abs is one of several extended math commands available in Tclx. It makes the expr abs function available as a command, and takes expressions as arguments. (Tcl 8.5 has its own copy that it puts in the ::tcl::mathfunc namespace.)
If you don't have that, it can easily be emulated:
proc abs x {expr {abs($x)}}AMG: In Tcl 8.4.7, expr {abs(-0.0)} returns -0.0. In Tcl 8.6b1, it returns 0.0.
See also: