SMutant: a tool for type-sensitive mutation testing in a dynamic language

A mutation testing tool takes as input a system under test and a test suite and produces as output the mutation score of the test suite. The tool systematically creates mutants by making small syntactic changes to the system under test and executes the test suite to determine which mutants give different results from the original system. Almost all mutation testing tools have been developed for statically typed languages. The lack of tools for dynamically typed languages may be rooted in additional challenges that are caused by the lack of precise type information until the program is executed. Existing tools for dynamically typed languages mostly focus on mutation of literals because the type of literals are known statically. This paper presents SMutant, the first mutation testing tool for Smalltalk programs. In addition to literal replacement, SMutant supports many mutation operators that are commonly seen in tools for statically typed languages, such as operator replacement. Instead of applying mutations statically, SMutant postpones mutating until execution and applies mutations dynamically, when the types are available. Also, SMutant enables the user to define new mutation operators by sending a single message. The tool automatically generates code to support new mutation operators.