https://www.stephanboyer.com/post/9/monads-part-1-a-design-pattern

monads.groovy

Closure bind
List.metaClass.rightShiftUnsigned = { return bind(delegate, it) }
List.metaClass.call = { return [delegate, ''] }

Closure squrt = { Double x ->
    if (x < 0.0d) return []
    if (x == 0.0d) return [0.0]
    return [Math.pow(x, 0.5), -Math.pow(x, 0.5)]
}

bind = { List x, Closure f ->
    return (x.collect{ Double d -> f(d) }).flatten()
}

assert [5.0, 0.0, 3.0] >>> squrt >>> squrt ==
        [1.4953487812212205, -1.4953487812212205, 0.0, 1.3160740129524924, -1.3160740129524924]

//////

Closure verbose_u  = { Integer x ->
    return [x + 4, "[verbose_u was called on $x]"]
}
Closure verbose_v  = { Integer x ->
    return [x * 2, "[verbose_v was called on $x]"]
}

bind = { List x, Closure f ->
    Integer result
    String output
    (result, output) = f(x[0])
    return [result, x[1] + output]
}

assert [4]() >>> verbose_v >>> verbose_u ==
    [12, '[verbose_v was called on 4][verbose_u was called on 8]']

Examples taken from haskell article "Monads, part 1: a design pattern":

https://www.stephanboyer.com/post/9/monads-part-1-a-design-pattern