Basics

This page covers the fundamental building blocks of a FlowLog program.

Variables

Variables are identifiers that start with a lowercase letter and may contain letters, digits, and underscores. Variables are local to the rule they appear in:

Path(x, y) :- Edge(x, y).
Path(x, z) :- Path(x, y), Edge(y, z).

Here x, y, and z are variables. The same variable name in different rules refers to independent bindings.

Constants

FlowLog supports four kinds of constants:

Kind	Example	Description
Integer	42, -1, 0	Decimal integer literal
String	"hello"	Double-quoted UTF-8 string
Boolean	True, False	Capitalized boolean literal

Constants can appear in rule heads and bodies:

Source(1).
Name(1, "alice").
Flag(1, True).

BigNode(x) :- Node(x, size), size > 100.

Placeholders

The underscore _ is a wildcard that matches any value without binding it to a variable. Use it when you don't need a column's value:

HasOutEdge(x) :- Edge(x, _).

Multiple underscores in the same rule are independent — each matches any value.

Comments

Comments start with // or # and extend to the end of the line:

// This is a comment
# This is also a comment
Reach(y) :- Source(y).  // inline comment

Whitespace

Whitespace (spaces, tabs, newlines) is not significant except to separate tokens. Rules can span multiple lines:

Path(x, y) :-
    Edge(x, z),
    Path(z, y).