[Cthulhu_]

One caveat; if `fn` is declared inline when calling that function, it's not very pretty because Go doesn't have a function shorthand (yet?):

    x.transform(func(value int) string { return fmt.Sprintf("%b", value) })

This quickly becomes more difficult to read, especially if you want to chain some operations this way.

But this applies to other languages as well, in JS (which has a function shorthand) I prefer to extract the predicates and give them a meaningful name.

[pdimitar]

I don't write much Golang -- mostly using it for my own needs because it allows quick iteration but haven't made a career out of it -- but for any such cases I just extract out the function. I deeply despise such inline declarations, they are a sign of somebody trying to be too clever and that inevitably ends up inconveniencing everyone else, their future selves included.