1. A method for previewing executable code in an automatic code generator, comprising: a) providing a unit of a hierarchical model on a first graphical user interface (GUI); b) receiving as input from a user an initial function signature for the unit of the hierarchical model on a preview GUI; c) automatically generating a function from the unit and the initial function signature; d) dynamically displaying the initial function signature and the generated function to the user on the preview GUI; e) receiving a modified function signature for the unit from the user on the preview GUI; f) automatically generating an updated function from the unit and the modified function signature; g) dynamically displaying the modified function signature and updated function to the user on the preview GUI; and h) automatically generating code for the unit of the hierarchical model from the updated function, the code for the unit of the hierarchical model comprising the updated function corresponding to the unit of the hierarchical model and the modified function signature.