ccall problem recognizing tuple types

ccall_test.jl

f(x) = typeof(x)==ASCIIString?Ptr{Uint8}:typeof(x)                                                   
                                                                                                     
args = ("%.1f\n", 2.3)                                                                               
args_type = map(f, args)                                                                             
                                                                                                     
return_value = ccall( (:printf, "libc"), Int32, (Ptr{Uint8}, Float64), args[1], args[2])             
                                                                                                     
println(isa(args_type, typeof((Ptr{Uint8}, Float64))))                                               
                                                                                                     
return_value = ccall( (:printf, "libc"), Int32, args_type, args[1], args[2])