= Coalesce Expression =

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

=== Grammar ===
{{{
<a> ? <b>
}}}

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'''.

=== Grammar ===
{{{
<a> ?= <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.

{{{
#!cobra
# 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' 
}}}