for use within the function’s body.
For information about class methods, see Logan's answer.
Please find the small code for understanding in swift 3+.
func sumInt(a:Int,b:Int){
print(a+b) // Displays 3 here
}
sumInt(a: 1, b: 2) // not like in other languages