On Dec 17, 4:30 am, Meikel Brandmeyer <m...@kotka.de> wrote:
> Hi,
>
> I'd like to propose/put up for discussion a change to the unquote
> handling.
> I basically ran into the following problem:
>
> I'd like to embed a DSL into my system. Using macros it should be
> possible
> to define a language, which looks like usual Clojure. Simply quote the
> form
> and pass it to a function, which knows how to handle the commands.
>
> However I hit a roadblock. Coming from Scheme, I tried the following
> approach.
>
> A hypothetical SQL embedding:
>
> (def x 5)
> (sql col1 from table1 where col2 = ~x)
>
> Or a hypothetical cljsh (inspired by scsh):
>
> (def *cmd* "/usr/bin/command")
> (run (~*cmd* -option 1 -another argument foo bar))
>
> In Scheme this is done, by quoting the macros argument using the
> quasiquote form. In Clojure, however, this style is not possible since
> we are
> missing quasiquote. Here a short list of examples on how the different
> quotations work:
>
> (quote (foo ~x bar))
> => Failure in Clojure.
> => In some Schemes: (foo (unquote x) bar), where , (aka. ~) translates
> to some unquote form. It just quotes everything. Also the unquote.
>
> (quasiquote (foo ~x bar))
> => Not available in Clojure.
> => (foo 5 bar) in Scheme
>
> (syntax-quote (foo ~x bar))
> => (assuming ` would have a syntax-quote form as ' does with quote)
> (my.ns/foo 5 maybe.some.other.ns/bar)
> => Not available in Scheme, AFAIK.
>
> So as you see, the above style is not possible, since in Clojure you
> only
> have the choice to be static or resolve all the symbols. If I want to
> use
> the style anyway, I have to jump through hoops and rewrite the quoted
> form in my handler function, be it replacing somehow marked or named
> symbols with values from the environment or be it renormalising all
> the
> symbols. (But what if the user injected a qualified symbol with ~?)
>
> So I came to the question of whether it is possible to include a
> similar
> quasiquote form, which does not resolve the symbols inside? It would
> be a philosophy of "Unquote, what the user wants to be unquoted, and
> leave the rest to me. I know how to handle it."
>
> And when were are at it: maybe also provide a syntax-quote, unquote
> and unquote-splice forms?
>
> Finally, I skipped the obvious question: Does this make sense at all?
> I like this way of including a sub-language. However, is it
> "idiomatic"?
> (for some definition of "idiomatic")
>
> Maybe there is already a different solution, which I just don't see at
> the moment.
>
Moving syntax-quote out of the reader might be a big deal. But I think
scenarios like this could be covered if:
~x not in a syntax-quote yielded the form (unquote x) from the reader
unquote would not be defined by Clojure, so still an error if allowed
to get evaluated.
Things like your sql would be macros that handled (unquote x)
internally.
Rich
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