$ crystal test.cr
Error in ./test.cr:30: error instantiating ->move_piece_down(UInt32, ::Pointer(Void))

SDL2.add_timer(1000_u32, ->move_piece_down(UInt32, Void*))
                         ^

instantiating 'move_piece_down(UInt32, Pointer(Void))'

in ./test.cr:17: type must be UInt32, not Int32

def move_piece_down(interval : UInt32, param : Void*) : UInt32
    ^~~~~~~~~~~~~~~