Hi Phil,
It's a good question.
Syntax quote is designed for generating code/syntax that is deterministic
and avoids accidental local name capture.
1. Automatic namespace qualification helps avoid accidental local name
capture in macros.
user=> `(let [a# 1
b# a]
a#)
(clojure.core/let [a__48963__auto__ 1
b__48964__auto__ user/a]
a__48963__auto__)
The binding for b# shows a common mistake. By namespace qualifying "a" to
"user/a",
we don't allow the possibility of accidentally capturing locals.
eg. "a" is not captured in the body
(let [a 'evil]
(clojure.core/let [a__48963__auto__ 1
b__48964__auto__ user/a]
a__48963__auto__))
2. Automatic namespace qualification helps make code deterministic with
respect to the namespace
of the consumer of a macro.
Syntax quote resolves vars in the namespace the *macro* is defined in. This
avoids odd situations where
different vars are invoked depending on which namespace we use the macro.
Bottom line: syntax quote is designed for macros by default. All the good
properties of syntax quote can be avoided using
combinations of quote, syntax quote and splice.
eg. `~'a
;=> a
Thanks,
Ambrose
On Thu, Jun 20, 2013 at 3:49 PM, Phillip Lord
<[email protected]>wrote:
> Jim <[email protected]> writes:
> > On 19/06/13 09:46, Phillip Lord wrote:
> >> With the ' paul is not namespace qualified, while with the ` paul is.
> >>
> >>
> >> Turns out to be a bit of a pain, actually, although I have worked around
> >> it. But mostly I am surprised. Is this expected?
> >
> > Yes, this is very much expected! :)
> >
> > How else would you be able to write macros without ` ?
>
> Well, that doesn't answer the question. The backtick is useful in
> writing macros (although the situation this caused me grief, actually, I
> wasn't). So is quote. But then I do not understand why the behaviour is
> different between the two.
>
> > Vars needs to resolve to something eventually...
>
> There are no vars involved here. `paul expands to a namespace qualified
> symbol irrespective of the existence of a var.
>
> Phil
>
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