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Using directives

The using directives mechanism lets you define configuration information within .scala source code files, eliminating the need for build tools to define a dedicated configuration syntax.

using directives are basically key-value pairs that let you provide multiple values to a single key. These directives need to be put in comments with a special syntax. For instance, this command:

//> using foo bar baz

Deprecated syntax

As a part of 0.0.x series we experimented with different syntaxes for using directives. Based on feedback and discussions with the Scala compiler team, we decided to remove @using (using annotations), // using (using within plain comment) and using code directives. Those syntaxes will keep working in the 0.1.x series and will be ignored starting from 1.0.x.

Semantics

using directives can be only declared before any other Scala code:

//> using scala 2.13
//> using platform scala-js
//> using options -Xasync

// package statements, import statements and other code follows ...

using directives contribute settings to the whole compilation scope where a given .scala file is defined. This means that a library or compiler option defined in one file applies to the whole application or test (depending on whether the source file is a test, or not).

The only exceptions are using target directives, which only apply to the given file. using target is a marker to specify requirements for the file to be used (e.g. Scala version, platform, or scope).

caution

The using target directives are an experimental feature, and may change in future versions of Scala CLI.

We believe that syntax similar to using directives should become a part of Scala in the future and will already be included within the Scala runner itself

using directives in the Scala CLI

Below is a list of the most important using directives that Scala CLI supports. The full list can be found in the Reference section of this documentation.

  • //> using scala <scala-version> - defines version of Scala used
  • //> using dep org::name:version - defines dependency to a given library more in dedicated guide
  • //> using dep org:name:version - defines dependency to a given java library, note the : instead of ::
  • //> using dep org::name:version,url=url - defines dependency to a given library with a fallback to its jar url
  • //> using resourceDir dir - marks directory as source of resources. Resources accessible at runtime and packaged together with compiled code.
  • //> using javaOpt opt - use given java options when running application or tests
  • //> using testFramework framework - select test framework to use

There are several reasons that we believe using directives are a good solution:

  • One of the main Scala CLI use cases is prototyping, and the ability to ship one or more source code files with a complete configuration is a game-changer for this use case.
  • Defining dependencies and other settings is common in Ammonite scripts as well.
  • From a teaching perspective, the ability to provide pre-configured pieces of code that fit into one slide is also beneficial.
  • Having configuration close to the code is beneficial, since often — especially in small programs — the given dependencies are only used within one source file.

We acknowledge that configuration distributed across many source files may be hard to maintain in the long term. Therefore, in the near feature we will introduce a set of lints to ensure that above a given project size or complexity, all configuration details will be centralized.

How can configuration that’s contained in source files be centralized? using directives can be placed in any .scala file, so it’s possible to create a .scala file that contains only configuration information. Therefore, when your project needs to centralize its configuration, we recommend creating a project.scala file, and placing the configuration there. We plan to add ways to Scala CLI to migrate these settings into a centralized location with one command or click.

We are aware that using directives may be a controversial topic, so we’ve created a dedicated space for discussing using directives.

Explicit handling of paths in using directives

The ${.} pattern in directive values will be replaced by the parent directory of the file containing the directive. This makes it possible for example to generate coverage output files relative to the source file location.

//> using options -coverage-out:${.}

However, if you want to include the ${.} pattern in the directive value without it being replaced, you can precede it with two dollar signs ($$), like this:

//> using options -coverage-out:$${.}

How to comment out using directives?

Using directives are part of the code so similarly, developers should be able to comment them out.

Commenting out comment-based directives does not cause any problems. Below, some examples how to do it:

// //> using dep "no::lib:123"
// // using dep "no::lib:123"

Directives with a test scope equivalent

Some directives have a test scope equivalent. For example, using dep has using test.dep, which allows to declare dependencies that are only used in tests outside test-specific sources.

For example, this way you can declare the dependency to munit in project.scala like this:

//> using test.dep org.scalameta::munit::0.7.29

The dependency will then only be available in test sources. It's effectively an equivalent to just using dep inside of a test source (except you can define it anywhere):

//> using dep org.scalameta::munit::0.7.29

Directives with a test scope equivalent:

//> using test.dep org.scalameta::munit::0.7.29
//> using test.jar path/to/dep.jar
//> using test.sourceJar path/to/some-sources.jar
//> using test.javaOpt -Dfoo=bar
//> using test.javacOpt source 1.8 target 1.8
//> using test.javaProp foo1=bar1
//> using test.option -Xfatal-warnings
//> using test.resourceDir testResources
//> using test.toolkit latest