The language might permit it now, but it isn't designed for it. I think if the Go designers had intended for map, filter, et al to replace most for loops, they would have designed a more concise syntax for anonymous functions. Something more along the lines of:

    colors := items.Filter(_.age > 20).Map(_.color)

Instead of

    colors := items.Filter(func(x Item){ return x.age > 20 }).Map(func(x Item){ return x.color })

which as best as I can tell is how you'd express the same thing in Go if you had a container type with Map and Filter defined.