11 ms·

Sweet.js – Hygienic Macros for JavaScript

76 points by ekzy 9y ago

longlho 9y ago
this is already covered in basically any AST transform systems (babylon, acorn, TS...)
- zeckalpha 9y ago
  This predates those, and is hygienic.
- adgasf 9y ago
  I believe that Babel lacks custom infix operators whilst Sweet.js provides them.
  - btown 9y ago
    You're absolutely right; Babel's parser is very much hand-coded, and adding syntax requires deep knowledge of the internals. Not impossible, but not for the faint of heart. My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled... https://github.com/babel/babylon/blob/master/src/parser/expression.js#L461 https://github.com/babel/babylon/blob/master/src/parser/expr...
    - scribu 9y ago
      That raises a question: What's stopping Babel from using Sweet.js instead of a custom parser? Are there any transpilers that do use Sweet.js?
      - zeckalpha 9y ago
        Contracts.coffee IIRC, uses Sweet for Contracts.
    - djfm 9y ago
      Well "a.b.c" violates the principle that you should never access a property more than one level deep anyway ;) But in this case it's pretty straightforward to write a function like "get('b', 'c')(a)" or use a lib like lodash [1] [1]: https://lodash.com/docs/#get https://lodash.com/docs/#get
      - btown 9y ago
        The Law of Demeter doesn't always apply neatly, especially when you're introspecting JSON sent over the wire and you don't have a "class" with object oriented behavior for each sub object (at best, they're strongly typed). And the runtime overhead AND verbosity of a function may not be ideal, with the string parsing required in lodash and the awkward syntax of the curried function. Something that compiles natively in a DRY way to a && a.b ... would be used widely imo.
    - chocolateboy 9y ago
      > My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled... Not for long: https://github.com/babel/babel/pull/5813 https://github.com/babel/babel/pull/5813
      - btown 9y ago
        You just made my day :)
- andreypopp 9y ago
  That's not just "AST transform system", Sweet.js provides a way to extend grammar with new productions. That allows to introduce new syntax constructs.
eatonphil 9y ago
Coming from a Scheme background, if there's one thing I can't believe we live without in Javascript, it's macros. Hell even C has macros... I've looked at Sweet.js a few times to add support for macros to the Linode Manager. Between m4, the gcc preprocessor, and something native, this seemed like the easiest potential sell. However, I've been turned off by how complicated this looks compared to any of the other options and I'm not sure it actually even supports the feature I want: allow some code to show up in "development" mode but for that code to not even exist in the compiled output of "production" mode. (The other feature that's just as important though is being able to wrap shared code up in a single function that gets compiled in at compile time, but I think Sweet.js supports that.)
- ykler 9y ago
  Who uses m4? It has been around forever, but I have never really seen it used except in autoconf. When I looked at it (a long time ago) it struck me as really tricky and confusing, even more than lisp macros (but maybe on a par with TeX). However, this was just a first impression -- I didn't go too deep.
  - chas 9y ago
    I have used it recently. I have a similar attitude towards m4 as shell scripts: they are good for quick hacks, proofs of concept, and very simple systems. When I need to sprinkle just a bit of macros on something, m4 is great. For example, a one-off piece of technical documentation I wrote is markdown interspersed with code examples. I wanted to make sure the code passed its unit tests, so it lives in separate files and I use m4 to include it into the bigger document. This is nicer than catting separate file chunks together because the text can all be contiguous, but if I needed to do anything more sophisticated I would probably switch to purpose-built templating or static site system.
  - dflock 9y ago
    I've used it for really simple templating stuff with Dockerfiles - which it works fine for.
  - kazinator 9y ago
couchand 9y ago
The in-the-wild code I've seen that uses Sweet.js has been atrocious. Of course I'm not saying that this is necessarily the case. I'd love to see some real world examples whter this is used effectively.
- btown 9y ago
  Separately, could you point to some examples of the bad Sweet code you've seen? If it's anything like C macro misuse, it'll at the very least be educational to see just how badly one can shoot oneself in the foot.
- joshschreuder 9y ago
  I must say I don't see the use case... What real world examples do you have?
d--b 9y ago
Also known as: 'how to build your own javascript dialect that you won't be able to hire for'
jaywunder 9y ago
The last thing the JS community needs is macros IMO. From what I've seen in other languages, macros are difficult to maintain and document, and they add syntax constructs to languages that aren't supported by code highlighters. The difference between this and babel, is documentation. Babel is working off of a specification of how the language should work, whereas I doubt someone will document their macros as well as TC39 does the JS syntax
- flavio81 9y ago
  Every language benefits from macros, and certainly JS.
lmeyerov 9y ago
We were looking into sweetjs for adding cross-module contracts to the graphistry stack, but nixed the project when it became too much to generate code that passed our basic linters. TS largely scratches that itch, so we've been moving to that. I'm still hopeful though :)
alnitak 9y ago
What would hygienic mean in this context?
- Risord 9y ago
  Not C macro-like source code manipulation as dummy string but as more smart structure like AST.
- girvo 9y ago
  It's to do with how variables leak into scopes from (and to) macros, iirc
devdad 9y ago
Can someone please explain macros, in an easy to grasp way?
- sscarduzio 9y ago
  Macros are a tool given to the user by PL designers to extend the language with new syntax. I.e. a macro program is a piece of code that defines new keywords, operators, etc. and their implementation.
- coltonv 9y ago
  Say you're creating a programming language and you know everyone is going to love it and use it. You add some nice features like if statements, switch statements, and for loops which allow the people that use your language to decide when certain code runs and shortens the syntax for common patterns. Five years down the line your language is losing popularity fast, because all the hip new languages have an "unless" statement which is the opposite of an if statement. They have for-each loops, cond statements, and their if/else statements return values, so you can write less code to do those common patterns in those new languages than your language. You rewind time 5 years using a time machine and this time, when you create your langauge, you add all those cool features, only to realize in another 10 years your language has lost popularity because there's a new set features they want, lambdas, argument piping, etc. So this time when you go back in time, you add macros to your language. Macros are bits of code that allow people using your language to write code that produces more code. Now, you don't even have to write an unless statement, you can let the community write one for you and release it as a package. Anyone at any time can effectively add features to the language in their own project. That sounds fantastic, and it often is, but macros need to not be abused. If a developer writes a bunch of code using lots of his own macros in your language, anyone reading that code later is going to be very confused seeing that many language features that aren't in the language docs, because that random developer wrote them himself. Functions are much more explicit than macros, so if you can write whatever you're trying to write as a function instead of a macro, you'll be better off. For every success story of a language becoming easier to use because of macros, there's another story of a different language becoming frighteningly complicated due to macro abuse.
- 9y ago
lamlam 9y ago
I personally prefer MetaScript [1] as it's comment based so JavaScript highlighters/linters don't have issues with it. It is also essentially JavaScript so no special syntax had to be learned. [1] https://github.com/dcodeIO/MetaScript https://github.com/dcodeIO/MetaScript

76 points by ekzy 9y ago

longlho 9y ago

this is already covered in basically any AST transform systems (babylon, acorn, TS...)

zeckalpha 9y ago
This predates those, and is hygienic.
adgasf 9y ago
I believe that Babel lacks custom infix operators whilst Sweet.js provides them.
- btown 9y ago
  You're absolutely right; Babel's parser is very much hand-coded, and adding syntax requires deep knowledge of the internals. Not impossible, but not for the faint of heart. My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled... https://github.com/babel/babylon/blob/master/src/parser/expression.js#L461 https://github.com/babel/babylon/blob/master/src/parser/expr...
  - scribu 9y ago
    That raises a question: What's stopping Babel from using Sweet.js instead of a custom parser? Are there any transpilers that do use Sweet.js?
    - zeckalpha 9y ago
      Contracts.coffee IIRC, uses Sweet for Contracts.
  - djfm 9y ago
    Well "a.b.c" violates the principle that you should never access a property more than one level deep anyway ;) But in this case it's pretty straightforward to write a function like "get('b', 'c')(a)" or use a lib like lodash [1] [1]: https://lodash.com/docs/#get https://lodash.com/docs/#get
    - btown 9y ago
      The Law of Demeter doesn't always apply neatly, especially when you're introspecting JSON sent over the wire and you don't have a "class" with object oriented behavior for each sub object (at best, they're strongly typed). And the runtime overhead AND verbosity of a function may not be ideal, with the string parsing required in lodash and the awkward syntax of the curried function. Something that compiles natively in a DRY way to a && a.b ... would be used widely imo.
  - chocolateboy 9y ago
    > My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled... Not for long: https://github.com/babel/babel/pull/5813 https://github.com/babel/babel/pull/5813
    - btown 9y ago
      You just made my day :)
andreypopp 9y ago
That's not just "AST transform system", Sweet.js provides a way to extend grammar with new productions. That allows to introduce new syntax constructs.

eatonphil 9y ago

Coming from a Scheme background, if there's one thing I can't believe we live without in Javascript, it's macros. Hell even C has macros... I've looked at Sweet.js a few times to add support for macros to the Linode Manager. Between m4, the gcc preprocessor, and something native, this seemed like the easiest potential sell. However, I've been turned off by how complicated this looks compared to any of the other options and I'm not sure it actually even supports the feature I want: allow some code to show up in "development" mode but for that code to not even exist in the compiled output of "production" mode. (The other feature that's just as important though is being able to wrap shared code up in a single function that gets compiled in at compile time, but I think Sweet.js supports that.)

ykler 9y ago
Who uses m4? It has been around forever, but I have never really seen it used except in autoconf. When I looked at it (a long time ago) it struck me as really tricky and confusing, even more than lisp macros (but maybe on a par with TeX). However, this was just a first impression -- I didn't go too deep.
- chas 9y ago
  I have used it recently. I have a similar attitude towards m4 as shell scripts: they are good for quick hacks, proofs of concept, and very simple systems. When I need to sprinkle just a bit of macros on something, m4 is great. For example, a one-off piece of technical documentation I wrote is markdown interspersed with code examples. I wanted to make sure the code passed its unit tests, so it lives in separate files and I use m4 to include it into the bigger document. This is nicer than catting separate file chunks together because the text can all be contiguous, but if I needed to do anything more sophisticated I would probably switch to purpose-built templating or static site system.
- dflock 9y ago
  I've used it for really simple templating stuff with Dockerfiles - which it works fine for.
- kazinator 9y ago

couchand 9y ago

The in-the-wild code I've seen that uses Sweet.js has been atrocious. Of course I'm not saying that this is necessarily the case. I'd love to see some real world examples whter this is used effectively.

btown 9y ago
Separately, could you point to some examples of the bad Sweet code you've seen? If it's anything like C macro misuse, it'll at the very least be educational to see just how badly one can shoot oneself in the foot.
joshschreuder 9y ago
I must say I don't see the use case... What real world examples do you have?

d--b 9y ago

Also known as: 'how to build your own javascript dialect that you won't be able to hire for'

jaywunder 9y ago

The last thing the JS community needs is macros IMO. From what I've seen in other languages, macros are difficult to maintain and document, and they add syntax constructs to languages that aren't supported by code highlighters. The difference between this and babel, is documentation. Babel is working off of a specification of how the language should work, whereas I doubt someone will document their macros as well as TC39 does the JS syntax

flavio81 9y ago
Every language benefits from macros, and certainly JS.

lmeyerov 9y ago

We were looking into sweetjs for adding cross-module contracts to the graphistry stack, but nixed the project when it became too much to generate code that passed our basic linters. TS largely scratches that itch, so we've been moving to that. I'm still hopeful though :)

alnitak 9y ago

What would hygienic mean in this context?

Risord 9y ago
Not C macro-like source code manipulation as dummy string but as more smart structure like AST.
girvo 9y ago
It's to do with how variables leak into scopes from (and to) macros, iirc

devdad 9y ago

Can someone please explain macros, in an easy to grasp way?

sscarduzio 9y ago
Macros are a tool given to the user by PL designers to extend the language with new syntax. I.e. a macro program is a piece of code that defines new keywords, operators, etc. and their implementation.
coltonv 9y ago
Say you're creating a programming language and you know everyone is going to love it and use it. You add some nice features like if statements, switch statements, and for loops which allow the people that use your language to decide when certain code runs and shortens the syntax for common patterns. Five years down the line your language is losing popularity fast, because all the hip new languages have an "unless" statement which is the opposite of an if statement. They have for-each loops, cond statements, and their if/else statements return values, so you can write less code to do those common patterns in those new languages than your language. You rewind time 5 years using a time machine and this time, when you create your langauge, you add all those cool features, only to realize in another 10 years your language has lost popularity because there's a new set features they want, lambdas, argument piping, etc. So this time when you go back in time, you add macros to your language. Macros are bits of code that allow people using your language to write code that produces more code. Now, you don't even have to write an unless statement, you can let the community write one for you and release it as a package. Anyone at any time can effectively add features to the language in their own project. That sounds fantastic, and it often is, but macros need to not be abused. If a developer writes a bunch of code using lots of his own macros in your language, anyone reading that code later is going to be very confused seeing that many language features that aren't in the language docs, because that random developer wrote them himself. Functions are much more explicit than macros, so if you can write whatever you're trying to write as a function instead of a macro, you'll be better off. For every success story of a language becoming easier to use because of macros, there's another story of a different language becoming frighteningly complicated due to macro abuse.
9y ago

lamlam 9y ago

I personally prefer MetaScript [1] as it's comment based so JavaScript highlighters/linters don't have issues with it. It is also essentially JavaScript so no special syntax had to be learned. [1] https://github.com/dcodeIO/MetaScript https://github.com/dcodeIO/MetaScript