Composition again

[Simon Marlow]

Simon Marlow wrote:

> Here are the possibilities for composition:
> 
> 0. do nothing
> 1. use a Unicode operator for composition
> 2. require spaces around . as an operator
> 3. require spaces around all operators
> 4. use another ASCII operator for composition, e.g. <<<
> 
> Nothing has been decided yet, but most of the committee tends to favour 
> (2), with some expressing a slight preference for (0).  We've pretty 
> much ruled out (1) and (3) as too radical, and as you say using Unicode 
> is still too impractical.
> 
> There is some uncertainty about the precise details of (2), and thinking 
> about that is what lead to my proposal about changing the syntax of 
> qualified operators.

Ok, I'm going to try to make some progress on this.  I think it's fair 
to say that the only possible options are (0) do nothing, or (2) require 
spaces around "." as an operator.

I'll explore how we might do (2).  We discussed this a bit on the 
committee mailing list, here's where we got to.  The proposal amounts to 
making certain sequences illegal.  Let's try to implement the informal rule

   dot is only allowed adjacent to a varid or conid when
   it is part of a qualified name

so that would make these illegal:

  length.lines
  (.f)
  (f.)
  ++.x
  x.++
  Just.(+1)

Here's an interesting case:

  [Monday..Friday]

which I imagine we would like to be legal. (currently it's illegal, or 
rather 'Monday..' is interpreted as a qualified operator; see separate 
proposal for changing the syntax of qualified operators).

The way to implement this is to add the following productions to the 
lexical syntax:

reservedlexeme ::= ({symbol} '.')<reservedop> (varid | conid)
                  | (varid | conid) ('.' {symbol})<reservedop>

The idea is that reservedlexemes are illegal, and work via the 
maximal-munch rule.  (note that this version relies on the alternate 
qualified operator syntax proposal, because it makes the existing 
qualified operator syntax into a reservedlexeme).

My personal view is that this is somewhat ugly.  However, it does have 
the benefit of releasing the 'foo.x' syntax for future use, e.g. in a 
new record system - but some people find that a bit tasteless too, and I 
might be inclined to agree.

Thoughts?

Cheers,
	Simon