Haskell and other call-by-name languages use lazy evaluation as their default evaluation strategy. For beginners and advanced programmers alike this can sometimes be confusing. The visualize-cbn tool is designed to help in such cases; it is a simple interpreter for a mini-Haskell-like language which outputs the state of the program at every step in a human readable format. It can also generate a HTML/JavaScript version with "Previous" and "Next" buttons to allow to step through a program.
Consider the following example program:
fac = (\n ->
if le n 1
then 1
else mul (@fac (sub n 1)) n
)
main = @fac 1The syntax is not quite Haskell, but hopefully it should be pretty self-explanatory. The parser is pretty finicky; look at some of the examples in examples/ to deduce what the syntax is. The only somewhat odd feature is the identifies marked with an at-sign (@); these corresponds to pointers in the heap. For programs in their initial state (i.e., as written down), the only heap pointers we expect are to CAFs (constant applicative forms; values defined at the top-level of the program).
We can step through the evaluation of this program using
visualize-cbn -i examples/fac.hs --show-trace --hide-prelude 0
This will result in something like
** 0
fac 1
(apply fac)
** 1
if 1 <= 1
then 1
else fac (1 - 1) * 1
(delta: 1 <= 1)
** 2
if True
then 1
else fac (1 - 1) * 1
(if True)
** 3
1
(whnf)
At every step it lists the current state of the program, as well as the reduction rules that apply. There are some options for tweaking the output; see --help.
The tool can also generate HTML/JavaScript:
cr visualize-cbn -i examples/fac.hs --javascript foo.js
The resulting .js file can be embedded in a HTML page (such as a blog post); a minimal HTML page illustrating how this is done is given by
<html>
<body>
<a onclick="cbnPrev()">Prev</a>
<a onclick="cbnNext()">Next</a>
(step <span id="cbn_step">Step</span>, <span id="cbn_status">Status</span>)
<table width="100%" border="1" cellpadding="5" style="border-collapse: collapse;">
<tr><td><div style="font-family: monospace;" id="cbn_term">Term</div></td></tr>
<tr><td><div style="font-family: monospace;" id="cbn_heap">Heap</div></td></tr>
</table>
<script type="text/javascript" src="foo.js"></script>
</body>
</html>(This .html file was not written to illustrate HTML best practices :-) ) See the Well-Typed blog post about the tool for an example output.