[julia-users] RetType in ccall as a variable --> 'not defined'

[J Luis]

I want to parameterize the return type of a ccall call that otherwise will 
return a pointer to void. So I tried


function create_data(API::Ptr{None}, family, geometry, mode, par,
        wesn::Ptr=C_NULL, inc::Ptr=C_NULL, reg=0, pad=2, data::Ptr{Void}=
C_NULL)

    if (family == GMT_IS_VECTOR)
        ret_type = Ptr{GMT_VECTOR}
    elseif (family == GMT_IS_IMAGE)
        ret_type = Ptr{GMT_IMAGE}
    else
        ret_type = Ptr{None}
    end

    ptr = ccall((:GMT_Create_Data,lib), ret_type, (Ptr{None}, Uint32, Uint32
, Uint32, Ptr{Uint64},
        Ptr{Cdouble}, Ptr{Cdouble}, Uint32, Cint, Ptr{None}),
        API, uint32(family),uint32(geometry),uint32(mode),par,wesn,inc,
uint32(reg),int32(pad), data)
end

but it errors saying that ret_type is not defined.

julia> V = create_data (API, 5, 1, 0, dim)
ERROR: ret_type not defined

How come that it's 'not defined' ?????