[catfacts]

I am not even a newbye in Rust and also this could be just nitpicking, but it seems that match is comparing strings and not characters, if this is the case then I think Common Lisp can optimize more, since there is a special comparison for characters in CL.

Edited: In the optimized version the author use bytes and generators and avoid using strings. I don't know if Rust generators are optimized for speed or memory, ideally you could define the length of the buffer according to the memory cache available.

Edited: I find strange using input = read_input_file()? and then using eval(&input), what happens when there is an error reading the file? Rust is supposed to be a high security language. In CL there are keyword like if-does-not-exists to decide what to do and also read accepts additional parameters for end-of-file and for expressing that this read is inside a recursive procedure inside another read.

I should stop comparing Rust to CL, better learn Rust first. I consider this kind of articles a very good way of learning Rust for those interested in parsing and optimizations. Rust seems to be a very nice language when you can afford the time to develop your program.

[VGHN7XDuOXPAzol]

> what happens when there is an error reading the file?

the question mark `?` denotes the fact that the error is bubbled up (kind of like an exception, but with stronger typing and less silent)

[tialaramex]

Specifically the ? symbol is currently implemented via the operator trait Try as Try::branch() which gets you a ControlFlow

If Try::branch gives us a ControlFlow::Break we're done here, return immediately with the value wrapped by Break [if any] inside an Err, otherwise we have ControlFlow::Continue wrapping a value we can use to continue with execution of this function.

This is type checked, so if the function says it returns Result<Goose, Dalek> then the type of the value wrapped in a ControlFlow::Break had better be Err(Dalek) or else we can't use our ? operator here.

Reifying ControlFlow here separates concerns properly - if we want to stop early successfully then control flow can represent that idea just fine whereas an Exception model ties early exit to failure.

[catfacts]

Thanks for the info. I imagine that in this care, since it seems the error is not captured, it should end producing panic. So a question mark is used when the expected result is of type Result or Error. Also the web page, https://doc.rust-lang.org/rust-by-example/error/result.html, describe the result type as Ok(T) or Err(E), and indicates that is a richer version of Option.

[VGHN7XDuOXPAzol]

Yeah, if `main` returns an error I think it exits with an error code and prints it out, so quite similar to a panic.

I think the blog post is not focussing on error handling too much, but in any case this is 'safe', just could likely be handled better in a real-world case.

[biorach]

> I should stop comparing Rust to CL, better learn Rust first

Yes

[phi-go]

The ? will directly return Err if there is one during read_input_file(). This is just some syntactic sugar.

[VGHN7XDuOXPAzol]

If you wanted to match on characters (`char`s) then you could do this with single quotes (`'+'`)

Or if you wanted to do it on bytes, you could also do this, with (`b'+'`).

Unsure if that would provide a meaningful boost or not

[epage]

Likely, comparing on `char` ('+') would be slower as it requires decoding the `&str` as a `char` which comes with some significant overhead (I've seen 9% on a fairly optimized parser). Ideally, when you grammar is 7-bit ASCII (or any 8-bit UTF-8 values are opaque to your grammar), you instead parse on `&[u8]` and do `u8` comparisons, rather than `char` or `&[u8]`.

[catfacts]

Thanks for all the information you provided. I will read Rust by Example and stop posting in this thread to avoid deviating from the OP. Anyway, perhaps other readers are learning Rust and having the same questions in their minds, so your answers are also welcome for them.

Edited: I will eliminate my catfacts username (changing passsord to a random one), I don't like being downvoted and I know I should not mention it, but things are what they are. Good bye catfacts !.