Second order functions in GLSL?

I'm looking for a way to use a function as an argument to another function in GLSL.

Short answer: you can't.

The closest thing to this kind of functionality you'll get in GLSL is shader subroutines. And that only allows the external OpenGL API to select which subroutine to use, not the shader itself.

So just do the switch/case statement and get it over with.