luke
/
sbt
mirror of https://github.com/sbt/sbt.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Revert bdc89b33f0f2fa99de4841fc2c11c869364e4a9a^ ... bdc89b33f0f2fa99de4841fc2c11c869364e4a9a

ds9999 2011-11-24 04:42:47 -08:00
parent bdc89b33f0
commit 804ebf767b
1 changed files with 750 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

750
FAQ.md Normal file

@ -0,0 +1,750 @@
[API Documentation]: http://harrah.github.com/xsbt/latest/api/index.html
[ChangeReport]: http://harrah.github.com/xsbt/latest/api/sbt/ChangeReport.html
[FileFunction.cached]: http://harrah.github.com/xsbt/latest/api/sbt/FileFunction$.html
[FileUtilities]: http://simple-build-tool.googlecode.com/svn/artifacts/latest/api/sbt/FileUtilities$object.html
[FilesInfo API]: http://harrah.github.com/xsbt/latest/api/sbt/FilesInfo$.html
[IO]: http://harrah.github.com/xsbt/latest/api/sbt/IO$.html
[Path 0.11]: http://harrah.github.com/xsbt/latest/api/sbt/Path$.html
[Path 0.7]: http://simple-build-tool.googlecode.com/svn/artifacts/latest/api/sbt/Path.html
[Path object]: http://simple-build-tool.googlecode.com/svn/artifacts/latest/api/sbt/Path$.html
[PathFinder 0.11]: http://harrah.github.com/xsbt/latest/api/sbt/PathFinder.html
[PathFinder 0.7]: http://simple-build-tool.googlecode.com/svn/artifacts/latest/api/sbt/PathFinder.html
[PathFinder object]: http://harrah.github.com/xsbt/latest/api/sbt/PathFinder$.html
[RichFile]: http://harrah.github.com/xsbt/latest/api/sbt/RichFile.html
[State]: http://harrah.github.com/xsbt/latest/api/sbt/State$.html
[checksum report]: https://issues.sonatype.org/browse/MVNCENTRAL-46
[hyperlinked sources]: http://harrah.github.com/xsbt/latest/sxr/index.html
[issue tracker]: https://github.com/harrah/xsbt/issues
[mailing list]: http://groups.google.com/group/simple-build-tool/
[mailing list]: http://groups.google.com/group/simple-build-tool/topics
[migration page]: https://github.com/harrah/xsbt/wiki/Migrating-from-SBT-0.7.x-to-0.10.x
[original proposal]: https://gist.github.com/404272
[sbt-launch.jar]: http://repo.typesafe.com/typesafe/ivy-releases/org.scala-tools.sbt/sbt-launch/0.11.1/sbt-launch.jar
[xsbt-web-plugin]: https://github.com/siasia/xsbt-web-plugin
[xsbt-webstart]: https://github.com/ritschwumm/xsbt-webstart
[xsbti.ComponentProvider]: http://harrah.github.com/xsbt/latest/api/xsbti/ComponentProvider.html
# Frequently Asked Questions
## Project Information
### How do I get help?
Please use the [mailing list] for questions, comments, and discussions.
* Please state the problem or question clearly and provide enough context. Code examples and build transcripts are often useful when appropriately edited.
* Providing small, reproducible examples are a good way to get help quickly.
* Include relevant information such as the version of sbt and Scala being used.
### How do I report a bug?
Please use the [issue tracker] to report confirmed bugs. Do not use it to ask questions. If you are uncertain whether something is a bug, please ask on the [mailing list] first.
### How can I help?
* Fix mistakes that you notice on the wiki.
* Make [bug reports][issue tracker] that are clear and reproducible.
* Answer questions on the [mailing list].
* Fix issues that affect you. [Fork, fix, and submit a pull request](http://help.github.com/fork-a-repo/).
* Implement features that are important to you. There is an [[Opportunities]] page for some ideas, but the most useful contributions are usually ones you want yourself.
For more details on developing sbt, see [Developing.pdf](http://harrah.github.com/xsbt/Developing.pdf)
## 0.7 to 0.10+ Migration
### How do I migrate from 0.7 to 0.10+?
See the [[migration page|Migrating-from-SBT-0.7.x-to-0.10.x]]
first and then the following questions.
### Where has 0.7's `lib_managed` gone?
By default, sbt 0.11 loads managed libraries from your ivy cache without copying them to a `lib_managed` directory. This fixes some bugs with the previous solution and keeps your project directory small. If you want to insulate your builds from the ivy cache being cleared, set `retrieveManaged := true` and the dependencies will be copied to `lib_managed` as a build-local cache (while avoiding the issues of `lib_managed` in 0.7.x).
This does mean that existing solutions for sharing libraries with your favoured IDE may not work. There are 0.11.x plugins for IDEs being developed:
* IntelliJ IDEA: [[https://github.com/mpeltonen/sbt-idea]]
* Netbeans: [[https://github.com/remeniuk/sbt-netbeans-plugin]]
* Eclipse: [[https://github.com/typesafehub/sbteclipse]]
### What are the commands I can use in 0.11 vs. 0.7?
For a list of commands, run `help`. For details on a specific
command, run `help <command>`. To view a list of tasks defined on
the current project, run `tasks`. Alternatively, see the
[[Running|Getting Started Running]] page in the Getting Started Guide for descriptions of common commands and tasks.
If in doubt start by just trying the old command as it may just work. The built in TAB completion will also assist you, so you can just press TAB at the beginning of a line and see what you get.
The following commands work pretty much as in 0.7 out of the box:
reload
update
compile
test
test-only
publish-local
exit
### Why have the resolved dependencies in a multi-module project changed since 0.7?
sbt 0.10 fixes a flaw in how dependencies get resolved in multi-module projects. This change ensures that only one version of a library appears on a classpath.
Use `last update` to view the debugging output for the last `update` run. Use `show update` to view a summary of files comprising managed classpaths.
### My tests all run really fast but some are broken that weren't in 0.7!
Be aware that compilation and tests run in parallel by default in sbt 0.11. If your test code isn't thread-safe then you may want to change this behaviour by adding one of the following to your `build.sbt`:
```scala
// Execute tests in the current project serially.
// Tests from other projects may still run concurrently.
parallelExecution in Test := false
// Execute everything serially (including compilation and tests)
parallelExecution := false
```
### How do I set log levels in 0.11 vs. 0.7?
`warn`, `info`, `debug` and `error` don't work any more.
The new syntax in the sbt 0.11.x shell is:
```text
> set logLevel := Level.Warn
```
Or in your `build.sbt` file write:
```scala
logLevel := Level.Warn
```
### What happened to the web development and Web Start support since 0.7?
Web application support was split out into a plugin. See the [xsbt-web-plugin] project.
For an early version of an xsbt Web Start plugin, visit the [xsbt-webstart] project.
### How are inter-project dependencies different in 0.11 vs. 0.7?
In 0.11, there are three types of project dependencies (classpath, execution, and configuration) and they are independently defined. These were combined in a single dependency type in 0.7.x. A declaration like:
```scala
lazy val a = project("a", "A")
lazy val b = project("b", "B", a)
```
meant that the `B` project had a classpath and execution dependency on `A` and `A` had a configuration dependency on `B`. Specifically, in 0.7.x:
1. Classpath: Classpaths for `A` were available on the appropriate classpath for `B`.
1. Execution: A task executed on `B` would be executed on `A` first.
1. Configuration: For some settings, if they were not overridden in `A`, they would default to the value provided in `B`.
In 0.11, declare the specific type of dependency you want. Read
about [[multi-project builds|Getting Started Multi-Project]] in
the Getting Started Guide for details.
### Where did class/object X go since 0.7?
| 0.7 | 0.11 |
| --- | --- |
| [FileUtilities] | [IO] |
| [Path class][Path 0.7] and [object][Path object] | [Path object][Path 0.11], `File`, [RichFile] |
| [PathFinder class][PathFinder 0.7] | `Seq[File]`, [PathFinder class][PathFinder 0.11], [PathFinder object][PathFinder object] |
### Where can I find plugins for 0.11?
See [[sbt 0.10 plugins list]] for a list of currently available plugins.
## Usage
### My last command didn't work but I can't see an explanation. Why?
sbt 0.11 by default suppresses most stack traces and debugging information. It has the nice side effect of giving you less noise on screen, but as a newcomer it can leave you lost for explanation. To see the previous output of a command at a higher verbosity, type `last <task>` where `<task>` is the task that failed or that you want to view detailed output for. For example, if you find that your `update` fails to load all the dependencies as you expect you can enter:
```text
> last update
```
and it will display the full output from the last run of the `update` command.
### How do I disable ansi codes in the output?
Sometimes sbt doesn't detect that ansi codes aren't supported and you get output that looks like:
```
[0m[ [0minfo [0m] [0mSet current project to root
```
or ansi codes are supported but you want to disable colored output. To completely disable ansi codes, set the `sbt.log.noformat` system property to `true`. For example,
```
sbt -Dsbt.log.noformat=true
```
## Build definitions
### What are the `:=`, `~=`, `<<=`, `+=`, `++=`, `<+=`, and `<++=` methods?
These are methods on keys used to construct a `Setting`. The Getting Started Guide covers all these methods, see [[.sbt build definition|Getting Started Basic Def]] and
[[more about settings|Getting Started More About Settings]] for example.
### What is the `%` method?
It's used to create a `ModuleID` from strings, when specifying
managed dependencies. Read the Getting Started Guide about
[[library dependencies|Getting Started Library Dependencies]].
### What is `ModuleID`, `Project`, ...?
To figure out an unknown type or method, have a look at the
[[Getting Started Guide|Getting Started Welcome]] if you have
not. Also try the [[Index]] of commonly used methods, values, and
types, the [API Documentation], and the [hyperlinked sources].
### How can one key depend on multiple other keys?
See [[More About Settings|Getting Started More About Settings]] in the Getting Started Guide,
scroll down to the discussion of `<<=` with multiple keys.
Briefly: You need to use a tuple rather than a single key by itself. Scala's syntax for a tuple is with parentheses, like `(a, b, c)`.
If you're creating a value for a task key, then you'll use `map`:
```scala
packageBin in Compile <<= (name, organization, version) map { (n, o, v) => file(o + "-" + n + "-" + v + ".jar") }
```
If you're creating a value for a setting key, then you'll use `apply`:
```scala
name <<= (name, organization, version) apply { (n, o, v) => "project " + n + " from " + o + " version " + v }
```
Typing `apply` is optional in that code, since Scala treats any object with an `apply` method as a function. See [[More About Settings|Getting Started More About Settings]] for a longer explanation.
To learn about task keys vs. setting keys, read [[.sbt build definition|Getting Started Basic Def]].
### How do I add files to a jar package?
The files included in an artifact are configured by default by a task `mappings` that is scoped by the relevant package task. The `mappings` task returns a sequence `Seq[(File,String)]` of mappings from the file to include to the path within the jar. See [[Mapping Files]] for details on creating these mappings.
For example, to add generated sources to the packaged source artifact:
```scala
mappings in (Compile, packageSrc) <++=
(sourceManaged in Compile, managedSources in Compile) map { (base, srcs) =>
import Path.{flat, relativeTo}
srcs x (relativeTo(base) | flat)
}
```
This takes sources from the `managedSources` task and relativizes them against the `managedSource` base directory, falling back to a flattened mapping. If a source generation task doesn't write the sources to the `managedSource` directory, the mapping function would have to be adjusted to try relativizing against additional directories or something more appropriate for the generator.
### How can I generate source code or resources?
sbt provides standard hooks for adding source or resource generation tasks. A generation task should generate sources in a subdirectory of `sourceManaged` for sources or `resourceManaged` for resources and return a sequence of files generated. The key to add the task to is called `sourceGenerators` for sources and `resourceGenerators` for resources. It should be scoped according to whether the generated files are main (`Compile`) or test (`Test`) sources or resources. This basic structure looks like:
```scala
sourceGenerators in Compile <+= <your Task[Seq[File]] here>
```
For example, assuming a method `def makeSomeSources(base: File): Seq[File]`,
```scala
sourceGenerators in Compile <+= sourceManaged in Compile map { outDir: File =>
makeSomeSources(outDir / "demo")
}
```
As a specific example, the following generates a hello world source file:
```scala
sourceGenerators in Compile <+= sourceManaged in Compile map { dir =>
val file = dir / "demo" / "Test.scala"
IO.write(file, """object Test extends App { println("Hi") }""")
Seq(file)
}
```
Executing 'run' will print "Hi". Change `Compile` to `Test` to make it a test source. To generate resources, change `sourceGenerators` to `resourceGenerators` and `sourceManaged` to `resourceManaged`. Normally, you would only want to generate sources when necessary and not every run.
By default, generated sources and resources are not included in the packaged source artifact. To do so, add them as you would other mappings. See the `Adding files to a package` section.
### How can a task avoid redoing work if the input files are unchanged?
There is basic support for only doing work when input files have changed or when the outputs haven't been generated yet. This support is primitive and subject to change.
The relevant methods are two overloaded methods called [FileFunction.cached]. Each requires a directory in which to store cached data. Sample usage is:
```scala
// define a task that takes some inputs
// and generates files in an output directory
myTask <<= (cacheDirectory, inputs, target) map {
(cache: File, inFiles: Seq[File], outDir: File) =>
// wraps a function taskImpl in an uptodate check
// taskImpl takes the input files, the output directory,
// generates the output files and returns the set of generated files
val cachedFun = FileFunction.cached(cache / "my-task") { (in: Set[File]) =>
taskImpl(in, outDir) : Set[File]
}
// Applies the cached function to the inputs files
cachedFun(inFiles)
}
```
There are two additional arguments for the first parameter list that allow the file tracking style to be explicitly specified. By default, the input tracking style is `FilesInfo.lastModified`, based on a file's last modified time, and the output tracking style is `FilesInfo.exists`, based only on whether the file exists. The other available style is `FilesInfo.hash`, which tracks a file based on a hash of its contents. See the [FilesInfo API] for details.
A more advanced version of `FileFunction.cached` passes a data structure of type [ChangeReport] describing the changes to input and output files since the last evaluation. This version of `cached` also expects the set of files generated as output to be the result of the evaluated function.
## Extending sbt
### How can I add a new configuration?
The following example demonstrates adding a new set of compilation settings and tasks to a new configuration called `samples`. The sources for this configuration go in `src/samples/scala/`. Unspecified settings delegate to those defined for the `compile` configuration. For example, if `scalacOptions` are not overridden for `samples`, the options for the main sources are used.
Options specific to `samples` may be declared like:
```scala
scalacOptions in Samples += "-deprecation"
```
This uses the main options as base options because of `+=`. Use `:=` to ignore the main options:
```scala
scalacOptions in Samples := "-deprecation" :: Nil
```
The example adds all of the usual compilation related settings and tasks to `samples`:
```text
samples:run
samples:run-main
samples:compile
samples:console
samples:console-quick
samples:scalac-options
samples:full-classpath
samples:package
samples:package-src
...
```
#### Example of adding a new configuration
`project/Sample.scala`
```scala
import sbt._
import Keys._
object Sample extends Build {
// defines a new configuration "samples" that will delegate to "compile"
lazy val Samples = config("samples") extend(Compile)
// defines the project to have the "samples" configuration
lazy val p = Project("p", file("."))
.configs(Samples)
.settings(sampleSettings : _*)
def sampleSettings =
// adds the default compile/run/... tasks in "samples"
inConfig(Samples)(Defaults.configSettings) ++
Seq(
// (optional) makes "test:compile" depend on "samples:compile"
compile in Test <<= compile in Test dependsOn (compile in Samples)
) ++
// (optional) declare that the samples binary and
// source jars should be published
publishArtifact(packageBin) ++
publishArtifact(packageSrc)
def publishArtifact(task: TaskKey[File]): Seq[Setting[_]] =
addArtifact(artifact in (Samples, task), task in Samples).settings
}
```
### How do I add a test configuration?
See the `Additional test configurations` section of [[Testing]].
### How can I create a custom run task, in addition to `run`?
This answer is extracted from a [mailing list discussion](http://groups.google.com/group/simple-build-tool/browse_thread/thread/4c28ee5b7e18b46a/).
Read the Getting Started Guide up to
[[custom settings|Getting Started Custom Settings]] for background.
A basic run task is created by:
```scala
// this lazy val has to go in a full configuration
lazy val myRunTask = TaskKey[Unit]("my-run-task")
// this can go either in a `build.sbt` or the settings member
// of a Project in a full configuration
fullRunTask(myRunTask, Test, "foo.Foo", "arg1", "arg2")
```
or, if you really want to define it inline (as in a basic `build.sbt` file):
```scala
fullRunTask(TaskKey[Unit]("my-run-task"), Test, "foo.Foo", "arg1", "arg2")
```
If you want to be able to supply arguments on the command line, replace `TaskKey` with `InputKey` and `fullRunTask` with `fullRunInputTask`.
The `Test` part can be replaced with another configuration, such as `Compile`, to use that configuration's classpath.
This run task can be configured individually by specifying the task key in the scope. For example:
```scala
fork in myRunTask := true
javaOptions in myRunTask += "-Xmx6144m"
```
### How can I delegate settings from one task to another task?
Settings [[scoped|Getting Started Scopes]] to one task can fall
back to another task if undefined in the first task. This is
called delegation.
The following key definitions specify that settings for `myRun` delegate to `aRun`
```scala
val aRun = TaskKey[Unit]("a-run", "A run task.")
// The last parameter to TaskKey.apply here is a repeated one
val myRun = TaskKey[Unit]("my-run", "Custom run task.", aRun)
```
In use, this looks like:
```scala
// Make the run task as before.
fullRunTask(myRun, Compile, "pkg.Main", "arg1", "arg2")
// If fork in myRun is not explicitly set,
// then this also configures myRun to fork.
// If fork in myRun is set, it overrides this setting
// because it is more specific.
fork in aRun := true
// Appends "-Xmx2G" to the current options for myRun.
// Because we haven't defined them explicitly,
// the current options are delegated to aRun.
// So, this says to use the same options as aRun
// plus -Xmx2G.
javaOptions in myRun += "-Xmx2G"
```
### How should I express a dependency on an outside tool such as proguard?
Tool dependencies are used to implement a task and are not needed by project source code. These dependencies can be declared in their own configuration and classpaths. These are the steps:
1. Define a new [[configuration|Configurations]].
2. Declare the tool [[dependencies|Library Management]] in that configuration.
3. Define a classpath that pulls the dependencies from the [[Update Report]] produced by `update`.
4. Use the classpath to implement the task.
As an example, consider a `proguard` task. This task needs the ProGuard jars in order to run the tool. Assuming a new configuration defined in the full build definition (#1):
```scala
val ProguardConfig = config("proguard") hide
```
the following are settings that implement #2-#4:
```scala
// Add proguard as a dependency in the custom configuration.
// This keeps it separate from project dependencies.
libraryDependencies +=
"net.sf.proguard" % "proguard" % "4.4" % ProguardConfig.name
// Extract the dependencies from the UpdateReport.
managedClasspath in proguard <<=
(classpathTypes in proguard, update) map { (ct, report) =>
Classpaths.managedJars(proguardConfig, ct, report)
}
// Use the dependencies in a task, typically by putting them
// in a ClassLoader and reflectively calling an appropriate
// method.
proguard <<= managedClasspath in proguard { (cp: Seq[File] =>
// ... do something with 'cp', which includes proguard ...
}
```
### How would I change sbt's classpath dynamically?
It is possible to register additional jars that will be placed on sbt's classpath (since version 0.10.1).
Through [State], it is possible to obtain a [xsbti.ComponentProvider], which manages application components.
Components are groups of files in the `~/.sbt/boot/` directory and, in this case, the application is sbt.
In addition to the base classpath, components in the "extra" component are included on sbt's classpath.
(Note: the additional components on an application's classpath are declared by the `components` property in the `[main]` section of the launcher configuration file `boot.properties`.)
Because these components are added to the `~/.sbt/boot/` directory and `~/.sbt/boot/` may be read-only, this can fail.
In this case, the user has generally intentionally set sbt up this way, so error recovery is not typically necessary (just a short error message explaining the situation.)
#### Example of dynamic classpath augmentation
The following code can be used where a `State => State` is required, such as in the `onLoad` setting (described below) or in a [[command|Commands]].
It adds some files to the "extra" component and reloads sbt if they were not already added.
Note that reloading will drop the user's session state.
```scala
def augment(extra: Seq[File])(s: State): State =
{
// Get the component provider
val cs: xsbti.ComponentProvider = s.configuration.provider.components()
// Adds the files in 'extra' to the "extra" component
// under an exclusive machine-wide lock.
// The returned value is 'true' if files were actually copied and 'false'
// if the target files already exists (based on name only).
val copied: Boolean = s.locked(cs.lockFile, cs.addToComponent("extra", extra.toArray))
// If files were copied, reload so that we use the new classpath.
if(copied) s.reload else s
}
```
### How can I take action when the project is loaded or unloaded?
The single, global setting `onLoad` is of type `State => State` (see [[Build State]]) and is executed once, after all projects are built and loaded. There is a similar hook `onUnload` for when a project is unloaded. Project unloading typically occurs as a result of a `reload` command or a `set` command. Because the `onLoad` and `onUnload` hooks are global, modifying this setting typically involves composing a new function with the previous value. The following example shows the basic structure of defining `onLoad`:
```scala
// Compose our new function 'f' with the existing transformation.
{
val f: State => State = ...
onLoad in Global ~= (f compose _)
}
```
#### Example of project load/unload hooks
The following example maintains a count of the number of times a project has been loaded and prints that number:
```scala
{
// the key for the current count
val key = AttributeKey[Int]("load-count")
// the State transformer
val f = (s: State) => {
val previous = s get key getOrElse 0
println("Project load count: " + previous)
s.put(key, previous + 1)
}
onLoad in Global ~= (f compose _)
}
```
## Errors
### Type error, found: `Initialize[Task[String]]`, required: `Initialize[String]` or found: `TaskKey[String]` required: `Initialize[String]`
This means that you are trying to supply a task when defining a
setting key. See
[[.sbt build definition|Getting Started Basic Def]] for the
difference between task and setting keys, and
[[more about settings|Getting Started More About Settings]] for
more on how to define one key in terms of other keys.
Setting keys are only evaluated once, on project load, while tasks
are evaluated repeatedly. Defining a setting in terms of a task does
not make sense because tasks must be re-evaluated every time.
One way to get a task when you didn't want one is to use the `map`
method instead of the `apply`
method. [[More about settings|Getting Started More About Settings]]
covers this topic as well.
Suppose we define these keys, in `./project/Build.scala` (For
details, see [[.scala build definition|Getting Started Full Def]]).
```
val baseSetting = SettingKey[String]("base-setting")
val derivedSetting = SettingKey[String]("derived-setting")
val baseTask = TaskKey[Long]("base-task")
val derivedTask = TaskKey[String]("derived-task")
```
Let's define an initialization for `base-setting` and `base-task`. We will then use these as inputs to other setting and task initializations.
```scala
baseSetting := "base setting"
baseTask := { System.currentTimeMillis() }
```
Then this will not work:
```
// error: found: Initialize[Task[String]], required: Initialize[String]
derivedSetting <<= baseSetting.map(_.toString),
derivedSetting <<= baseTask.map(_.toString),
derivedSetting <<= (baseSetting, baseTask).map((a, b) => a.toString + b.toString),
```
One or more settings can be used as inputs to initialize another setting, using the `apply` method.
```
derivedSetting <<= baseSetting.apply(_.toString)
derivedSetting <<= baseSetting(_.toString)
derivedSetting <<= (baseSetting, baseSetting)((a, b) => a.toString + b.toString)
```
Both settings and tasks can be used to initialize a task, using the `map` method.
```
derivedTask <<= baseSetting.map(_.toString)
derivedTask <<= baseTask.map(_.toString)
derivedTask <<= (baseSetting, baseTask).map((a, b) => a.toString + b.toString)
```
But, it is a compile time error to use `map` to initialize a setting:
```
// error: found: Initialize[Task[String]], required: Initialize[String]
derivedSetting <<= baseSetting.map(_.toString),
derivedSetting <<= baseTask.map(_.toString),
derivedSetting <<= (baseSetting, baseTask).map((a, b) => a.toString + b.toString),
```
It is not allowed to use a task as input to a settings initialization with `apply`:
```
// error: value apply is not a member of TaskKey[Long]
derivedSetting <<= baseTask.apply(_.toString)
// error: value apply is not a member of TaskKey[Long]
derivedTask <<= baseTask.apply(_.toString)
// error: value apply is not a member of (sbt.SettingKey[String], sbt.TaskKey[Long])
derivedTask <<= (baseSetting, baseTask).apply((a, b) => a.toString + b.toString)
```
Finally, it is not directly possible to use `apply` to initialize a task.
```
// error: found String, required Task[String]
derivedTask <<= baseSetting.apply(_.toString)
```
### On project load, "Reference to uninitialized setting"
Setting initializers are executed in order. If the initialization
of a setting depends on other settings that has not been
initialized, sbt will stop loading. This can happen using `+=`,
`++=`, `<<=`, `<+=`, `<++=`, and `~=`. (To understand those
methods, [[read this|Getting Started More About Settings]].)
In this example, we try to append a library to `libraryDependencies` before it is initialized with an empty sequence.
```
object MyBuild extends Build {
val root = Project(id = "root", base = file("."),
settings = Seq(
libraryDependencies += "commons-io" % "commons-io" % "1.4" % "test"
)
)
}
```
To correct this, include the default settings, which includes `libraryDependencies := Seq()`.
```
settings = Defaults.defaultSettings ++ Seq(
libraryDependencies += "commons-io" % "commons-io" % "1.4" % "test"
)
```
A more subtle variation of this error occurs when using
[[scoped settings|Getting Started Scopes]].
```
// error: Reference to uninitialized setting
settings = Defaults.defaultSettings ++ Seq(
libraryDependencies += "commons-io" % "commons-io" % "1.2" % "test",
fullClasspath ~= (_.filterNot(_.data.name.contains("commons-io")))
)
```
Generally, all of the update operators can be expressed in terms of `<<=`. To better understand the error, we can rewrite the setting as:
```
// error: Reference to uninitialized setting
fullClasspath <<= (fullClasspath).map(_.filterNot(_.data.name.contains("commons-io")))
```
This setting varies between the test and compile scopes. The solution is use the scoped setting, both as the input to the initializer, and the setting that we update.
```
fullClasspath in Compile <<= (fullClasspath in Compile).map(_.filterNot(_.data.name.contains("commons-io")))
// or equivalently
fullClasspath in Compile ~= (_.filterNot(_.data.name.contains("commons-io")))
```
## Dependency Management
### How do I resolve a checksum error?
This error occurs when the published checksum, such as a sha1 or md5 hash, differs from the checksum computed for a downloaded artifact, such as a jar or pom.xml. An example of such an error is:
```
[warn] problem while downloading module descriptor:
http://repo1.maven.org/maven2/commons-fileupload/commons-fileupload/1.2.2/commons-fileupload-1.2.2.pom:
invalid sha1: expected=ad3fda4adc95eb0d061341228cc94845ddb9a6fe computed=0ce5d4a03b07c8b00ab60252e5cacdc708a4e6d8 (1070ms)
```
The invalid checksum should generally be reported to the repository owner (as [was done][checksum report] for the above error). In the meantime, you can temporarily disable checking with the following setting:
```scala
checksums in update := Nil
```
See [[Library Management]] for details.
## Miscellaneous
### How do I use the Scala interpreter in my code?
sbt runs tests in the same JVM as sbt itself and Scala classes are not in the same class loader as the application classes. Therefore, when using the Scala interpreter, it is important to set it up properly to avoid an error message like:
```
Failed to initialize compiler: class scala.runtime.VolatileBooleanRef not found.
** Note that as of 2.8 scala does not assume use of the java classpath.
** For the old behavior pass -usejavacp to scala, or if using a Settings
** object programatically, settings.usejavacp.value = true.
```
The key is to initialize the Settings for the interpreter using _embeddedDefaults_. For example:
```scala
val settings = new Settings
settings.embeddedDefaults[MyType]
val interpreter = new Interpreter(settings, ...)
```
Here, MyType is a representative class that should be included on the interpreter's classpath and in its application class loader. For more background, see the [original proposal] that resulted in _embeddedDefaults_ being added.
Similarly, use a representative class as the type argument when using the _break_ and _breakIf_ methods of _ILoop_, as in the following example:
```scala
def x(a: Int, b: Int) = {
import scala.tools.nsc.interpreter.ILoop
ILoop.breakIf[MyType](a != b, "a" -> a, "b" -> b )
}
```