There are a few barriers in the way of understanding how a computer algebra system works. Commercial systems like Mathematica, Maple, and the engines in the TI-89 and HP-50 calculators are closed source. Some open source ones such as Axiom and Maxima can be incredibly complex.
Joel S. Cohen, professor emeritus at the University of Denver, authored two volumes which can help dispel the mystery of CAS internals:
In these texts, Cohen uses a “Mathematical Pseudo Language” (MPL) to illustrate some basic algorithms used in computer algebra systems.
The mpl Scheme libraries are implementations of some of these algorithms.
Up till now, I’ve hacked on the system while in Minneapolis Minnesota, so perhaps I should call the project ‘mpls’. ;-)
The mpl libraries are designed for Scheme systems which implement the R6RS standard.
The mpl testsuite successfully runs in:
Sympy endows Python with symbolic math capabilities.
JACAL is a CAS written in R5RS Scheme which runs in many implementations that support SLIB.
MIT Scheme based scmutils
A tour of mpl
In standard Scheme, if you try to add two symbols, you’ll get an error:
:> (+ ‘x ‘x) :Unhandled exception :…
Let’s try again with the help of mpl:
:> (import (mpl arithmetic)) :> (+ ‘x ‘x) :(* 2 x)
The ‘(mpl arithmetic)’ library offers symbolic versions of + - * / ^.
The ‘(mpl trig)’ library offers symbolic versions of sin and cos.
mpl comes with a basic infix expression mechanism.
:> (import (mpl alge)) :> (alge “2+x*y+z^2”) :(+ 2 (* x y) (^ z 2))
You can use ‘vars’ to create a bunch of self-quoting symbols. For example, if you know you’ll be working with x, y, and z, alot and don’t want to quote them manually, you can do:
:> (import (mpl misc)) :> (vars x y z) :> (/ (+ x y) z)
‘collect-terms’ knows how to handle basic collection of terms:
:> (import (mpl collect-terms)) :> (collect-terms (alge ” 2 a x y + 3 b x y + 4 a x + 5 b x “)
:(+ (* (+ (* 4 a) (* 5 b)) x) (* (+ (* 2 a) (* 3 b)) x y))
:> (import (mpl algebraic-expand)) :> (algebraic-expand (alge ” (2 a + 3 b) x y + (4 a + 5 b) x “)) :(+ (* 4 a x) (* 5 b x) (* 2 a x y) (* 3 b x y))
The test suite
Take a look at the file ./test.sls to get more ideas of what is possible.