The discussion of [If we had no if] got me thinking - what core commands can be written in terms of other core commands, and which are just syntactic sugar or optimizations?  Maybe if we could hook into [eval] to redo tcl's read-eval-print loop then things like [proc] become a simple lookup into a hashtable, a few [set]s and another eval..

Maybe something like
 proc eval {command args} {
   if {[info procs $command] == $command} {
     foreach [info args $command] $args break
     eval [info body $command]
   } elseif {[info commands $command] == $command} {
     $command $args
   }
 }

A little rough around the edges, but I never claimed to have fully thought it through ...
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Are there not already [core] [tcl] commands that are defined as tcl procs?

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TIP 90 removed the last barrier to being able to create
a proc for each built-in Tcl command that behaves exactly
like that built-in Tcl command.  That is, for every built-in
[[foo]], you can now create a corresponding [[myFoo]] that
is a proc and behaves the same.

The proc replacement ''may'' need to call the original
built-in command, if it exposes functionality not available
any other way, for example consider [[socket]].
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[category internals]