The coalesce binary expression evaluates to the first non-nil value. There is an augmented assignment version of it as well.

The expression evaluates to a unless that value is nil, in which case, it evaluates to b.

Although uncommon, nothing prevents b itself from also being nil. Neither expression will be evaluated more than once and if a is non-nil then b will not be evaluated at all.

The type of the coalesce expression is the greatest common denominator between the type of a and the type of b.

In the augmented assignment version, the result is assigned back to a. This requires that b is type compatible with a or a compilation error will occur.

# Example 1
print name ? 'NONAME'

# Example 2
def foo(factor as decimal?)
    factor ?= 1  # 'normalize' factor before proceeding with the rest of implementation
    ...

# Example 3
get name as String
    return _name ? .getType.name

# Example 4
# this:
name = if(employee.manager.name<>nil, employee.manager.name, 'NONAME')
# can evaluate the key expression twice and is less succinct than:
name = employee.manager.name ? 'NONAME'