[Haskell-cafe] Automatic fixity allocation for symbolic operators

Henning Thielemann lemming at henning-thielemann.de
Mon Oct 16 06:20:00 EDT 2006

On Sat, 14 Oct 2006, Jim Apple wrote:

> On 10/14/06, Brian Hulley <brianh at metamilk.com> wrote:
> > User defined fixities are an enormous problem for
> > an interactive editor
> This is the second or third time you've proposed a language change
> based on the editor you're writing. I don't think this is a fruitful
> avenue.

I assume that editor developers, compiler writers and language tool
writers (documentation extraction, source code formatting) get much more
insight into syntactic issues than other users. Finding some problem when
implementing one of these tools often reveals weak points in the language
syntax. I repeat my example of a source code formatting tool which must
decide whether to format

 a +
   b * c


 a + b *

 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. 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!
 You may argue that difficult language syntaxes like that of C++ push the
parser technique forward. However this seems to me like Windows pushes
memory development.
 Concerning the automatic precedence assigment according to the characters
in an infix operator, I think that it is difficult to find a reasonable
algorithm, because that algorithm would also limit the kind of operator
schemes that can be used. If the operations can be associated with basic
mathematical operations like +, *, =, then it would work, but what about
different structures? How would you translate lattice operations "up" and
"down", how operations like "parallel" and "serial" composition? Summed
up, I think infix handling must be somehow improved, but I don't know how.

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