Using the minus-sign in identifiers

Novices, especially with a Lisp background assume that this-is-an-indentifier is a valid Prolog identifier. It is not. Using the display/1 primitive, we see that

?- display(this-is-an-indentifier). -(-(-(this, is), an), indentifier)

Of course, it is is prolog term and therefore no syntax error is raised. Even for the most common operations such as test for equivalence or `unify' the result is generally as expected. Some go wrong, such as sorting or breaking the identifier into its characters (atom_chars/2).

The same people write programs like this:

bruce-lee.

godzilla.

chuck-norris.

They get the warning Clauses of (-)/2 are not together in the source-file because there are two clauses of the term (-)/2 with one of godzilla/0 in between. SWI-Prolog (any many others) anticipate you might be re-using the same predicate name for a different definition and warns you (if you want a `discontigous' predicate, use the discontigous/1 declaration).

What to do?

In Prolog, multi-word identifiers are written as this_is_an_identifier or thisIsAnIdentifier. As we have variables and constants, it is good and quite widely accepted practice to write

• this_is_an_atom
• ThisIsAVariable

-- JanWielemaker - 17 Feb 2002 (after remarks from Adrian Holzwarth)