Nov 11, 2014

Introduction to the Swift REPL

Xcode 6.1 introduces yet another way to experiment with Swift in the form of an interactive Read Eval Print Loop, or REPL. Developers familiar with interpreted languages will feel comfortable in this command-line environment, and even experienced developers will find a few unique features. To get started, launch Terminal.app (found in /Applications/Utilities) and type “swift” at the prompt in OS X Yosemite, or “xcrun swift” in OS X Mavericks. You’ll then be in the Swift REPL:

Welcome to Swift version 1.1 (swift-600.0.20.0). Type :help for assistance.
  1>   

All you need to do is type Swift statements and the REPL will immediately execute your code. Expression results are automatically formatted and displayed along with their type, as are the results of both variable and constant declarations. Console output flows naturally within the interactive session:

  1> "100".toInt()
$R0: Int? = 100
  2> let name = "Katherine"
name: String = "Katherine"
  3> println("Hello, \(name)")
Hello, Katherine

Note that the result from line one has been given a name by the REPL even though the result of the expression wasn’t explicitly assigned to anything. You can reference these results to reuse their values in subsequent statements:

  4> $R0! + 200
$R1: Int = 300

The Swift compiler recognizes incomplete code, and will prompt for additional input when needed. Your code will even be indented automatically as it would in Xcode. For instance, starting a function:

5> func timesTwo() {
6.		 

The prompt for continuation lines is a line number followed by a period instead of the angle bracket that indicates a new statement, so you can tell at a glance when you’re being asked to complete a code fragment. At this point you can keep typing remaining lines in the method:

5> func timesTwo() {
6.		return value * 2
7. }  

There are three noteworthy points to make here: The first is that line six was originally indented, but the REPL automatically unindented when we typed the closing brace. The second is that the function references a parameter we forgot to declare and needs a return type, so you’ll need to add both to the declaration. The last is that even if you did press return after the last line, it’s not too late to fix it.

Multi-Line History

When code is submitted to the compiler it’s also recorded in the REPL history, which makes correcting mistakes trivial. If you pressed return at the end of the incomplete function declaration above, you’d be presented with the following message:

error: use of unresolved identifier 'value'

Like most history implementations, you can call up your last entry by pressing up arrow from the prompt. The REPL brings back all three lines in our example, and places the cursor at the end. You can now proceed with editing the code to correct your mistake as described in the next section.

Your history is preserved between sessions and will record hundreds of code fragments. Each time you move up from the top line you’ll move to an earlier history entry. Each time you move down from an empty line at the bottom of an entry you’ll move to a more recent history entry. The empty line that opens up before moving to the next entry comes in handy for reasons discussed below.

Multi-Line Editing

Even though the REPL behaves like a traditional line editor, it also provides convenient features for dealing with multi-line input like most class or function declarations. In the example above, before pressing return on the final line you can press up arrow to move the cursor up to the declaration line, then use the left arrow to move the cursor just after the opening parenthesis for the parameter list:

5> func timesTwo() {
6.		return value * 2
7. }

Type the parameter declaration, press the right arrow to move past the closing parenthesis and add the return type as well:

5> func timesTwo(value: Int) -> Int {
6.		return value * 2
7. }

You can’t press return to complete the declaration at this point because you’re in the middle of a block of text. Pressing return here would insert a line break, which can be useful if you’re trying to insert additional lines in a function or method body, but what you want here is to move to the end of the declaration. You can press down arrow twice to get there, or use the Emacs sequence ESC > (the escape key followed by a closing angle bracket). Pressing return at the end of the last line will compile the newly declared function so it’s ready for use:

  8>  timesTwo(21)
$R2: (Int) = 42

Automatic detection of statement completion means that you can just type code and the REPL will do the right thing the vast majority of the time. There are occasions, however, where it’s necessary to submit more than one declaration at the same time because they have mutual dependencies. Consider the following code:

func foo() {
	bar()
}
func bar() {
	foo()
}

Typing everything above line by line will result in trying to compile the first function once the third line is complete, and of course this produces an error:

error: use of unresolved identifier 'bar'

You could declare both functions on a single line to get around automatic completion detection that takes place when you press return, but there’s a better solution. After typing the third line above you can press the down arrow to move to create a fourth line manually, and type the remainder normally. The two declarations are compiled together, achieving the desired goal of mutual recursion.

Quick Reference

To help you get started, here’s a handy chart with some of the most commonly used editing and navigation keys:

Arrow Keys		Move cursor left/right/up/down
Control+F		Move cursor right one character, same as right arrow
Control+B		Move cursor left one character, same as left arrow
Control+N		Move cursor to end of next line, same as down arrow
Control+P		Move cursor to end of prior line, same as up arrow
Control+D		Delete the character under the cursor
Option+Left		Move cursor to start of prior word
Option+Right	Move cursor to start of next word
Control+A		Move cursor to start of current line
Control+E		Move cursor to end of current line
Delete			Delete the character to the left of the cursor
Esc <			Move cursor to start of first line
Esc >			Move cursor to end of last line

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