A debugging tool pauses or stops the execution of a computer program midway through the evaluation of a single line of source code that contains multiple expressions. A debugging tool pauses or stops the execution of a computer program after evaluating and executing one or more expressions within a particular line of source code, but before evaluating and executing one or more other expressions within that same particular line of source code. Thus, programmers can use such a debugging tool to debug a computer program at a level of granularity that is finer than source code line-level. Programmers can use such a debugging tool to debug a computer program, using breakpoints and/or stepping, at an expression-level of granularity.