$reducer = {$a + $b} # an 'add' function
1..10 | Compress-Object $reducer

# Compress-Object will internally call
# 1. &$reducer  1 2 => 3
# 2. &$reducer  3 3 => 6
# 3. &$reducer  6 4 => 10
# 4. &$reducer 10 5 => 15
# 5. ...