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Writing a JPEG Decoder in Rust – Part 2: Implementation I

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Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#51

Post by premium-concern »

gmfawcett wrote: post #50
cfallin wrote: post #48

Earlier quoted context omitted.

Not at all -- there's a lot of commonality in the language designs. Core language features: both have algebraic datatypes like ML ("case classes" in Scala, enums in Rust), and a 'match' statement that makes working with them easy. Both have pervasive destructuring that works with match arms, 'let' bindings, etc. Data structures and mutation: both encourage a pragmatic version of immutability -- use values and provide…

I'm aware that Rust's bootstrap compiler was written in Ocaml. And yes, there are lots of ML inspirations in the language -- but not enough to make an ML language. Where are the module functors, for example? Many would consider that an essential element of an ML language, and it is a stretch to call a language an ML family member without them. I don't want this to devolve into a No True Scotsman debate, I just feel t…

I think ML-family is fine. Scala is much closer to ML than what people sort into the Algol-family or C-family groups.

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#52

Post by saosebastiao »

gmfawcett wrote: post #50
cfallin wrote: post #48

Earlier quoted context omitted.

Not at all -- there's a lot of commonality in the language designs. Core language features: both have algebraic datatypes like ML ("case classes" in Scala, enums in Rust), and a 'match' statement that makes working with them easy. Both have pervasive destructuring that works with match arms, 'let' bindings, etc. Data structures and mutation: both encourage a pragmatic version of immutability -- use values and provide…

I'm aware that Rust's bootstrap compiler was written in Ocaml. And yes, there are lots of ML inspirations in the language -- but not enough to make an ML language. Where are the module functors, for example? Many would consider that an essential element of an ML language, and it is a stretch to call a language an ML family member without them. I don't want this to devolve into a No True Scotsman debate, I just feel t…

A module functor is nothing more than a higher order module...one that takes as a parameter some data structure that conforms to an interface (signature). Rust may not have constructs called module functors, but it definitely has constructs that are capably equivalent, as well as some that are even more powerful (a claim that applies to Scala as well)

For me, type inference, strong typing, and ADTs with pattern matching puts a language into the ML family quite easily.

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#53

Post by pslam »

If you scan through the source (on github), the most notable thing is the lack of the "unsafe" keyword. I've seen too many people basically transliterate from C to Rust, along with all the unsafe operations. This one is pretty much unoptimized, but still seems to be performant enough, and doesn't do anything "unsafe".

That's not to say you could just throw this into the back-end of a public-facing website. It's still going to panic (abort) if someone unexpected happens, and it's still going to chew indeterminate amounts of CPU, memory or storage unless sandboxed (maybe even just ulimit). And there's still the danger one of the Rust standard library functions has a flaw in it. But this is the kind of starting point you wouldn't get with the plain C equivalent.

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#54

Post by gmfawcett »

saosebastiao wrote: post #52
gmfawcett wrote: post #50

Earlier quoted context omitted.

I'm aware that Rust's bootstrap compiler was written in Ocaml. And yes, there are lots of ML inspirations in the language -- but not enough to make an ML language. Where are the module functors, for example? Many would consider that an essential element of an ML language, and it is a stretch to call a language an ML family member without them. I don't want this to devolve into a No True Scotsman debate, I just feel t…

A module functor is nothing more than a higher order module...one that takes as a parameter some data structure that conforms to an interface (signature). Rust may not have constructs called module functors, but it definitely has constructs that are capably equivalent, as well as some that are even more powerful (a claim that applies to Scala as well) For me, type inference, strong typing, and ADTs with pattern match…

All right, I concede.:) Thanks for sharing your views.

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#55

Post by doomrobo »

grayrest wrote: post #34
0x0 wrote: post #2

I don't know rust, but what happens if you get a 0xff as the last byte here, wouldn't i+1 go out of array bounds? while i

Since Rust people are reading this. I find myself wanting to walk an iterator pairwise regularly for stuff like this and I'm using: for (cur, next) in (& vec).iter().zip(vec.iter().skip(1)) { encoded_data.push(cur); if cur == 0xff && next == 0x00 { // ... } } Is there a better way to write this? What I'd really like is the equivalent to Clojure's partition[1]. I know that the stdlib has windows `(partition n (dec n)…

If you can afford the allocation at every iteration, the iterslide crate might work for you.

https://github.com/slapresta/rust-iterslide

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#57

Post by eddyb »

pcwalton wrote: post #22
lmm wrote: post #16

Earlier quoted context omitted.

It's not, but they're both ML-family languages. I had hoped that Rust would be able to offer a similar level of elegance to Scala.

The biggest difference between Scala and Rust here is ".iter()", which is necessary mainly because you need to specify whether the iterator iterates over references (".iter()") or passes results by value (".into_iter()"). Because Java doesn't have value types and has no concept of move semantics, it has no need for the distinction, while Rust does. In effect, the "elegance" of Scala here is really the product of its…

The example can be written like this though:

  let codes: Vec = data_table.iter()
            .zip(&code_lengths)
            .zip(&code_table)
            .map(|((&value, &length), &code)| {
                HuffmanCode {
                    length: length,
                    code: code,
                    value: value,
                }
            })
            .collect();

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#59

Post by wahern »

Manishearth wrote: post #19
quotemstr wrote: post #8

Earlier quoted context omitted.

Yes: now we all forget how to correctly manage memory. Rust: where resources are unlimited and aborting when you run out of them is okay

Rust has provided ways to not abort on oom for a while now.

If by "a while" you mean several months. But, yes, the recent addition of catch_unwind answered one of my biggest gripes with Rust.

  https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
Does anybody know if the standard library allocator aborts the process or throws a catchable exception? When they added catch_unwind they officially split panics into two types: abort panics and unwind panics. Code can choose one or the other when panic'ing. Also, programs can be built in such a way that all panics become aborts. That kind of sucks but it's probably one of the compromises needed to assuage those Rust developers who don't believe it's practical to handle OOM conditions.

Re: Writing a JPEG Decoder in Rust – Part 2: Implementation I

#60

Post by wahern »

vertex-four wrote: post #43
quotemstr wrote: post #8

Earlier quoted context omitted.

Yes: now we all forget how to correctly manage memory. Rust: where resources are unlimited and aborting when you run out of them is okay

The vast majority of programs that you run on a day to day basis will abort when they run out of memory. On a default Linux system, there's not even any way to prevent that - you need to faff with the config to make it actually return errors when allocating memory.

That's all irrelevant for a language which is supposed to be a systems programming language, however that term is defined.
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