[Haskell-cafe] Automatic fixity allocation for symbolic operators

Arie Peterson ariep at xs4all.nl
Mon Oct 16 08:12:51 EDT 2006


Henning Thielemann wrote:

> [...] I repeat my example of a source code formatting tool which must
> decide whether to format
>  a +
>    b * c
> or
>  a + b *
>    c
>  It needs to know the precedences of the used operators, which, as Brian
> pointed out, is possibly not even defined somewhere. Alternatively
> consider a compiler which must have a parser that must adapt the grammar
> to the module contents (infix statements) in order to parse the module
> correctly.  Even worse: The same symbol can have different precedences,
> because infix operators can be declared locally.

I would suggest to first parse modulo fixity and precedence (keep all
operator applications in flat lists) and only later, if/when precedence
and fixity information is available, parse those operator lists.

>                                                  The same problem for a
> human: In order to analyse the meaning of an expression with infix
> operators he must know the precedences. That is, problems with infix
> operators are by far not bound to a text editor!

Quite so. I wouldn't consider it a problem though. The meaning of the
expression also depends on the meaning (definition) of the operators
involved, and of other functions used in the expression. That information
may also be located elsewhere or even not yet available, just as
precedence declarations. Locality has been wittingly sacrificed for




Mr. Pelican Shit may be Willy.


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