Are there any static duck-typed languages?

Question

Can I specify interfaces when I declare a member?

After thinking about this question for a while, it occurred to me that a static-duck-typed language might actually

Answer 1

Crystal is a statically duck-typed language. Example:

def add(x, y)
  x + y
end

add(true, false)

The call to add causes this compilation error:

Error in foo.cr:6: instantiating 'add(Bool, Bool)'

add(true, false)
^~~

in foo.cr:2: undefined method '+' for Bool

  x + y
    ^