LazyArg

Transforms a given object's toString with a given function lazily.

This class is intended to be used with the mapResult method of MatcherProducers, which you can use to modify error messages when composing matchers. This class exists to enable those error messages to be modified lazily, so that toString is invoked on the given arg, and its result transformed by the given function f, only when and if the toString method is invoked on the LazyArg. As a performance optimization, ScalaTest's MatchResult avoids invoking toString on objects until and unless an error message is actually needed, to minimize unecessary creation and concatenation of strings. The LazyArg class enables this same performance optimization when composing matchers.

The other design goal of LazyArg is to make the internal arg available for inspection in an IDE. In a future version of ScalaTest, the args of MatchResult that were used to create the error message will be included in the TestFailedException, so they can be inspected in IDEs. This is why the arg field of LazyArg is public.

For an example of using LazyArg, see the Composing matchers section in the main documentation for trait Matcher.