14. A non-transitory computer-readable medium whose contents enable a computing system to provide for debugging of interpreted code in a visual debugger using breakpoints that facilitate step-based evaluation of expressions, by performing a method comprising: presenting a user interface for receiving debugging commands; in response to a received debug command to set or change an indicated breakpoint in a segment of code that is expressed in an interpreted programming language and that defines one or more expressions, determining the nearest enclosing expression to the indicated breakpoint, including when the nearest enclosing expression begins on a line of source code associated with the code segment that is different from a line of the associated source code that contains the indicated breakpoint; and generating a breakpoint address that indicates a location in the segment of code that corresponds to the beginning of the determined enclosing expression when the code segment is evaluated; causing the code segment to be evaluated using an interpreter for the interpreted programming language, such that evaluation automatically stops when the location in the code segment that corresponds to the breakpoint address is encountered; and causing one or more source code statements that correspond to and indicate the location in the code segment where the evaluation stopped to be displayed in the visual interface.
14. A non-transitory computer-readable medium whose contents enable a computing system to provide for debugging of interpreted code in a visual debugger using breakpoints that facilitate step-based evaluation of expressions, by performing a method comprising: presenting a user interface for receiving debugging commands; in response to a received debug command to set or change an indicated breakpoint in a segment of code that is expressed in an interpreted programming language and that defines one or more expressions, determining the nearest enclosing expression to the indicated breakpoint, including when the nearest enclosing expression begins on a line of source code associated with the code segment that is different from a line of the associated source code that contains the indicated breakpoint; and generating a breakpoint address that indicates a location in the segment of code that corresponds to the beginning of the determined enclosing expression when the code segment is evaluated; causing the code segment to be evaluated using an interpreter for the interpreted programming language, such that evaluation automatically stops when the location in the code segment that corresponds to the breakpoint address is encountered; and causing one or more source code statements that correspond to and indicate the location in the code segment where the evaluation stopped to be displayed in the visual interface. 18. The computer-readable medium of claim 14 wherein the contents are instructions that, when executed, cause the computing system to perform the method.