You can build categories just by connecting objects with arrows. You can imagine starting with any directed graph and making it into a category by simply adding more arrows. First, add an identity arrow at each node. Then, for any two arrows such that the end of one coincides with the beginning of the other (in other words, any two composable arrows), add a new arrow to serve as their composition. Every time you add a new arrow, you have to also consider its composition with any other arrow (except for the identity arrows) and itself. You usually end up with infinitely many arrows, but that’s okay.
Another way of looking at this process is that you’re creating a category, which has an object for every node in the graph, and all possible chains of composable graph edges as morphisms. (You may even consider identity morphisms as special cases of chains of length zero.)
Such a category is called a free category generated by a given graph. It’s an example of a free construction, a process of completing a given structure by extending it with a minimum number of items to satisfy its laws (here, the laws of a category). We’ll see more examples of it in the future.
type CAT k = k -> k -> Type data FreeCat :: CAT k -> CAT k where Nil :: FreeCat cat a a Cons :: cat a b -> FreeCat cat b c -> FreeCat cat a c instance Category (FreeCat cat) where id :: (FreeCat cat) a a id = Nil (.) :: (FreeCat cat) b c -> (FreeCat cat) a b -> (FreeCat cat) a c xs . Nil = xs xs . Cons y ys = Cons y (xs . ys)
The most trivial category is one with zero objects and, consequently, zero morphisms. It’s a very sad category by itself, but it may be important in the context of other categories, for instance, in the category of all categories (yes, there is one). If you think that an empty set makes sense, then why not an empty category?
data ZeroTemplate :: Cat Void type Zero = FreeCat ZeroTemplate
-- Generate a free category from a graph with one node and no edges data OneTemplate :: CAT () type One = FreeCat OneTemplate id_unit :: One () () id_unit = Nil
-- Graph with two nodes and a single arrow between them data AB = A | B data TwoTemplate :: CAT AB where A_to_B :: TwoTemplate A B type Two = FreeCat TwoTemplate idA :: Two A A idA = Nil idB :: Two B B idB = Nil ab :: Two A B ab = Cons A_to_B Nil
data XYZ = X | Y | Z data ThreeTemplate :: CAT XYZ where X_to_Y :: ThreeTemplate X Y Y_to_Z :: ThreeTemplate Y 'Z type Three = FreeCat ThreeTemplate xy :: Three X Y xy = Cons X_to_Y Nil yz :: Three Y 'Z yz = Cons Y_to_Z Nil xz :: Three X 'Z xz = xy >>> yz