## Introduction

Although I am primarily a category theorist (my PhD research was at the border between category theory and homological algebra), I also studied algebraic geometry and computer algebra for my master degree at the University of Kaiserslautern. Computer algebra is a diverse subject encompassing many fields of symbolic computations. Our lectures in Kaiserslautern focused on computations in polynomial rings with an eye towards applications to algebraic geometry.

Functional programming languages are a particularly good fit for symbolic computations. My goal in this blog post is to illustrate this by showing you an implementation in Haskell of Buchberger’s algorithm for computing Gröbner bases of polynomial ideals.

Haskell is also a great host language for embedded domain-specific languages. To illustrate this, we are going to implement Buchberger’s algorithm as part of a tiny EDSL for polynomial computations.

Before we dive in, I would like to make a couple of remarks about the desiderata that guided the implementation.

First, my goal was to design a toy EDSL, not a fully-fledged computer algebra system, with the syntax that is as close to the mathematical notation as possible, possibly at the cost of efficiency. Don’t expect this implementation to be competitive with the advanced computer algebra systems like Singular, Macaulay, Maple etc.

Second, the whole point of using Haskell, and not say Common Lisp, as the host language is to leverage Haskell’s highly expressive type system. For example, in the computer algebra system Singular, developed in Kaiserslautern, polynomial rings are described by three pieces of information: the ground field, list of variables, and monomial ordering. In my EDSL this information is encoded in types, so that polynomials over different fields, or in different sets of variables, or of different monomial orderings are values of different types and can’t be accidentally mixed in programs. This particular desideratum has influenced many design choices we are about to discuss.

Today we are going to look at how variables, monomials, and monomial orderings are represented, leaving the representation of polynomials and actual implementation of the Gröbner basis algorithm for the next post. The complete code of the EDSL is available at GitHub.

## Variables

Somewhat unconventionally, I have chosen to represent variables by enumeration types. For example, the set of three variables `X`, `Y`, `Z` is represented by the following data type:

```data XYZ = X | Y | Z deriving (Eq, Ord, Show)
```

Every constructor is nullary and simply names a variable. If you want to compute with polynomials in $n$ variables, you first need to define an enumeration type with n constructors, e.g., if $n = 5$:

```data Vars = X1 | X2 | X3 | X4 | X5 deriving (Eq, Ord, Show)
```

Defining enumeration types by hand is arguably tedious and ugly (having to invent names for enumeration types is particularly obnoxious). We shall see later how we can make declaring variables more pleasant using some Template Haskell.

## Monomials

A monomial over a finite set of variables represented by an enumeration type `v` is represented internally by a `Map` from variables to integers (exponents):

```newtype Monomial v o = M (Map v Int) deriving Eq
```

The type `Monomial` is a phantom type: the type variable `o` does not appear on the right hand side of the definition of `Monomial`. We are going to use `o` later as a tag allowing us to define different instances of the `Ord` typeclass (i.e., different monomial orderings) on `Monomial`.

We have a number of helper functions to construct monomials. A variable can be viewed as a monomial:

```inject :: Eq v => v -> Monomial v o
inject x = M \$ Map.singleton x 1
```

We can convert a monomial to a list of variable-exponent pairs, and we can build a monomial from such a list:

```toList :: Ord v => Monomial v o -> [(v, Int)]
toList (M m) = [ p | p@(x, n) <- Map.toList m, n /= 0 ]

fromList :: Ord v => [(v, Int)] -> Monomial v o
fromList xs = M \$ Map.fromList [ p | p@(x, n) <- xs, n /= 0 ]
```

Note that the variables with zero exponents are dropped. We can look up the exponent of a given variable in a monomial:

```exponent :: Ord v => v -> Monomial v o -> Int
exponent x (M m) = fromMaybe 0 (Map.lookup x m)
```

We can also collect the variables occurring in a monomial with non-zero exponents:

```variables :: Ord v => Monomial v o -> [v]
variables = map fst . toList
```

Monomials are shown as power products of variables:

```instance (Ord v, Show v) => Show (Monomial v o) where
show m
| null support
= "1"
| otherwise
= concat [ show x ++ suffix
| (x, n) <- support
, let suffix = if n == 1
then ""
else "^" ++ show n
]
where
support = toList m
```

Monomials over a set of variables `v` are naturally a monoid:

```instance Ord v => Monoid (Monomial v o) where
mempty = M Map.empty
M a `mappend` M b = M \$ Map.unionWith (+) a b
```

Monomials have degree. Because polynomials also have degree, it is convenient to have an overloaded function `degree`:

```class HasDegree a where
degree :: a -> Int
```

The degree of a monomial is the sum of the exponents of its variables:

```instance Ord v => HasDegree (Monomial v o) where
degree (M m) = Map.fold (+) 0 m
```

We can test whether one monomial is divisible by another:

```isDivisibleBy :: Ord v => Monomial v o -> Monomial v o -> Bool
isDivisibleBy (M a) (M b) = Map.isSubmapOfBy (<=) b a
```

We can divide one monomial by another:

```div :: Ord v => Monomial v o -> Monomial v o -> Monomial v o
div (M a) (M b) = M \$ Map.differenceWith sub a b
where
sub x y | x > y     = Just (x - y)
| otherwise = Nothing
```

We are going to use the function div only when one monomial is known to be divisible by the other, but the above definition gives a plausible answer also when this is not the case.

The least common multiple of monomials is defined as follows:

```lcm :: Ord v => Monomial v o -> Monomial v o -> Monomial v o
lcm (M a) (M b) = M \$ Map.unionWith max a b
```

Finally, `complement m n` computes the product of factors in `n` that are missing in `m`:

```complement :: Ord v => Monomial v o -> Monomial v o -> Monomial v o
complement m n = lcm m n `div` m
```

## Monomial Orderings

A monomial ordering on the set of monomials is a total ordering that is compatible with multiplication of monomials, i.e., $m_1 \le m_2$ implies $m_1n \le m_2n$.

We equip monomials `Monomial v o` over the set of variables `v` with different orderings by supplying different tags `o`:

```instance (Show v, Enumerable v) => Ord (Monomial v Lex) where
(<=) = lex
instance (Show v, Enumerable v) => Ord (Monomial v RevLex) where
(<=) = revlex
instance (Show v, Enumerable v) => Ord (Monomial v DegLex) where
(<=) = deglex
instance (Show v, Enumerable v) => Ord (Monomial v DegRevLex) where
(<=) = degrevlex
```

Here `Lex`, `RevLex`, `DegLex`, and `DegRevLex` are empty data types:

```data Lex         -- Lexicographic ordering
data RevLex      -- Reverse lexicographic ordering
data DegLex      -- Degree lexicographic ordering
data DegRevLex   -- Reverse degree lexicographic ordering
```

that are used as tags so that we can define different `Ord` instances on the `Monomial` type. Instead of making `Monomial` a phantom type, we could also define `newtype` wrappers around `Map v Int`, but then the definition of polynomials would have to become more involved.

The definitions of orderings are written in a slightly unintuitive style because they define `(<=)`, not `(>)` or `(<)` as is customary. This is necessary because a minimal instance declaration of `Ord` requires either `compare` or `(<=)`. In particular, if we define only `(>)`, then the default implementation of `(<=)` isn’t in terms of `(>)` but in terms of `compare`, which in turn by default is defined in terms of `(<=)`, leading to an infinite loop.

```lex' :: (Ord v, Show v) => Monomial v o -> Monomial v o -> [v] -> Bool
lex' a b []     = True
lex' a b (x:xs) = exponent x a <= exponent x b
&& (exponent x a /= exponent x b || lex' a b xs)

lex, revlex, deglex, degrevlex :: (Enumerable v, Show v)
=> Monomial v o -> Monomial v o -> Bool
lex       a b = lex' a b enumerate
revlex    a b = lex' a b (reverse enumerate)
deglex    a b = degree a <= degree b
&& (degree a /= degree b || a `lex` b)
degrevlex a b = degree a <= degree b
&& (degree a /= degree b || b `revlex` a)
```

The definitions of orderings rely on the order of variables and the knowledge which variables can occur in monomials. We encode this information in the typeclass `Enumerable`:

```class Ord a => Enumerable a where
enumerate :: [a]
```

We have added the `Ord` constraint to the context in order to save some typing (and because it makes sense). For each type `v` of variables, we have to define an instance of `Enumerable`. For example:

```data XYZ = X | Y | Z deriving (Eq, Ord, Show)

instance Enumerable XYZ where enumerate = [X, Y, Z]
```

## Summary

That’s it for this time. Thanks for reading this far! In this blog post we have discussed how variables, monomials, and monomial orderings are represented in our EDSL. In the next post we shall consider how polynomials are represented in the EDSL and implement the Gröbner basis algorithm.