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
I've used the Picolisp REPL like that, though not as a login shell proper, but as the shell where I actually do stuff. Mainly due to the ease with which it integrates with the below shell through 'in, 'out and 'fork.
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