You can look at it as heavily customized Scheme REPL, where everything not inside parentheses is parsed and executed as shell syntax,
and everything inside parentheses is parsed and executed as Scheme syntax.
Having arithmetic and procedure definition within the login shell definitely feels liberating, at least to me
In my case, I use my interactive shell https://github.com/cosmos72/schemesh every day as login shell.
You can look at it as heavily customized Scheme REPL, where everything not inside parentheses is parsed and executed as shell syntax, and everything inside parentheses is parsed and executed as Scheme syntax.
Having arithmetic and procedure definition within the login shell definitely feels liberating, at least to me