The '''S''' combinator is a sort-of generalised function application from combinator theory, the other most basic one being [K].
  proc S {f g x} {
     $f $x [$g $x]
  }

See [Hot curry] or [Combinator engine] for much more.


[[Can someone provide some context here on this page as to how this would be used ''in the real world''?]]
[RS]: It certainly is less evidently usable than [K]. You can for example compose identity out of S and two calls to K:
 I = S K K
but in the "real Tcl world" one would of course code identity (which is useful at times) as
 proc I x {set x}