LISP has an insidious parenthetical notation, and is well known to be a "write only" language, where nobody but the author can understand large programs written in LISP. It is avoided commercially for this reason.
Also, LISP is a language that's been dead since 1970s. Did you perhaps mean Lisp?
More seriously, I have seen a lot of badly written Lisp/Scheme, as much as in any other language. Lisp DOES scale quite well, if the code is well-designed.
However, that's not how Lispers usually read Lisp code. Lispers would connect to a REPL and start "dissecting" the code evaluating it piece by piece. Lisp is unlike many other PLs where you usually have to follow "write -> save -> lint|compile -> run -> check the results" routine, boils down to "write -> eval -> check the results".
You can even say that coding in Lisp is dealing with a "living," "breathing" program, whereas in other PLs - your program is "dead" and comes to life only after it's compiled and ran.
> It is avoided commercially for this reason.
Lisp is being used today by Apple, Walmart, Cisco, Amazon, Netflix, and even NASA and many other companies. Huge banks and FinTech startups around the world actively using it. Clojure is the most popular programming language in its category of PLs with strong FP emphasis. It is more popular than Haskell, OCaml, Elm, Elixir, Purescript, and recently surpassed Scala.
Not a single guru, CS veteran or renowned programmer ever complained about Lisp being hard to read, because they understand - if they know anything about programming at all, publicly saying shit like that would raise many eyebrows.
But every single time someone mentions Lisp on HN, there will be at least one dyslisplexic, who has never developed enough patience or inner curiosity to learn it.
If Lisp were mainstream I'd probably see that as a red flag
Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.
What are these amazing 'commercial' programs?
And why are they so large? And, do they need to be so large?
Does the entire team understand all the inner workings of the commercial program?
The Lisp eval function is easier to read, understand, and modify than to start over and rewrite it from scratch.
( As for those brackets, well ... https://xkcd.com/859/ https://xkcd.com/541/ https://xkcd.com/297/
Sure, because C and Fortran are extremely limited languages - no first-class functions, no sum types, pitiful error handling. In a world where language design was extremely-primitive, user-defined macros made sense. Once languages that offered standard ways of doing the constructs that most programs need were invented (i.e. ML in 1973), there were better alternatives.
> The Lisp eval function is easier to read, understand, and modify than to start over and rewrite it from scratch.
That's not actually true though. The definition in the link uses a bunch of obscure lisp-specific terminology and doesn't really gain anything from doing so. It genuinely is easier to reimplement it from scratch (perhaps with the help of a more neutral reference) than to understand the Lisp version.