[RS] 2007-02-23: ''Inf'' is for infinity. From Tcl 8.5 it is a valid double (though not integer) value, as these experiments in an 8.5a4 tclkit show:

 (bin) 3 % set x [expr 1/0.]
 Inf
 (bin) 4 % expr $x+1
 Inf
 (bin) 5 % expr $x*$x
 Inf
 (bin) 6 % expr $x-$x
 domain error: argument not in valid range
 (bin) 7 % expr -$x
 -Inf
 (bin) 8 % expr -$x+$x
 domain error: argument not in valid range
 (bin) 9 % expr {$x+1 == $x}
 1

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[LV] Besides [expr]. where else does a Tcl programmer need to worry about
Inf (or [NaN] for that matter?) - [RS]: [binary scan] may return it for a double. A general test for Inf and Nan would be
 proc is'special'float x {expr {$x != $x || [catch {expr $x-$x}]}}

[LV] Can I also assume that [if], because it does some expr like stuff, 
is another place one should be aware that Inf or NaN might show up? 

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[Category Glossary]