# Call-by-name interpretation and visualization tool
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.
## Example
Consider the following example program:
``` haskell
fac = (\n ->
if le n 1
then 1
else mul (@fac (sub n 1)) n
)
main = @fac 1
```
The 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).
## Stepping through
We can step through the evaluation of this program using
```
visualize-cbn -i examples/fac.hs --show-trace --hide-prelude
```
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`.
## Generating HTML/JavaScript
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
Prev
Next
(step Step, Status)
```
(This `.html` file was not written to illustrate HTML best practices :-) ) See the [Well-Typed blog post about the tool](http://www.well-typed.com/blog/2017/09/visualize-cbn/) for an example output.