final case classLazyArg(arg: Any)(f: (Any) ⇒ String) extends Product with Serializable
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.
arg
the argument
f
a function that given the arg will produce a String
Transforms a given object's
toStringwith a given function lazily.This class is intended to be used with the
mapResultmethod ofMatcherProducers, which you can use to modify error messages when composing matchers. This class exists to enable those error messages to be modified lazily, so thattoStringis invoked on the givenarg, and its result transformed by the given functionf, only when and if thetoStringmethod is invoked on theLazyArg. As a performance optimization, ScalaTest'sMatchResultavoids invokingtoStringon objects until and unless an error message is actually needed, to minimize unecessary creation and concatenation of strings. TheLazyArgclass enables this same performance optimization when composing matchers.The other design goal of
LazyArgis to make the internalargavailable for inspection in an IDE. In a future version of ScalaTest, theargsofMatchResultthat were used to create the error message will be included in theTestFailedException, so they can be inspected in IDEs. This is why theargfield ofLazyArgis public.For an example of using
LazyArg, see the Composing matchers section in the main documentation for traitMatcher.the argument
a function that given the
argwill produce aString