Global Variables in Prolog
It is not the skilful poet that bemoans the rigid rules of the sonnet.
When you start learning Prolog, the language will initially seem
very limited to you. Where are all the features you know from
other programming languages? In particular, where are
the global variables that you can use in other
languages to pass information around?
In Prolog, we pass information around in
predicate arguments. We are not using global
Why is this? Think about it: Suppose there were a global state
that is modified by a predicate. This would have consequences that
clash fundamentally with how we expect relations to behave. For
example, when programming in terms of relations, we expect:
Therefore, we avoid the use of global state in Prolog.
Instead, we describe relations between states of interests.
When you are stuck, consider adding an argument to your predicate
that lets you express the state of interest.
See Thinking in States for
- to be able to use them in reverse too
- to be able to reason about them in isolation from each other, allowing declarative testing
- that repeated use of the same relation has exactly the same meaning
- that all changes are undone on backtracking
- thread-safety for parallel evaluation
That being said, there are of course ways to modify the
global state of your programs. However, using them should be
the exception, not the norm. For example, you can modify
the global database using assertz/1. The global
database is very good for reading information, but quite
bad for often modifying the data. This is because the
global database performs indexing over its entries, and you
pay the price for indexing every time you assert new clauses.
In addition, you cannot use this kind of modifications in the
other direction. On backtracking, the data will still be there,
and running the same code twice may also add the data redundantly.
Using predicate arguments to pass around information
therefore more naturally leads to Prolog programs that
are pure and efficient.
lets you pass around information without the need to introduce
many additional predicate arguments.
More about Prolog