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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HEAD>
<TITLE>Introduction to the prototype Haskell Tracer </TITLE>
</HEAD>
<BODY>
<H1>Introduction to the prototype Haskell Tracer</H1>
<H3>Overview</H3>
<p>
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 <em>redex trails</em> 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.
<p>
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 <em>nhc98</em> 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.
<H3><a href="paper.dvi">Tracing Lazy Functional Computations (.dvi)</a></H3>
<H3>User Interface</H3>
<p>
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).
<p>
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 <em>the particular occurrence</em> of the item.
By shift-right-clicking on an item, the source code window will display
the source code of <em>the definition</em> of the item.
<p>
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.
<p>
<img src=compiler2.gif>
</BODY>
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