Add permutation operator about parboiled

[jason-s]

important (and probably obvious) nuance: actions should be disallowed: they're ok in sequences, but not permutations, because the order may be scrambled

[sirthias]

Actually, Jason, I have been thinking about this a bit more and I find it really hard to come with a good general construct for permutations. The reason is that there are really quite a few dimensions along which one permutation application can differ from another:

• Do all the elements have to appear or are partial permutations, where only a subset of all elements is present, ok? Is the empty match (zero elements) ok?
• How are permutation elements separated? Only empty matches or things like commas, colons, and the like?
• Should a custom action be run after matching an element?

After having looked at a few of my own application I realized that sometimes I need a "partial, non-zero, comma separated permuation with a custom element action" and sometimes I need a "complete, unseparated permutation without element action".

So in total, this construct really seems very hard to generalize properly and I am actually inclined to not offer it but require applications to implement it themselves, to really fit there needs...

Cheers,
Mathias

[jason-s]

Do all the elements have to appear or are partial permutations, where only a subset of all elements is present, ok? Is the empty match (zero elements) ok?

If you want an element to be optional, you wrap it in an Optional() rule, the same way you would in a Sequence().

How are permutation elements separated? Only empty matches or things like commas, colons, and the like?

I can see this as being a feature, but right now you don't offer this feature with other rules.

Should a custom action be run after matching an element?

No, if you want that, you wrap it in a Sequence().

For example:

  Permutation(
      Year(),
      Sequence(FirstOf(WordMonth(),NumericMonth()), runSomeActionOnMonth())
      Optional(NumericDayOfMonth())
  )

I could see wanting a separator, but that's a feature, not a core property of a permutation. There's probably a way to implement it in other ways, but I can't think of it at the top of my head.

I still think this is a valuable primitive that is very difficult for users of Parboiled to implement (as compared to within the Parboiled library), and you should offer primitive constructs with a minimum of extra features.

[sirthias]

Ok, I see your logic.
If the production elements can be complex rules and not just primitives (e.g. Strings or Chars) than most problems seem solvable. The separator rule could be attached to a Permutation with a "withSeparator" modifier, like this:

Permutation(
    Year(),
    Sequence(FirstOf(WordMonth(),NumericMonth()), runSomeActionOnMonth())
    Optional(NumericDayOfMonth())
).withSeparator(Sequence(',', OptWhiteSpace))

Looks doable...

I'll give it a first implementation with the next release...

Cheers and thanks for your ideas,
Mathias

[jason-s]

Cool! I like the withSeparator idea, I had no idea you could do things like that with rule objects.

[sirthias]

Actually, you can't generally.
But the Permutation rule would support 'withSeparator' to apply one...

---

[email protected]
http://www.parboiled.org

On 21.04.2011, at 14:09, jason-s wrote:

Cool! I like the withSeparator idea, I had no idea you could do things like that with rule objects.

Reply to this email directly or view it on GitHub:
#18 (comment)

[fge]

I have implemented something like this:

https://github.com/fge/parboiled-css/blob/master/src/main/java/org/eel/kitchen/css/base/RuleSetRunner.java

But it is not really "generic". I suppose it could be made generic enough if you registered rules instead of parser classes as I do now, but the effect is:

• I register n parsers/rules;
• some of them are required to match;
• I check that a rule only matches once.

It works beautifully well for me. But I don't know parboiled enough to make it general.