One fundamental principle in tcl is that everything is a string (EIAS) [1]. Therefore, an empty string "" is the same thing as a "null value" or 0-length list,

One fundamental principle in tcl is that everything is a string (EIAS) [1]. Therefore, an empty string "" is the same thing as a "null value" or 0-length list, but generally doesn't require any special treatment (your function that takes a list as input generally should know how to handle empty lists). [1] In practice things are typed in the interpreter internals for performance reasons, but the language behaves as if EIAS.