The Lisp version can also be more readable with a macro (like https://github.com/quil-lang/cmu-infix): #I(a*(1.0-t) + b*t). Or something else that would let you write GP's preferred syntax. One of the things that makes Lisp Lisp is that if the parens are over-cumbersome, you have the tools to take them away. See also CL:LOOP.