transcendent value null

null (transcendent value)

null is a transcendent value that represents the absence of a value

#if target_platform == null
    #print_warning "Assuming UNIX-like platform"
#end

Difference from undef

null should be used instead of undef to represent the absence of a value

#set has_feature null

undef is only meant as the value for transcendent variables that haven't been assigned a value