proc unless {condition body} {
uplevel [list if "!($condition)" $body]
}example:
unless {$tcl_version >= 8.4} {
error "package requires Tcl version 8.4 or greater"
}Lars H: An alternative implementation, which avoids shimmering:
proc unless {condition body} {
uplevel 1 [list if $condition then {} else $body]
}glennj: additionally, provide an else clause...
proc unless {cond body args} {
set else_body {}
if {([llength $args] == 2) && ([lindex $args 0] eq "else")} {
set else_body [lindex $args 1]
} elseif {[llength $args] > 0} {
error "usage: [lindex [info level 0] 0] expr body1 ?else body2?"
}
uplevel 1 [list if $cond then $else_body else $body]
}