ecma262 VS virgil

These aren't valid JavaScript (@@iterator would throw an error). They are actually internal Symbols used in JavaScript. They are used to implement features like iteration, instanceOf, and such internally. They actually might get removed or changed

Note that in previous editions, the ECMAScript® Language Specification used the term "lexical environment" before it decided to rename it to "Environment Record" so you might encounter this term in other definitions and tutorials.

For a new specification to be written, you need two things, a_ technical committee_, and a standard. The standard specification for JavaScript is called ECMA-262, and the technical committee is Technical Committee-39(TC39).

4. Proposed something else [ https://github.com/tc39/proposal-ptc-syntax ]

While apple is against Syntactic tail calls, they’re mainly just opposed to versions of it that would remove/unrequire the tail-call optimisation they already do: https://github.com/tc39/ecma262/issues/535

For the version of it that is backwards compatible, they wouldn’t need to do anything other than recognise it as valid syntax. Their main concern is that it "could add confusion with very little benefit."

The spec for STC has a critique of PTC:

- performance

- developer tools

- Error.stack

- cross-realm tail calls

- developer intent

See: https://github.com/tc39/proposal-ptc-syntax#issues-with-ptc

Apple's 2016 response as to why they won't implement STC is here: https://github.com/tc39/ecma262/issues/535

- STC is part of the spec and will take too long to change.

- Now that they've implemented support for PTC, they don't want to regress web pages that rely on it.

- They don't want to discourage vendors from implementing PTC by agreeing to STC.

- They don't want to introduce confusion.

Some of these arguments about confusion and delays seem wrong hindsight, since on every point things would have been better if they'd just agreed to the compromise of STC.

- It would have been part of the spec years ago

- STC would have had a clear way for web pages to know when tail calls could be relied on (and PTC would have been optional)

- Other vendors didn't implement PTC in any case, despite no agreement on STC

- There's even more confusion as things are now

For (2) Virgil has several features that allow you to layout memory with various levels of control. I assume you meaning "array of structs", and you can do that with arrays of tuples, which will naturally be flattened and normalized based on the target (i.e. will be array-of-structs on native targets). You can define byte-exact layouts[1] (mostly for interfacing with other software and parsing binary formats), unbox ADTs, and soon you can even control the exact encoding of ADTs.

Virgil is GC'd.

[1] https://github.com/titzer/virgil/blob/master/doc/tutorial/La...

Virgil doesn't use frame pointers. If you don't have dynamic stack allocation, the frame of a given function has a fixed size can be found with a simple (binary-search) table lookup. Virgil's technique uses an additional page-indexed range that further restricts the lookup to be a few comparisons on average (O(log(# retpoints per page)). It combines the unwind info with stackmaps for GC. It takes very little space.

The main driver is in (https://github.com/titzer/virgil/blob/master/rt/native/Nativ... the rest of the code in the directory implements the decoding of metadata.

I think frame pointers only make sense if frames are dynamically-sized (i.e. have stack allocation of data). Otherwise it seems weird to me that a dynamic mechanism is used when a static mechanism would suffice; mostly because no one agreed on an ABI for the metadata encoding, or an unwind routine.

I believe the 1-2% measurement number. That's in the same ballpark as pervasive checks for array bounds checks. It's weird that the odd debugging and profiling task gets special pleading for a 1% cost but adding a layer of security gets the finger. Very bizarre priorities.

This paper is the first time I seen mention of the Virgil programming language, from the same author:

https://github.com/titzer/virgil

JavaScript handles the "no identity hash" with WeakMap and WeakSet, which are language built-ins. For Virgil, I chose to leave out identity hashes and don't really regret it. It keeps the language simple and the separation clear. HashMap (entirely library code, not a language wormhole) takes the hash function and equality function as arguments to the constructor.

[1] https://github.com/titzer/virgil/blob/master/lib/util/Map.v3

This is partly my style too; I try to avoid using maps for things unless they are really far flung, and the things that end up serving as keys in one place usually end up serving as keys in lots of other places too.

Whoa, interesting. I didn't know Kotlin had all those constructs.

In Virgil, a method on an object (or ADT) can declare its return type as "this". Then the method implicitly returns the receiver object. That trick is very useful to allow a chain of calls such as object.foo().bar().baz(). I find it readable and easy to explain:

https://github.com/titzer/virgil/blob/master/doc/tutorial/Re...

I hadn't written one until ~30 mins ago [1]. I cheated and looked at a Java quine (not particularly elegant, but easy to see what is going on.), but I wrote one for Virgil. Just think string substitution; a string with a hole in it and you substitute a copy of the string, quoted into the hole. Just one substitution suffices.

[1] https://github.com/titzer/virgil/blob/master/apps/Quine/Quin...

Virgil has a family of completely well-defined (i.e. no UB) fixed-size integer types with some hard-fought rules that I eventually got around to documenting here:

https://github.com/titzer/virgil/blob/master/doc/tutorial/Fi...

One of the key things is that values are never silently truncated (other than 2's-complement wrap-around) or values changed; only promotions. The only sane semantics for over-shifts (shifts larger than the size of the type) is to shift the bits out, like a window.

The upshot of all that is that Virgil has a pretty sane semantics for fixed-size integers, IMHO.

LLVM and other compilers that use SSA but target a stack machine can run a stackification phase. Even without reordering instructions, it seems to work well in practice.

In Virgil I implemented this for both the JVM and Wasm. Here's the algorithm used for Wasm:

https://github.com/titzer/virgil/blob/master/aeneas/src/mach...

Virgil: A fast and lightweight programming language that compiles to WASM\ (54 comments)