Issue with a tutorial

[digamma F]

Hi.

I'm a total beginner in Prolog, and I'm following a tutorial. It says that :

Prolog file :

sunny.

Interactive session :

 ?- sunny.

true

 ?- snowy.

false

However, when I try it, given the same Prolog file :

 ?- sunny.

true

 ?- snowy.

ERROR: Undefined procedure: snowy/0 (DWIM could not correct goal)

Why ?

[Boris Vassilev]

It must be a very outdated tutorial. As the old joke goes:

> 1972 - Alain Colmerauer designs the logic language Prolog. His goal is to create a language with the intelligence of a two year old. He proves he has reached his goal by showing a Prolog session that says "No." to every query.

Modern Prolog correctly throws an exception when a rule is not found at all.

You can implement "Negation as failure" like this:

Prolog file:

weather(sunny).

Now:

?- weather(sunny).
true.

?- weather(snowy).
false.

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[digamma F]

Thanks a lot !