[Haskell-cafe] Fixity parsing, Template Haskell

John A. De Goes john at n-brain.net
Sat Nov 22 16:23:16 EST 2008

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.

In this case, the most important thing is following the principle of  
least surprise: A Haskell expression inside a quasiquote should work  
the same as a Haskell expression outside a quasiquote.

Violating the "principle of least surprise" is one of the most  
grievous mistakes language (and interface) designers make.


John A. De Goes
[n minds are better than n-1]

On Nov 22, 2008, at 9:02 AM, Reiner Pope wrote:

> It seems to me that fixity information behaves more like semantics
> than like syntax. For instance, fixities may be imported, and obey
> namespacing rules. Knowing and correctly handling these rules seems
> beyond the scope of a mere parser: I would hope that a single Haskell
> file could be parsed without reference to any files, and fixity
> declarations seem to be just about the only thing which prevent this
> -- hence my suggestion to change the AST in order to regain this
> property.
> The use I envision of it is as I described: writing a quasiquoter
> using HSE to parse the user's Haskell expressions. The problem is
> that, for such a case, HSE (or any other parser) is forced to parse
> infix expressions for which it cannot possibly know the correct
> fixities. Any result with more information than the list form I gave
> would be a lie.
> I realise that I don't know how fixities are implemented in Haskell
> compilers, so perhaps I'm misunderstanding how they are treated.
> Cheers,
> Reiner
> On Sat, Nov 22, 2008 at 11:54 PM, Niklas Broberg
> <niklas.broberg at gmail.com> wrote:
>>> Of course, this would require a change to Template Haskell, so a
>>> second-best solution would be to forbid unparenthesised  
>>> expressions in
>>> my quasiquoter. Then, parsing can proceed correctly without knowing
>>> the fixities. This would be easiest to do if haskell-src-exts  
>>> changed
>>> its AST in a similar way to described above for Template Haskell.
>> I'm not sure I follow you here. In what way would it be simpler if  
>> HSE
>> changes its AST to a less-information constructor? I won't do that,
>> for the same reason you point out with TH and disadvantages when  
>> using
>> it as a library, though I'm still curious what uses you envision and
>> how it would be made easier. I'm still in the process of designing  
>> the
>> fixity support for HSE, and all input is valuable. :-)
>> Cheers,
>> /Niklas
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