Plan 9’s mk isn’t really a radical departure from Make, it is more of an adaptation of it to the (decidedly non-POSIX) Plan 9 shell, rc, with modest extensions. One design mistake in make that mk fixes is variables in recipes: those are now passed as environment variables, with no prior substitution, so no more writing $$$$ to get the current PID in a recipe (and it’s written $pid in rc anyway).

Unfortunately, mk inherits from rc the principle of having the list of strings as the fundamental datatype (also used by Jam). That works, but it’s noticeably more limiting than Tcl’s route of having everything be strings but with robust quoting and unquoting procedures for putting lists inside them—at which point Tcl starts to look like a Lisp-2 with a slight propensity for stringiness and a mildly unusual syntax.