Om
From the SETL Wiki
om, an abbreviation for omega, represents the absence of a value in SETL. Syntactically, it may appear in a program anywhere that a literal could appear.
Examples of expressions with no values:
- An uninitialised variable (including a wr parameter of a procedure)
- A key that is absent from a map
- A tuple element whose index is greater than the size of the tuple
- The "result" of a procedure call that does not return a value
e.g. after
var a, b, c, d, m, t;
sqrtMap := {[1, 1], [4, 2], [9, 3], [16, 4], [25, 5]};
b := sqrtMap(2);
t := ['a', 'b', 'c'];
c := t(4);
d := p();
proc p();
print("Hello, world!");
end proc;
a, b, c and d are all om.
In general an expression having no value (for whatever reason) is equivalent to it having a value of om.
Similarly, assigning om to a variable is equivalent to unsetting the variable and assigning om to a map reference is equivalent to removing the key from the map, e.g. after
v := om;
m := {[1, 1], [2, 4]};
m(1) := om;
v is uninitialised and m has the value {[2, 4]}, the key 1 having been removed.
Another thing you can do with om is test it for equality with a variable, e.g.
var a, b, c, m;
b := om;
c := {[1, 2]};
d := c(1);
e := c(2);
print(a = om, b = om, c /= om, d /= om, e = om);
outputs #T #T #T #T #T
However, om differs from "proper" values in some ways:
