Comment by lexi-lambda
6 years ago
I think this is a reasonable point, and your split() example is a good one. I wasn’t sure while I was writing the blog post if I considered isomorphisms to be parsers, and I’m still not entirely sure if I do or not, but there’s an argument to be made that they are simply parsers where the set of possible failures is empty. I don’t have strong feelings about that distinction one way or the other, though.
Further nit-picking: even if there are no errors, I don't think it's an isomorphism unless the parser preserves all information? Usually parsers treat some input strings as equivalent (by stripping whitespace and comments), so there are fewer outputs than valid inputs.
I can't think of an easy way to make string splitting an isomorphism in both directions. You can serialize any list of strings to a comma-separated list, but only if you use escape sequences or encode the length of each item, and then there are some input strings that are errors.
I guess you could define a type as the set of all valid strings in the input language, but that's going out of your way to make sure parsing has to be done more than once.
Parsers that report no errors don't seem desirable anyway; error-checking is usually good.
Some other random feedback: the short phrase makes me think that I should parse instead of validating. Might I suggest: "Don't just validate: parse"?
The idea really is that parsing subsumes validation. If you’re parsing, you don’t have to validate, because validation is a natural side-effect of the parsing process. And indeed, I think that’s the very thesis of the blog post: parsing is validation, just without throwing away all the information you learned once you’ve finished!
I understood the article, and I see your point, but my main feedback is that when I read the title I assumed that you were going to describe some way where you would parse input without validating it.
3 replies →
Structure is orthogonal to partiality.
Partiality is what you get when you choose you only accept one branch of a disjunctive structure.