On Sat, Mar 24, 2012 at 1:28 AM, Sean Corfield <seancorfi...@gmail.com> wrote:
> On Fri, Mar 23, 2012 at 8:44 PM, Cedric Greevey <cgree...@gmail.com> wrote:
>> #{foo bar baz} is somewhat ugly. It occurs to me that one could modify
>> the reader to additionally accept
>>
>> {{foo bar baz}}
>
> My concern is that {{1 2 3 4}5} is currently legal - a map with a map
> as a key and 5 as the value. That means that the reader couldn't tell
> if {{ introduced your set notation or the start of a nested map
> without looking arbitrarily far ahead. And that problem gets worse if
> you encounter {{{ or {{{{ or... {{{ could introduce a nested map, a
> map of your sets or one of your sets containing a map...
The other objections are fair enough -- matters of taste. But the
above is a technical problem with a simple fix: make one pass over the
source, converting it into a rudimentary AST whose nodes just look
like (thing thing thing), #(thing thing thing), [thing thing thing],
{thing thing thing}, and #{thing thing thing} (the file as a whole can
be regarded as implicitly wrapped in (do ... )); then (assuming no
unbalanced delimiters) make a second pass doing the rest of the
reader's job. The second pass will see a {...} node with one direct
child that is also a {...} node in, and only in, the case of the
proposed set syntax. (It will also allow { {foo bar baz}} and
similarly as sets.)
As for the aesthetics, what I like about {{...}} is that the
delimiters are symmetrical, unlike #{...}, and it would allow one to
reserve use of the # mark to, mostly, closures, so # would stand out
more as typically indicating a lambda. (The #{...} syntax then remains
comparatively desirable in the specific case that the set literal is
being used as a predicate, as it's both a set and a lambda in that
context.)
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