Error Code: ARITY MISMATCH

This guide explains why an arity mismatch error may occur and how to fix it.

If you think of relations as tables, the arity of a relation in Rel is the number of columns. For instance, the constants true and false are relations of arity 0, a single element is a relation of arity 1, and so on. See Relations in Rel Primer: Basic Syntax for more details.

Consider the following example that returns an arity mismatch error:

def friends = {("Mike", "Anna"); ("Zoe", "Jenn")}
def zoes_friends = friends("Zoe") 

The first relation, friends, contains tuples of friends. The second relation, zoes_friends, defines Zoe’s friends. The error occurs because:

1. The Rel compiler expects a different number of arguments. The relation friends("Zoe") has only one argument, while friends is defined by tuples of arity 2.

2. The second relation has parentheses () instead of square brackets []. You can think of the operator () as a special case of partial relational application used as a boolean filter, with output arity 0 — i.e., true or false. This means that when you write friends("Zoe"), the Rel compiler expects a second argument to check if that tuple is within the relation friends. See Arity in Rel Primer: Basic Syntax for more details.

Here’s one way to write the example above correctly:

// read query
 
def friends = {("Mike", "Anna"); ("Zoe", "Jenn")}
def zoes_friends = friends["Zoe"]
def output = zoes_friends

This returns Zoe’s friends list.

You can verify the output above:

// read query
 
def friends = {("Mike", "Anna"); ("Zoe", "Jenn")}
def zoes_friends = friends("Zoe", "Jenn")
def output = zoes_friends

This returns (), i.e., true, confirming that the tuple is within the relation friends.