Mathics is implementation of Mathematica (actually it is now called Wolfram language) using sympy, ply, and django.<p>It also provides jupyter kernel.<p>The original author was hired by Wolfram, but the project still lives on!<p><a href="http://mathics.github.io" rel="nofollow">http://mathics.github.io</a><p>Maxima is older than Mathematica and written in LISP. One of its contributors has written MockMMA to implement Mathematica in LISP:<p><a href="https://people.eecs.berkeley.edu/~fateman/lisp/" rel="nofollow">https://people.eecs.berkeley.edu/~fateman/lisp/</a>
I've written a Mathematica clone twice before. This guy got there much faster ( I was writing in C++).<p>However both times I got the basic language going then realized it was the standard library that made Mathematica gold and abandoned it.<p>Kudos to him and it's good work!
I've written a toy TeX interpreter/macro-expander as my first "large" Haskell program. It really made me think hard about both Haskell and TeX. I think this is a great way to learn two languages at the same time if you already have experience.