# dharmatech/mpl

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# Introduction

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:

Computer Algebra and Symbolic Computation: Elementary Algorithms

Computer Algebra and Symbolic Computation: Mathematical Methods

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’. ;-)

# Implementation support

The mpl libraries are designed for Scheme systems which implement the R6RS standard.

The mpl testsuite successfully runs in:

• Ikarus
• Ypsilon
• Chez
• Mosh

# Other projects

## sympy

Sympy endows Python with symbolic math capabilities.

## JACAL

JACAL is a CAS written in R5RS Scheme which runs in many implementations that support SLIB.

## MIT Scheme based scmutils

Software used for the book Structure and Interpretation of Classical Mechanics.

# A tour of mpl

## Symbolic arithmetic

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 + - * / ^.

## Symbolic trigonometry

The ‘(mpl trig)’ library offers symbolic versions of sin and cos.

## Infix expressions

mpl comes with a basic infix expression mechanism.

:> (import (mpl alge)) :> (alge “2+x*y+z^2”) :(+ 2 (* x y) (^ z 2))

## vars

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

‘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 “)

```                '(x y))
```

:(+ (* (+ (* 4 a) (* 5 b)) x) (* (+ (* 2 a) (* 3 b)) x y))

## algebraic-expand

:> (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.