A first example

Running datatoad (cargo run), you are dropped into a REPL. If you would like it to go faster, you can use cargo’s --release flag.

Let’s compute transitive closure of a small graph:

> edge(1, 2) :- .
> edge(2, 3) :- .
> edge(3, 1) :- .

> reach(x, y) :- edge(x, y).
> reach(x, y) :- edge(x, z), reach(z, y).

Datatoad evaluates the rules to a fixpoint, and then returns a prompt.

The .list command prints all relations and their sizes. You should see something like

> .list
         3 edge  [forms: Identity, Permute-1-0]
         9 reach [forms: Identity]

To inspect the results you’ll want to use the built-in :print relation:

> temp(x,y) :- reach(x,y), :print(x,y).

This prompts a query that consults the :print relation, which contains all facts but prints each fact it is queried about.

> temp(x,y) :- reach(x,y), :print(x,y).
0x00000001      0x00000001
0x00000001      0x00000002
0x00000001      0x00000003
0x00000002      0x00000001
0x00000002      0x00000002
0x00000002      0x00000003
0x00000003      0x00000001
0x00000003      0x00000002
0x00000003      0x00000003

The output is hexadecimal, because the underlying data model is [u8], and integer literals are parsed as 32 bit integers.