NEM Transparent values in Tcl, are those which directly represent the value, as opposed to handle values which are more like pointers or references in other languages. Transparent values include things like lists, dicts (in 8.5) and strings. Transparent values are first-class in that they can be passed directly to commands, and returned from commands. Handles are first-class, but the things that they are handles for (what is being pointed to) are not first-class, as they can only be passed around via a handle. For instance, commands are a handle value - you pass around the name of the command, rather than the command itself (Tcl uses a hashtable or similar to map the name to the actual command internally). On the particular issue of commands, see TIPs 187, 194, and 196 which all try to introduce first-class transparent commands into Tcl.
[ Category ?? ]