Style Guide

Var vs Val

By default prefer using val over var. Reasonable exceptions include:

Lazy Loading

When initialization of a class object needs to be decoupled from allocation (e.g., because of a cyclic allocation dependency). Example: Objects in Database are initialized as part of db.init(). Before using var in this way, consider whether the same effect might be accomplished by a lazy val.

Staged Transformation

Some parts of the code iteratively apply a sequence of transformations to a value. For example, Compiler's deploy method conditionally applies several transformations to the oper object. val-based alternatives are generally inferior:

• Unique names for the variable at every stage: Every time you add a new stage, you need to remember to update names both before and after the new stage. This has caused numerous bugs.
• A big giant match statement: Exponential code complexity with tons of repetition.

Staged transformations are a case where var is preferred.