Contexts in the typer

The type checker passes contexts through all methods and adapts fields where necessary, e.g.

case tree: untpd.Block => typedBlock(desugar.block(tree), pt)(ctx.fresh.withNewScope)

A number of fields in the context are typer-specific (mode, typerState).

In other phases

Other phases need a context for many things, for example to access the denotation of a symbols (depends on the period). However they typically don't need to modify / extend the context while traversing the AST. For these phases the context can be simply an implicit class parameter that is then available in all members.

Careful: beware of memory leaks. Don't hold on to contexts in long lived objects.

Using contexts

Nested contexts should be named ctx to enable implicit shadowing:

scala> class A

scala> def foo(implicit a: A) { def bar(implicit b: A) { println(implicitly[A]) } }
<console>:8: error: ambiguous implicit values:
 both value a of type A
 and value b of type A
 match expected type A
       def foo(implicit a: A) { def bar(implicit b: A) { println(implicitly[A]) } }

scala> def foo(implicit a: A) { def bar(implicit a: A) { println(implicitly[A]) } }
foo: (implicit a: A)Unit