Higher kinded polymorphism

[tiffany352]

Rust doesn't support higher kinded polymorphism, despite being a common feature in many functional programming languages. As far as I know, it is a commonly requested feature from people in the FP crowd.

[nikomatsakis]

cc me. I've been thinking a lot about this and have a vague plan. I think we'll want it going forward to make dealing with user-defined pointer types work out. I'll try to find the time to write up my plans shortly.

[thehydroimpulse]

/cc me

[rizo]

Any updates on this?

[erickt]

One idea I was toying around with today was that if we did have Higher Order Types, we could design the std::io to optimize away the error branches for Writers that are statically guaranteed to never error, like MemWriter. We could declare the Writer trait to be:

trait Writer<M> {
    fn write(&mut self, buf: &[u8]) -> M<()>;
}

impl Writer<Identity> for MemWriter {
    fn write(&mut self, buf: &[u8]) -> Identity<()> {
        self.buf.push_all(buf);
        Identity(())
    }
} 

impl FdWriter<IoResult> for FileWriter {
    fn write(&mut self, buf: &[u8]) -> IoResult<()> {
        let len = libc::write(self.fd, buf.as_ptr());
        if len == -1 {
           Err(io_error_from_errno())
        } else {
           Ok(())
        }
    }
}

[bjadamson]

Does anyone have a 'collection' on all the different discussions/blog posts about supporting HKT's in rust? Maybe a collection of relevant links that would be useful for implementing this feature? I think it would be really helpful if we started pooling our ideas together on how to implement HKT in rust.

[bjadamson]

http://www.hydrocodedesign.com/2014/04/02/higher-kinded-types/

[nikomatsakis]

I don't think there have been that many. I know I at least have had a lot of thoughts and discussions but relatively little time to write them out.

[thehydroimpulse]

Should this be moved to the rfc issue tracker?

[aturon]

@thehydroimpulse Yes -- please ping @nick29581 (nrc on IRC)

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#324