Document the role of PhantomType/PhantomData about combine HOT 2 CLOSED

Not a problem!

Consider Take which uses PhantomData for the I : RangeStream parameter since Rust requires that all type paramters of a type is actually used in the type itself.

pub struct Take<I>(usize, PhantomData<fn(I) -> I>);
impl<I> Parser for Take<I>
    where I: RangeStream,
          I::Range: ::primitives::Range
{
    type Input = I;
    type Output = I::Range;

    fn parse_lazy(&mut self, mut input: Self::Input) -> ConsumedResult<Self::Output, Self::Input> {
        ...
    }
}

So why can't we just have pub struct Take(usize); ?

Well, the Parser implementation uses I for its associated Input type and all associated types must be fully specified from the Self type (Take<I>). If I did not appear in Take Rust rejects the impl as you could have Take : Parser<Input = &str> and Take : Parser<Input = &[u8]> etc.

It is actually possible to be a bit more clever and move Input to a trait parameter instead as below which will let use remove the I parameter and I have actually tried doing that in the past. However it turns out that if Parser is implemented as below rust can start failing to infer the correct type for Input in not to uncommon cases and the only way to resolve that is to force users to add type annotations which is not particularly user friendly.

impl<I> Parser<I> for Take
    where I: RangeStream,
          I::Range: ::primitives::Range
{
    type Output = I::Range;

    fn parse_lazy(&mut self, mut input: I) -> ConsumedResult<Self::Output, I> {
        ...
    }
}

Anyways, that is the gist of it. Do you have any reason other than curiosity towards this? I feel its more of an implementation detail so I am not sure it belongs in the README since it is really just an implementation detail. (Not that implementation details are worth documenting. I guess as long as it is not front and center it can belong there).

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@Marwes wow, great answer. Thanks for being so responsive!

Do you have any reason other than curiosity towards this?

Not really. I wrote a small combinator and really had to dig into the source to understand some subtleties (confounded by this being my "learn Rust!" project), so I wanted to really understand what was going on in the guts.

I feel its more of an implementation detail so I am not sure it belongs in the README since it is really just an implementation detail. (Not that implementation details are worth documenting. I guess as long as it is not front and center it can belong there).

I'm fine with this. I prefer to phrase tickets/issues as "complete this action" or "solve this problem", so I phrased my question as "document this curiousity" rather than "answer my question". But you've answered my question and we can link to this discussion rather than fold it into the README as you see fit.

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