How to paste and load blocks of code in the Scala REPL

This is an excerpt from the Scala Cookbook. This is Recipe 14.2, “How to paste and load blocks of code in the Scala REPL.”

Problem

You want to experiment with some code in the Scala REPL, and typing it in or trying to paste it into the REPL won’t work.

Solution

The REPL is “greedy” and consumes the first full statement you type in, so attempting to paste blocks of code into it can fail. To solve the problem, either use the :paste command to paste blocks of code into the REPL, or use the :load command to load the code from a file into the REPL.

The :paste command

Attempting to paste the following if/else block into the REPL will cause an error:

if (true)
    print("that was true")
else
    print("that was false")

But by issuing the :paste command before pasting in the code, the code will be interpreted properly:

scala> :paste
// Entering paste mode (ctrl-D to finish)
if (true)
    print("that was true")
else
    print("false")
[Ctrl-D]

// Exiting paste mode, now interpreting.
that was true

As shown, follow these steps to paste your code into the REPL:

  1. Type the :paste command in the REPL.
  2. Paste in your block of code (Command-V on a Mac, Ctrl-V on Windows).
  3. Press Ctrl-D, and the REPL will evaluate what you pasted in.

The :load command

Similarly, if you have source code in a file that you want to read into the REPL environment, you can use the :load command. For example, assume you have the following source code in a file named Person.scala in the same directory where you started the REPL:

case class Person(name: String)

You can load that source code into the REPL environment like this:

scala> :load Person.scala
Loading /Users/Al/ScalaTests/Person.scala...
defined class Person

Once the code is loaded into the REPL, you can create a new Person instance:

scala> val al = Person("Alvin Alexander")
al: Person = Person(Alvin Alexander)

Note, however, that if your source code has a package declaration:

// Person.scala source code
package com.alvinalexander.foo
case class Person(name: String)

the :load command will fail:

scala> :load /Users/Al/ProjectX/Person.scala
Loading /Users/Al/ProjectX/Person.scala...
<console>:1: error: illegal start of definition
       package com.alvinalexander.foo
       ^
defined class Person

You can’t use packages in the REPL, so for situations like this, you’ll need to compile your file(s) and then include them on the classpath, as shown in Recipe 14.3, “Adding JAR Files and Classes to the REPL Classpath”.

Discussion

Although the REPL is incredibly helpful, its greedy nature can cause multiline statements to fail. Imagine that you want to type the following block of code into the REPL:

if (true)
    't'
else
    'f'

If you try typing this code in one line at a time, the REPL will cut you off as soon as it sees a complete statement:

scala> if (true)
     |    't'
res0: AnyVal = t

In this simple example, you can get around the problem by adding curly braces to the expression, in which case the REPL recognizes that the expression isn’t finished:

scala> if (true) {
     |     't'
     | } else {
     |     'f'
     | }
res0: Char = t

But you can’t always do this. In the cases where this fails, use one of the approaches shown in the Solution.

Scala’s -i option

Another approach you can use is to load your source code with the -i argument when starting the Scala REPL. See Recipe 14.4, “Running a Shell Command from the Scala REPL” for more information on that approach.

See Also

The Scala Cookbook

This tutorial is sponsored by the Scala Cookbook, which I wrote for O’Reilly:

You can find the Scala Cookbook at these locations:

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