The Haskell Tracer Project
This page describes the current state of our Haskell tracer. The
theory about the tracing model, the implementation of the tracer, as
well as a user guide is included. We also provide a demonstration of
the tracer in action.
Short introduction
We describe the design and implementation of 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 embedded in
functional data structures that also include redex trails showing
how the values were obtained. When a final result is obtained in a
computation with redex trails, 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 to build a tracer for full Haskell, and ultimately a tracer that
can be applied to large computations. Our present tracer, hosted by the
nhc compiler, handles most of Haskell.
Snapshots
This pictures below shows the tracer user interface when tracing the nhc
compiler. The top part of the window displays the interactive redex trail,
and the bottom part shows either the source code (the first picture) or
the program output (the second picture).
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 display in the redex trail window.
