Re: [Haskell-cafe] Fixity parsing, Template Haskell

On Sun, Nov 23, 2008 at 8:23 AM, John A. De Goes wrote:

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Though many see it as "losing" information, I agree wholeheartedly with your proposal to change the AST.

It's better to have an AST that conveys less information, but truthfully, than to have an AST that purports to convey more information, when in fact that information is false.

In most languages, some things just can't be known at parse time. They need to be resolved later.

While I agree with you in principle, that the rule of least surprise should be adhered to, I don't agree in this instance. You are assuming that the AST is only used by the parser, and if that was true I would agree with you. But HSE is just as much a library for *building* an AST using combinators. If I changed the AST to one carrying less information, it would be impossible to *ever* get the fixities right in the AST, even for those use cases where the information is known and possible to use.

I want this information to be used somehow when creating the Template Haskell AST, so that the operators used have the correct fixities. If I use HSE for parsing Haskell expressions, then I want it to tell me where it unsure of the fixities, so that I can insert the correct ones there. For this use case, I would want HSE to use an AST like I suggested, because it allows the parser to say, "I'm not sure what the correct fixity is".

As noted above, I really don't like that change. If you use HSE for parsing expressions, it would *never* know the correct fixities, and you would always get a completely left-biased tree. Why would that be harder to work with? I understand the argument about least surprise, and that this feature must be strongly documented (which it currently isn't), but for practical purposes I don't see why the current state would be problematic. It should even be trivial to convert the left-biased tree into an intermediate structure exactly like the one you suggest, no? Cheers, /Niklas