Building a tiny little broken calculator with parser combinators(blog.jfo.click)

37 pointsby todsacerdoti4 anni fa8 comments

blog.jfo.click

Building a tiny little broken calculator with parser combinators

https://blog.jfo.click/building-a-tiny-little-broken-calculator-with-parser-combinators/

8 comments

jakear4 anni fa

Those interested may find Pratt parsing compelling, it is easy to implement and works well for hobby parsers (production quality error messaging tends to require custom code). As an example here’s a quick and dirty expression evaluator I made a whole back that supports D&D-style die roll codes as literals/first class variables: https://github.com/JacksonKearl/RollDice-Discord

http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-e...

https://github.com/JacksonKearl/PrattParse

userbinator4 anni fa

I believe Pratt, TDOP, and precedence climbing are all just different names for the same technique that involves comparing precedences in a loop. The fact that it's basic principle is so simple is probably what lead to multiple independent reinventions.

If anything, after reading about the aforementioned algorithms, the technique in the article seems to show that using parser combinators is a lot of work for very little gain --- I've read much shorter articles on recursive descent, and those do handle precedence correctly.

jakear4 anni fa

What is your complaint re precedence in Pratt parsing? The linked code properly implements precedence fairly trivially.

userbinator4 anni fa

I think you misunderstood. I'm referring to the original article of this item, not your linked code.

jakear4 anni fa

That’s more a critique of the author than the concept - I gave up midway through the article but order combinators remain a useful tool, particularly as a library. To my eyes, the logic here https://github.com/JacksonKearl/RollDice-Discord/blob/master... is much simpler than the equivalent recursive descent code would be. I’d be interested in an article comparing them in greater detail.

lioeters4 anni fa

I'm a fan of Pratt parsers too. In addition to the article by @munificent you linked above, I found the following helpful.

- How Desmos Uses Pratt Parsers (2018) - TypeScript - https://engineering.desmos.com/articles/pratt-parser/ - Source: https://github.com/desmosinc/pratt-parser-blog-code

- Simple But Powerful Pratt Parsing (2020) - Rust - https://matklad.github.io/2020/04/13/simple-but-powerful-pra... - Source: https://github.com/matklad/minipratt

21434 anni fa

To the original author:

Under "Parser Generator":

```

   const parseChar = char =>
  input => [
    input[0] === char,
    input.slice(1, input.length)
  ];

   const parseA = parseChar('A');
   const parseA = parseChar('B');

   parseA(testString) // [ true, 'bcd123' ]
   parseB(testString) // [ false, 'Abcd123' ]

```

There seems to be a typo: the 2nd "parseA" should really be a "parseB"?

jfo4 anni fa

Thanks, I have fixed it