Missing utilities about parsy HOT 6 OPEN

Hi, just wonder if there is anyone working on this? I've just finished one project using Parsy and willing to contribute.

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I'm not working on this, contributions would be welcome. notFollowedBy is already implemented in the form of should_fail.

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I've just sent a PR implementing the lookAhead #29

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And here's a proposal for manyTill : #31

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I've got a concern about my implementation of lookAhead. peek is a function rather than a method on Parser. It's something I find confusing when I'm working with Parsy: Is stuff a method or a function that takes a parser?

I did make a function, because it's how I implemented this as a helper function in a project of mine, no other reason.

Is there a rule of thumb for this dilemma, method or function?

Note that peek(p) is the exact equivalent of p.should_fail().should_fail() (and we should benchmark this to decide wich implementation is better). Maybe it makes sense to make it a method on Parser.

Same question for #31.

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@lisael - yes, it is a bit hard to decide in some cases.

I think many_until makes more sense as a method, because it is similar to other things like many, and peek for a similar argument comparing to should_fail. I guess it's about a consistent fluent API.

For things that don't take a parser as an argument, or can take more than one, we use functions. I think that's a good rule of thumb.

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