CODEWITHSUNDEEP
prolog
sentinel
sentinel

Reputation: 421

Question mark, plus, minus preceding Prolog variable names

I've searched for an explanation to this but haven't found one. What do the question mark, plus sign, and minus sign that sometimes precede variable names in the description of a Prolog predicate mean?

Example:

predicate(?Variable1,+Variable2,-Variable3)

Upvotes: 24

Views: 9738

Answers (2)

Roland Illig
Roland Illig

Reputation: 41625

  • ? means: This variable can be either instantiated or not. Both ways are possible.
  • + means: This variable is an input to the predicate. As such it must be instantiated.
  • - means: This variable is an output to the predicate. It is usually non-instantiated, but may be if you want to check for a specific "return value".

Source: Chapter 4 of the SWI Prolog documentation.

Upvotes: 36

user206428
user206428

Reputation:

+ means that Variable2 is expected to be bound (to a term, or perhaps just some variable) -- you can think of this as input to predicate/3, which the predicate won't attempt to modify in execution.

- means that Variable3 is expected to be bound by predicate/3 in it's execution -- you can think of this as output from predicate/3. This doesn't mean it can't be bound, however, particularly if you know what to expect and are checking for success, but predicate/3 is described as potentially binding (unifying) Variable3 to something.

? means that Variable1 can be either be bound (+, input) or not (-, output) - predicate/3 should deal with both cases, if it accepts either.

Upvotes: 5

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