Writing a mini-language

Question

I have an application that needs to allow users to write expressions similar to excel:

(H1 + (D1 / C3)) * I8

and more complex things like

If(H1 = \'True

Answer 1

I've got a counter-example of how not to do it: Will o’ the Wisp (since this is my own code I feel confident criticizing it).

What's good about the Code?

1. It uses a design pattern consequently: The interpreter pattern
2. It has a rather clean design
3. It uses attributes in a nice way.
4. It produces nice graphics. ;-)

Turtle graphics http://i3.codeplex.com/Project/Download/FileDownload.aspx?ProjectName=wisp&DownloadId=34823

What's bad about the code?

1. It's slow!
2. The language is ill-defined with regards to lists (data vs. code).