4 ms·

> free-form records, dynamically-sized arrays, no types AFAIK, free form records and lack of types do not an easier compilation target make. It's pretty easy t

> free-form records, dynamically-sized arrays, no types AFAIK, free form records and lack of types do not an easier compilation target make. It's pretty easy to compile an untyped language to a typed language. > How so? - There are a ton of implicit casts that are almost certainly not what you want. So, for example, if you're going to use addition in the compiled code, it's a 10-step process [1, section 12.8.3.1], and you should be careful not to trigger the 9 steps that do things other than addition. - There is, last I heard, no way to determine the size of the stack. - No integers. - Say you have a demo page for some language, and someone using the page writes an infinite loop. Don't want the page to crash? Welcome to advanced compilation techniques like CPS transformations and trampolining. Overall, JS has a ~850 page spec, and any part of the language you target for which you don't fully understand the spec is a potential bug. Instead, you want your target language to be dumb, tiny, and explicit. [1] http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf http://www.ecma-international.org/publications/files/ECMA-ST... Disclaimer: I have done research on JS, but I do not study or develop compilers.

DiThi 9y ago
> There are a ton of implicit casts Very easy to avoid if you're compiling a statically typed language to JS. > No integers Not in the JS specification (outside typed arrays), but every JS JIT works in a way you can actually declare variables as 32 bit signed integers, and made its way into the asm.js specification. They are declared like this: var a = value|0; Where the |0 is a no-op so not actually done, just a type hint. If what worries you is precision and not performance, doubles allow 53 bit integers (not counting sign bit) with full precision. > Say you have a demo page for some language, and someone using the page writes an infinite loop. Don't want the page to crash? Welcome to advanced compilation techniques like CPS transformations and trampolining. Or just use a web worker, which you can terminate if you haven't heard back in a while (pun intended). > and any part of the language you target for which you don't fully understand the spec is a potential bug It heavily depends on what type of language you do. If your compiler tracks the types and doesn't mix them, edge cases are much, much easier to avoid. Source: I did make a compiler for my own language that targeted JS.
- merb 9y ago
  > Where the |0 is a no-op so not actually done, just a type hint. if value is null / undefined / empty object / empty array, empty string... | 0 wouldn't be no-op it would actually assign 0 to a.
  - dbaupp 9y ago
    When JS is a compiler target, that truck will generally only be used if the source language excludes all those edge cases, i.e. the value is guaranteed to be an integer.
- flavio81 9y ago
  >Not in the JS specification (outside typed arrays), but every JS JIT works in a way you can actually declare variables as 32 bit signed integers, and made its way into the asm.js specification. They are declared like this: The "|0" trick you mention is not for javascript; it is for asm.js; to be able to declare such a "true integer" variable, your code would need to be in asm.js, not javascript. Javascript has no integers, only floating point numbers. This is a very strong limitation.
  - DiThi 9y ago
    What would entail to declare a "true integer"? Correctness? The |0 after each operation makes it correct. All bitwise operations in JS operate on signed 32 bit integers. Speed? All JS JITs have optimizations for integers, and it's the reason asm.js uses that trick, not the other way around. Precision? If you don't use bitwise operations, you have up to 53 bits of perfect integer precision, plus bit sign. Guaranteed by the standard.
michaelmior 9y ago
> It's pretty easy to compile an untyped language to a typed language Did you mean the other way around? Certainly it's possible to compile an untyped language to a typed language, but it's nontrivial especially in the presence of duck typing.
- comex 9y ago
  I think they mean by using a single generic type for everything, like you'd see in an interpreter for an untyped language. It's pretty easy, but it's also very slow...
  - michaelmior 9y ago
    Fair point. That approach is pretty easy :)
deleted 9y ago
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qualitytime 9y ago
"want your target language to be dumb, tiny, and explicit." What would be your choices?
- fasquoika 9y ago
  Not the parent, but I've been reading the WebAssembly spec and that sounds like a perfect description. It's honestly probably one of the more ideal compiler targets
- dualogy 9y ago
  Any one language of your choosing that you extract a dumb-tiny-and-explicit sub-set out of to target =)

DiThi 9y ago

> There are a ton of implicit casts Very easy to avoid if you're compiling a statically typed language to JS. > No integers Not in the JS specification (outside typed arrays), but every JS JIT works in a way you can actually declare variables as 32 bit signed integers, and made its way into the asm.js specification. They are declared like this: var a = value|0; Where the |0 is a no-op so not actually done, just a type hint. If what worries you is precision and not performance, doubles allow 53 bit integers (not counting sign bit) with full precision. > Say you have a demo page for some language, and someone using the page writes an infinite loop. Don't want the page to crash? Welcome to advanced compilation techniques like CPS transformations and trampolining. Or just use a web worker, which you can terminate if you haven't heard back in a while (pun intended). > and any part of the language you target for which you don't fully understand the spec is a potential bug It heavily depends on what type of language you do. If your compiler tracks the types and doesn't mix them, edge cases are much, much easier to avoid. Source: I did make a compiler for my own language that targeted JS.

merb 9y ago
> Where the |0 is a no-op so not actually done, just a type hint. if value is null / undefined / empty object / empty array, empty string... | 0 wouldn't be no-op it would actually assign 0 to a.
- dbaupp 9y ago
  When JS is a compiler target, that truck will generally only be used if the source language excludes all those edge cases, i.e. the value is guaranteed to be an integer.
flavio81 9y ago
>Not in the JS specification (outside typed arrays), but every JS JIT works in a way you can actually declare variables as 32 bit signed integers, and made its way into the asm.js specification. They are declared like this: The "|0" trick you mention is not for javascript; it is for asm.js; to be able to declare such a "true integer" variable, your code would need to be in asm.js, not javascript. Javascript has no integers, only floating point numbers. This is a very strong limitation.
- DiThi 9y ago
  What would entail to declare a "true integer"? Correctness? The |0 after each operation makes it correct. All bitwise operations in JS operate on signed 32 bit integers. Speed? All JS JITs have optimizations for integers, and it's the reason asm.js uses that trick, not the other way around. Precision? If you don't use bitwise operations, you have up to 53 bits of perfect integer precision, plus bit sign. Guaranteed by the standard.

michaelmior 9y ago

> It's pretty easy to compile an untyped language to a typed language Did you mean the other way around? Certainly it's possible to compile an untyped language to a typed language, but it's nontrivial especially in the presence of duck typing.

comex 9y ago
I think they mean by using a single generic type for everything, like you'd see in an interpreter for an untyped language. It's pretty easy, but it's also very slow...
- michaelmior 9y ago
  Fair point. That approach is pretty easy :)

deleted 9y ago

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qualitytime 9y ago

"want your target language to be dumb, tiny, and explicit." What would be your choices?

fasquoika 9y ago
Not the parent, but I've been reading the WebAssembly spec and that sounds like a perfect description. It's honestly probably one of the more ideal compiler targets
dualogy 9y ago
Any one language of your choosing that you extract a dumb-tiny-and-explicit sub-set out of to target =)