[plt-dev] #hash vs. #s

At Fri, 6 Feb 2009 17:47:23 -0500, Ryan Culpepper wrote:
> 
> On Feb 6, 2009, at 5:32 PM, Matthew Flatt wrote:
> 
> > At Fri, 06 Feb 2009 14:10:54 -0500, Dave Herman wrote:
> >>> Unquote for the value part of a `#hash' could work. I'm less sure  
> >>> that
> >>> it's a good idea for the key position, since the reader constructs a
> >>> hash table.
> >>
> >> Good point. For example:
> >>
> >>     (define table
> >>       `#hash((,(read) . foo)
> >>              (,(read) . bar)
> >>              (,(read) . baz)))
> >>
> >> The reader constructs a hash with a single entry whose key is the
> >> S-expression (unquote (read)), and it's too late for `qq' to  
> >> reconstruct
> >> the source.
> >>
> >>> Aside from the possible constraint on keys, it's not so much a  
> >>> reader
> >>> issue as a question of what the `quasiquote' macro recognizes. The
> >>> source of `quasiquote' is "collects/scheme/qq-and-or.ss".
> >>
> >> I've mostly implemented this on my machine. I can submit a patch, but
> >> which behavior do you prefer?
> >>
> >> 1) qq leaves the key position quoted and interprets the value  
> >> position.
> >> 2) qq errors on any occurrence of `unquote' in a key position.
> >> 3) scrap this whole extension and leave `qq' as is.
> >
> > I like 2. It seems like a sensible and useful extension. The change
> > could potentially break some old code, but that seems unlikely.

Oops --- I meant to vote for "1", not "2".

> Doesn't that mean that some hash tables that should have literal  
> representations won't?

Yes, at least in terms of quasiquote. That's a good reason against 2.