Force .Chain() to apply at least once about superpower HOT 7 CLOSED

Hi! I think there are two possible approaches. I would tend towards enforcing this by post-processing the resulting AST, but it also should be possible to use something like:

public static readonly TokenListParser<Token, Expression> Conditional =
    Parse.Chain(Boolean, Operand, MakeBinary)
        .Where(e => e is BinaryExpression);

Alternatively, you could modify Chain() to create your own combinator ChainAtLeastOnce().

Do any of the above help? Cheers!

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@nblumhardt Sorry about the delay in getting back to you, I did try to do something similar to both of those, but I was hoping there would be a way that in either of those methods I would be able to have access to the character/line number for the error message. The latter method does throw an ParseException automatically, but the message isn't helpful, and I haven't figured out how to get access to the positions within the different parts during post-processing.

Is it possible to do either of those? Preferably I would have it generate the error message, so it can happen earlier on during the parsing, but it would also be useful to know how to access the line/char/abs position after the parser runs.

If I were to modify Chain(), how would you suggest I go about that? I'm still fairly new to working with C# and libraries.

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Hi @RPiAwesomeness - some experimentation might be needed, but I think it should be enough to add a line at: https://github.com/datalust/superpower/blob/dev/src/Superpower/Parse.cs#L135

if (!operatorResult.HasValue) return operatorResult;

This would be before the while loop begins, and would have the effect of failing the parse if the first attempt to parse an operator fails.

HTH!

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So I could just edit that file in my local copy of Superpower? How would that work with future releases?

If I were to add a combinator like you mentioned (ex. ChainAtLeastOnce) how best should I go about submitting the pull request/handling testing/and all that?

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Hi! You'd just add ChainAtLeastOnce() to a new static class in your own project. As long as future Superpower versions don't break the signatures of underlying parser/result types, this would just keep working.

I don't think we'd accept this via a PR currently, as although it's the right thing to do in your scenario, it's probably not general enough to warrant adding to the core library right now (there's a huge space of possible parsing tactics, so it'd be easy to bloat Superpower out unintentionally).

HTH, and let me know if you need a hand getting ChainAtLeastOnce() up and running! 👍

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I finally have had time to get back around to working on this again and in trying what you suggested I ran into two issues when I attempted to build it:

1>Parser\Mixins.cs(60,54,60,68): error CS0029: Cannot implicitly convert type 'Superpower.Model.TokenListParserResult<TKind, TOperator>' to 'Superpower.Model.TokenListParserResult<TKind, T>'
1>Parser\Mixins.cs(60,54,60,68): error CS1662: Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type
1>Parser\Mixins.cs(62,66,62,75): error CS1061: 'TokenListParserResult<TKind, TOperator>' does not contain a definition for 'IsPartial' and no accessible extension method 'IsPartial' accepting a first argument of type 'TokenListParserResult<TKind, TOperator>' could be found (are you missing a using directive or an assembly reference?)

I wasn't sure exactly what you meant, so I tried copying the Chain() function code directly and adding the line as you suggested. I assume there is a better way to approach this, but that made the most sense to me as the first thing to try.

The first error makes sense to me, I just need to be able to return the correct type when I return early and the result from @operator() doesn't match the second type.

As for the second error, is this something that I can resolve by adding a reference to a certain part of Superpower? According to Visual Studio it's an internal method that I don't have access to in my code.

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Hope you found a solution to this; closing as the thread's now a bit stale 👋

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