gistfile1.groovy

def ds1 = [1, 2, [3, 4, [5, "a"]]];

def isSequential(x) {
  x instanceof Collection
}

def mapcat(f, coll) {
  coll.inject([]) { x, y ->
    def res = f(y)
    if (isSequential(res)) {
      x + res 
    } else {
      x << f(y)
    }
  }
}

def flat2(ds) {
  if (isSequential(ds)) {
    mapcat(this.&flat2, ds);
  } else {
    [ds]
  }
}


def flat3(ds) {
  ds.inject([]) { x, y ->
    if (isSequential(y)) {
      x = x + flat3(y)
    } else {
      x << y; 
    }
  }
}

println flat2(ds1);
println flat3(ds1);