$ mlr repl
Miller 6.13.0 REPL for linux/amd64/go1.23.2
Docs: https://miller.readthedocs.io
Type ':h' or ':help' for online help; ':q' or ':quit' to quit.
[mlr] xprint 123   # not exit if invalid keyword
mlr: cannot parse DSL expression.
…
[mlr] xvar   # not exit if invalid variable
(absent)
[mlr] xlength(123)   # but exit if invalid function!
mlr: function name not found: xlength

$ echo $?
1