Austin Fatheree, January 19 2022
This bounty gives the opportunity to
Prettier is an opinionated code formatter used by CICD, coding IDEs, and other tools to help make code easier to read.
We have some source material that may help you out. Forum users and current/past DFINITY employees have weighed in and worked on a similar system at points in the past.
The function you need to expose is likely here: https://github.com/dfinity/motoko/blob/d8e13023dfdc965907512432c0181c8d137d84ad/src/pipeline/pipeline.ml#L118(Thanks @PaulYoung). This will need to be converted to return an object instead of a filename, so you may have to do some oCaml or solicit the help of someone who does.
A few important comments:
I never used prettier before, but from a cursory look, this path feels a lot of work.
The general idea here is to be able to parse some input and then print it out exactly the same.
This is ultimately the approach taken in PureScript after wanting a formatting tool for a long time. The links below contain some good info on motivation and should provide some insight.
Perhaps a CST could be implemented using Tree Sitter via the OCaml bindings. That way, other languages can make use of it as well.
FWIW, the Motoko parser gives both full position info (in the AST nodes) as well as lexical comments and even white space (in the trivia table).
That said, it’s probably not a good idea to interface the compiler’s parser and AST. They are implementation details of the compiler, not public APIs. As such, they can change with any release, and have done so frequently in the past.
The Motoko grammar actually is deliberately simple (as can be witnessed by its semicolon rules :slight_smile: ), and the parser is generated with (ocaml)yacc. It would be quite straightforward to take the yacc grammar and convert it to C-yacc or another parser generator of your choosing.
Pretty-printing an AST is not difficult either. Moreover, Motoko’s syntax has been designed to allow reasonable simple formatting conventions (some of them codified in the style guide).
So, a dev proficient in OCaml could probably hack both up in a day or two.
However, as @chenyan pointed out, what’s hard is layout of non-AST elements like comments. That’s been what kept the Motoko team from providing a moformat so far. I’d estimate that being 90% of the actual work, and it has very little to do with parsing.
Another complication would be formatting of code fragments instead of just entire files. That’s something you’d want in an IDE. For that, you first need to recognise what kind of syntactic context you’re in.
Produce a Prettier Plug-in that formats motoko code using best practices.
Produce a prettier printer that outputs the code in a best-practices format. What are the best practices? You should work with the motoko community to decide these. We’d recommend using the forum post for this bounty to discuss what the formatting should look like.
Finally, create a UI canister that takes motoko code in one code editor window and renders it in a mirrored window with the prettier formatter applied to it. We suggest using the same monaco editor that Motoko Playground uses as you may be able to find a good bit of helper code in the motoko playground repo.
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DFINITY Foundation Grant: - $3750 USD of ICP at award date Anonymous - 2 ICP -> $250 USD of ICP at award date