Creating and working with an explicit list vs enumeration through fail

Question

I come up against this all the time, and I'm never sure which way to attack it. Below are two methods for processing some `season` facts. What I'm trying to work out is whether to use method 1 or 2, and what are the pros and cons of each, especially large amounts of facts. `methodone` seems wasteful since the facts are available, why bother building a list of them (especially a large list). This must have memory implications too if the list is large enough ? And it doesn't take advantage of Prolog's natural backtracking feature. `methodtwo` takes advantage of backtracking to do the recursion for me, and I would guess would be much more memory efficient, but is it good programming practice generally to do this? It's arguably uglier to follow, and might there be any other side effects? One problem I can see is that each time `fail` is called, we lose the ability to pass anything back to the calling predicate, eg. if it was `methodtwo(SeasonResults)`, since we continually fail the predic…

Accepted Answer

method one seems wasteful since the facts are available, why bother building a list of them (especially a large list). This must have memory implications too if the list is large enough ?

Yes, method 1 takes Θ(n) memory. It's primary benefit is that it is declarative, i.e. it has a straightforward logical meaning.

Method 2, the "failure-driven loop" as Prolog programmers call it, takes constant memory, is procedural and may be preferred when you're doing procedural (extra-logical) things anyway; i.e., in I/O code it's ok to use it.

Note that SWI-Prolog has a third way of writing this loop:

```
forall(season(S), showseason(S)).
```

This only works if `showseason` succeeds for each binding of `season(S)`.

Creating and working with an explicit list vs enumeration through fail

Solution

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