max(2,3,4,5,6,-99)

Various ways: in a loop (works for numeric and string values):
 proc max list {
    set res [lindex $list 0]
    foreach element [lrange $list 1 end] {
       if {$element > $res} {set res $element}
    }
    set res
 }
By sorting (this is for numeric values only):
 proc max list {lindex [lsort -real $list] end}

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See also [Reinhard Max]