sbt Coding Guideline 

This page discusses the coding style and other guidelines for sbt 1.0.

General goal 

sbt 1.0 will primarily target Scala 2.11. We will cross build against Scala 2.10.

Clean up old deprecation 

Before 1.0 is release, we should clean up deprecations.

Aim for zero warnings (except deprecation) 

On Scala 2.11 we should aim for zero warnings. One exception may be deprecation if it’s required for 2.10 cross building.

Modular design 

Aim small 

The fewer methods we can expose to the build user, the easier it becomes to maintain it.

Public APIs should be coded against “interfaces“ 

Code against interface.

Hide implementation details 

The implementation details should be hidden behind sbt.internal.x package, where x could be the name of the main package (like io).

Depend less 

Making independent modules with fewer dependent libraries make it easier to reuse them.

Hide external classes 

Avoid exposing external classes out to API, except for standard Scala and Java classes.

Hide internal modules 

A module may be declared internal if there’s no use for public.

Compiler flags 

-encoding utf8

The -Xfatal-warnings may be removed if there are unavoidable warnings.

Package name and organization name 

Use the package name appended with the layer name, such as for IO layer. The organization name for the publish artifacts should remain org.scala-sbt.

Binary resiliency 

A good overview on the topic of binary resiliency is Josh's 2012 talk Binary resiliency. The guideline here applies mostly to publicly exposed APIs.


Use MiMa.

Public traits should contain def declarations only 

Abstract classes are also useful 

To trait, or not to trait?. It is less flexible compared to trait, but easier to maintain binary compatibility. It also has better Java interop.

Seal the traits and abstract classes 

If there’s no need to keep the classes open, seal it.

Finalize the leaf classes 

If there’s no need to keep the classes open, finalize it.

Typeclass and subclass inheritance 

Typeclass pattern with pure trait might be easier to maintain binary compatibility than subclassing.

Avoid case classes, use sbt-datatype 

case class involves various codegen, which makes it harder to maintain binary compatibility over time.

Prefer method overloading over default parameter values 

The default parameter values are effectively codegen, which makes it difficult to maintain.

Other public API matter 

Here are other guidelines about public API.

Avoid Stringly-typed programming 

Define datatypes.

Avoid overuse of def apply 

def apply should be reserved for factory methods in a companion object that returns type T.

Use specific datatype (Vector, List, or Array) rather than Seq. 

scala.Seq is scala.collection.Seq, which is not immutable. Default to Vector. Use List if constant prepending is needed. Use Array if Java interop is needed. Note using mutable collection is perfectly fine within the implementation.

Avoid calling toSeq or anything side-effecty on Set 

Set is fine if you stick with set operations, like contains and subsetOf. More often than not, toSeq is called explicitly or implicitly, or some side effecting method is called from map. This introduces non-determinism to the code.

Avoid calling toSeq on Map 

Same as above. That will introduce non-determinism.

Avoid functions and tuples in the signature, if Java interop is needed 

Instead of functions and tuples, turn them into a trait. This is if Java interop is a concern, like implementing incremental compiler.

Style matter 

Use scalariform 

sbt-houserules comes with scalariform.

Avoid procedure syntax 

Return Unit.

Typeclass instances are encouraged to be defined in the companions 

final class FooID {}
object FooID {
  implicit val fooIdPicklerUnpicker: PicklerUnpickler[FooID] = ???

Implicit converter for syntax (enrich-my-library) should be imported 

Avoid defining implicit converters in companion objects and package objects.

Suppose IO module introduces URL enrichment called RichURI; and LibraryManagement introduces String enrichment called GroupID (for ModuleID syntax). These implicit converters should be defined in an object named syntax in respective packge:


object syntax {
  implicit def uriToRichURI(uri: URI): RichURI = new RichURI(uri)

When all the layers are available, sbt package should also define an object called syntax, which forwards all the implicit converters from all the layers:

package sbt

object syntax {
  implicit def uriToRichURI(uri: URI): io.RichURI = io.syntax.uriToRichURI(uri)


sbt Reference Manual
    1. sbt Coding Guideline