Code:

# rename the expr command to oexprrename expr oexpr#write our new expr commandproc expr {args} {  set newargs [list]# let's create a new function that computes the sum between 1..N# sum(N)  foreach item $args {    if {[regexp {sum\((.*)\)} $item - num]} {      set sres 0      for {set i 1} {$i <= $num} {incr i} {        incr sres $i      }# add our result to the new arguments that we'll call the original# expr command with      lappend newargs $sres    } else {# not interested in this.. just add it to the arguments to call original# expr command with      lappend newargs $item    }  }  if {([llength $newargs] == 1) && [info exists sres]} {# only our command was found in the expression, so just return the result    return [lindex $newargs 0]  } else {# call the original expr command    return [oexpr [join $newargs]]  }}

Code:

% expr sum(10)55% expr 5 + sum(10)60

Code:

proc tcl::mathfunc::sum {num} {   set sres 0  for {set i 1} {$i <= $num} {incr i} {     incr sres $i   }  return $sres }

Code:

% expr sum(10)55