Endianess should be passed to the load_from(stream) method. Default should be network byte order.

realize it with an extra validator?

The format behaves like this:
d = DataFormat.description do
uint :length
string :name, :length => :length
end

Giving the length an extra name is not necessary. Solve it like this:
d = DataFormat.description do
nstring :name, :n => 8.bits
end

Major redesign

Old

uint :x

optional(->{ x > 5}) do
    ushort :y, min: 3, max: 6
end

Disadvantages

• Ugly code for switches and conditionals (major disadvantage because the data-format specification code would become very ugly)

New

int :x, 4.bytes

if(x == 5)
    int :y, 2.bytes, range: 3..6
end

Disadvantages

• cant print the code (not important, since this was not used anyway)
• no format modification or inspection at runtime (not important, since this was not used anyway)

a:
optional(:x.gt => 5) do

end

b:
optional(->{ x > 5}) do

end

c:
???