The prototype Haskell Tracer is an experimental system for tracing Haskell computations. The basis of our tracing method is a program transformation carried out by the compiler: transformed programs compute the same values as the original, but in addition compute functional data structures containing redex trails which show how the values were obtained. After computing a result, or when an error occurs, the programmer can use a special-purpose interactive display program to investigate the derivation in detail, with cross-links to the source program.
Similar schemes attempted by others have typically been limited, for example, with respect to the language used to express traced programs, the speed of traced execution, the detail recorded in the trace, and the size of computations for which traces can be built and explored. We aim in the ART project to build a tracer for full Haskell'98, and ultimately a tracer that can be applied to very large computations. Our prototype tracer, hosted by the nhc98 compiler, currently handles most of the Haskell'98 language, with a subset of the Standard Prelude and Libraries. Various techniques have already been incorporated for dealing with larger computations.
The picture below illustrates the tracer user interface, here shown tracing the nhc98 compiler itself. The top part of the window displays the interactive redex trail, and the bottom part shows either the source code or the program output (selected via the notebook tabs).
The user can click on any expression, identifier or constant in the redex trail display to explore the redex trail of that item. By right-clicking on an item, the source code window will display the source code position of the particular occurrence of the item. By shift-right-clicking on an item, the source code window will display the source code of the definition of the item.
If the output window is selected, the user can click on any part of the program output, and the redex trail for the expression responsible for that piece of the output will be displayed in the redex trail window.