Comment by andrewray

Taking shader programs (they have a main() functions), hacking up their source code / AST to turn the into modularized code packages, and then enabling composition of shaders by injecting the output of one module into another (essentially calling the main function of one shader in another). This enables arbitrary shader composition. One application is Photoshop-style stacking of shaders as layers, like https://shaderfrog.com/2/editor/cm6y90vai002mpaxio3zjhfvq

Or getting real freaky with it by composing many effects deep https://shaderfrog.com/2/editor/cm1s7w23w000apar738s9d1x0