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Using the REPL

If you run swift repl without any other arguments, you'll launch the REPL, an interactive shell that will read, evaluate, and print the results of any Swift code you enter.

$ swift repl
Welcome to Apple Swift version 5.7 (swiftlang-5.7.0.127.4 clang-1400.0.29.50).
Type :help for assistance.
  1>

Interacting with the REPL is a great way to experiment with Swift. For example, if you enter the expression 1 + 2, the result of the expression, 3, is printed on the next line:

  1> 1 + 2
$R0: Int = 3

You can assign values to constants and variables, and use them in subsequent lines. For instance, the String value Hello, world! can be assigned to the constant greeting, and then passed as an argument to the print(_:) function:

  2> let greeting = "Hello!"
greeting: String = "Hello!"
  3> print(greeting)
Hello!

If you enter an invalid expression, the REPL will print an error showing where the problem occurred:

let answer = "forty"-"two"
error: binary operator '-' cannot be applied to two 'String' operands
let answer = "forty"-"two"
             ~~~~~~~^~~~~~

You can use the up-arrow and down-arrow keys (↑ and ↓) to cycle through previous lines entered into the REPL. This allows you to make a slight change to a previous expression without retyping the entire line, and is especially convenient for fixing errors like the one in the previous example:

let answer = "forty-two"
answer: String = "forty-two"

Another useful feature of the REPL is that it can automatically suggest functions and methods that can be used in a particular context. For example, if you enter re after a dot operator on a String value and then hit the tab key (⇥), the REPL will give a list of available completions like remove(at:) and replaceSubrange(bounds:with:):

5> "Hi!".re⇥
Available completions:
	remove(at: Index) -> Character
	removeAll() -> Void
	removeAll(keepingCapacity: Bool) -> Void
	removeSubrange(bounds: ClosedRange<Index>) -> Void
	removeSubrange(bounds: Range<Index>) -> Void
	replaceSubrange(bounds: ClosedRange<Index>, with: C) -> Void
	replaceSubrange(bounds: ClosedRange<Index>, with: String) -> Void
	replaceSubrange(bounds: Range<Index>, with: C) -> Void
	replaceSubrange(bounds: Range<Index>, with: String) -> Void
	reserveCapacity(n: Int) -> Void

If you start a block of code, such as when iterating over an array with a for-in loop, the REPL will automatically indent the next line, and change the prompt character from > to . to indicate that code entered on that line will only be evaluated when the entire code block is evaluated.

  6> let numbers = [1,2,3]
numbers: [Int] = 3 values {
  [0] = 1
  [1] = 2
  [2] = 3
}
  7> for n in numbers.reversed() {
  8.     print(n)
  9. }
3
2
1

All of the functionality of Swift is available to you from the REPL, from writing control flow statements to declaring and instantiating structures and classes.

You can also import any available system modules, such as Darwin on macOS and Glibc on Linux:

On macOS

1> import Darwin
2> arc4random_uniform(10)
$R0: UInt32 = 4

On Linux

1> import Glibc
2> random() % 10
$R0: Int32 = 4

On Windows

The REPL depends on Python bindings. You must ensure that Python 3.7 is available in the path. The following command adds Python 3.7 from Visual Studio to %PATH% so that it can be used:

path %ProgramFiles(x86)%\Microsoft Visual Studio\Shared\Python37_64;%PATH%

Because the Windows installation separates out the SDK from the toolchain, a few extra parameters must be passed to the REPL. This allows you to use multiple different SDKs with the same toolchain.

set SWIFTFLAGS=-sdk %SDKROOT% -I %SDKROOT%/usr/lib/swift -L %SDKROOT%/usr/lib/swift/windows
swift repl -target x86_64-unknown-windows-msvc %SWIFTFLAGS%