Tap

tap.groovy

@Grab( 'com.bloidonia:groovy-common-extensions:0.5' )

def m = (1..10)                         .tap { println "original ${it}" }
               .findAll { it % 2 == 0 } .tap { println "evens    ${it}" }
               .collect { it * it }     .tap { println "squares  ${it}" }
// prints:
//    original [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
//    evens    [2, 4, 6, 8, 10]
//    squares  [4, 16, 36, 64, 100]

// and returns:
assert m == [4, 16, 36, 64, 100]