syntax taste: use of unquote in macros

syntax taste: use of unquote in macros

[Matt Wette]

Hi All,

I'm not sure if you know about this, but there is a discrepancy in the
way some folks define macros to use unquote (aka ,).   For example,

 > (use-modules (system base pmatch))
 > (pmatch '(foo "bar")  ((foo ,val)  (write val) (newline)))
=> "bar"

 > (use-modules (ice-9 match))
 > (match '(foo "bar")  (`(foo ,val)  (write val) (newline)))
=> "bar"

Note the difference in the use of quasiquote (aka `) in the pattern
for (foo ,val): match syntax uses it, pmatch does not.
In Scheme, quasiquote and unquote always come together.

Is pmatch syntax in bad taste?  I'm looking for opinions.

Another example is sxml-match, which omits use of quasiquote.
I have written a variation of sxml-match.  Should I keep the sxml-match
usage, which keeps it compatible with sxml-match,  or adopt that
used by (ice-9 match)?

Matt

Re: syntax taste: use of unquote in macros

[Zelphir Kaltstahl]

On 3/29/20 5:11 PM, Matt Wette wrote:
> Hi All,
>
> I'm not sure if you know about this, but there is a discrepancy in the
> way some folks define macros to use unquote (aka ,).   For example,
>
> > (use-modules (system base pmatch))
> > (pmatch '(foo "bar")  ((foo ,val)  (write val) (newline)))
> => "bar"
>
> > (use-modules (ice-9 match))
> > (match '(foo "bar")  (`(foo ,val)  (write val) (newline)))
> => "bar"
>
> Note the difference in the use of quasiquote (aka `) in the pattern
> for (foo ,val): match syntax uses it, pmatch does not.
> In Scheme, quasiquote and unquote always come together.
>
> Is pmatch syntax in bad taste?  I'm looking for opinions.
>
> Another example is sxml-match, which omits use of quasiquote.
> I have written a variation of sxml-match.  Should I keep the sxml-match
> usage, which keeps it compatible with sxml-match,  or adopt that
> used by (ice-9 match)?
>
> Matt

Hi Matt!

I'm not sure where I first read about pmatch doing the quasiquote
internally automatically and I of course had already forgotten about it,
until I read your e-mail. I remember though: When I read about it, I
thought something like: "Ehhh? That's confusing. Everywhere else you
have to quasiquote to unquote, but in pmatch it's different. Easy to
forget that and I need to remember more. I would not mind having to
write a simple quasiquote myself."

So, with my limited experience, I would definitely vote for the, in my
opinion more natural way, to have the user write the quasiquote, instead
of doing that internally. It's not saving that much time or writing
really. I understand the idea, that "if it is always done anyway, one
could save the user some typing of the same thing over and over again",
but in my opinion it is not worth it to deviate from, what one would
expect without knowing about this behavior.

Regards,
Zelphir

Re: syntax taste: use of unquote in macros

[tomas]

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On Mon, Mar 30, 2020 at 12:07:07AM +0200, Zelphir Kaltstahl wrote:
> 
> On 3/29/20 5:11 PM, Matt Wette wrote:
> > Hi All,
> >
> > I'm not sure if you know about this, but there is a discrepancy in the
> > way some folks define macros to use unquote (aka ,).   For example,

[pmatch vs. match]

> I'm not sure where I first read about pmatch doing the quasiquote
> internally automatically and I of course had already forgotten about it,

[...]

I didn't even know about `pmatch'. Is it supposed to be equivalent
to `match' (except that outer quasiquote, that is)?

With the caveat that I don't have much of a stylistic feeling for
Scheme, I'd clearly prefer `match': the writer has the choice of
quote or quasiquote, as appropriate to the case -- and the reader
sees what's going on (after all, quote and quasiquote are "low
weight" primitives: everyone more or less knows what they do.

But perhaps I misunderstood what you're after?

Cheers
-- tomás

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Re: syntax taste: use of unquote in macros

[Matt Wette]

On 3/30/20 3:34 AM, tomas@tuxteam.de wrote:
> On Mon, Mar 30, 2020 at 12:07:07AM +0200, Zelphir Kaltstahl wrote:
>> On 3/29/20 5:11 PM, Matt Wette wrote:
>>> Hi All,
>>>
>>> I'm not sure if you know about this, but there is a discrepancy in the
>>> way some folks define macros to use unquote (aka ,).   For example,
> [pmatch vs. match]
>
>> I'm not sure where I first read about pmatch doing the quasiquote
>> internally automatically and I of course had already forgotten about it,
> [...]
>
> I didn't even know about `pmatch'. Is it supposed to be equivalent
> to `match' (except that outer quasiquote, that is)?
>
> With the caveat that I don't have much of a stylistic feeling for
> Scheme, I'd clearly prefer `match': the writer has the choice of
> quote or quasiquote, as appropriate to the case -- and the reader
> sees what's going on (after all, quote and quasiquote are "low
> weight" primitives: everyone more or less knows what they do.
>
> But perhaps I misunderstood what you're after?
>
> Cheers
> -- tomás

The other thing about match that is attractive is that use of
quasiquote+unquote is "reflexive" (or maybe self-adjoint?)
in the following sense:

'(foo "bar") => (match-lambda (`(foo ,val) `(foo ,val))) => '(foo "bar")

Re: syntax taste: use of unquote in macros

[tomas]

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On Mon, Mar 30, 2020 at 04:43:47PM -0700, Matt Wette wrote:

[...]

> The other thing about match that is attractive is that use of
> quasiquote+unquote is "reflexive" (or maybe self-adjoint?)
> in the following sense:
> 
> '(foo "bar") => (match-lambda (`(foo ,val) `(foo ,val))) => '(foo "bar")

Sorry, my head just exploded :-)

Will take it more slowly later today. Thanks for the riddle.

Cheers
-- t

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Re: syntax taste: use of unquote in macros

[Arne Babenhauserheide]

Matt Wette <matt.wette@gmail.com> writes:

> Hi All,
>
> I'm not sure if you know about this, but there is a discrepancy in the
> way some folks define macros to use unquote (aka ,).   For example,
>
>> (use-modules (system base pmatch))
>> (pmatch '(foo "bar")  ((foo ,val)  (write val) (newline)))
> => "bar"
>
>> (use-modules (ice-9 match))
>> (match '(foo "bar")  (`(foo ,val)  (write val) (newline)))
> => "bar"
>
> Note the difference in the use of quasiquote (aka `) in the pattern
> for (foo ,val): match syntax uses it, pmatch does not.
> In Scheme, quasiquote and unquote always come together.
>
> Is pmatch syntax in bad taste?  I'm looking for opinions.

I did not know about pmatch, but it gives me the feeling that this is a
pattern that will appear again for other embedded domain specific
languages.

I had a usecase for that in dryads-wake¹ for embedding instructions in
otherwise declarative code. It looks like this:

(Enter (Speaker))
(Speaker
    (My Text, line 1)
    (Second line)
    (very ,(color 'red) important ,(color #f) line))

At this point the difference to quasiquote doesn’t look big. It gets
much bigger when this code is used from wisp:²

Enter : Speaker
Speaker
    My Text, line 1
    Second line
    very ,(color 'red) important ,(color #f) line

With written quasiquote there would either be line-noise on every line
or this would have structure-noise because it would have to be wrapped
in a list instead of working like arguments to a procedure call:

(Enter (Speaker))
(Speaker
  `((My Text, line 1)
    (Second line)
    (very ,(color 'red) important ,(color #f) line)))

or

(Enter (Speaker))
(Speaker
    `(My Text, line 1)
    `(Second line)
    `(very ,(color 'red) important ,(color #f) line))

Best wishes,
Arne

¹: https://hg.sr.ht/~arnebab/dryads-wake/browse/default/dryads-wake.w#L231
²: https://www.draketo.de/english/wisp
-- 
Unpolitisch sein
heißt politisch sein
ohne es zu merken

Re: syntax taste: use of unquote in macros

[Ludovic Courtès]

Hi Matt,

Matt Wette <matt.wette@gmail.com> skribis:

> I'm not sure if you know about this, but there is a discrepancy in the
> way some folks define macros to use unquote (aka ,).   For example,
>
>> (use-modules (system base pmatch))
>> (pmatch '(foo "bar")  ((foo ,val)  (write val) (newline)))
> => "bar"
>
>> (use-modules (ice-9 match))
>> (match '(foo "bar")  (`(foo ,val)  (write val) (newline)))
> => "bar"
>
> Note the difference in the use of quasiquote (aka `) in the pattern
> for (foo ,val): match syntax uses it, pmatch does not.
> In Scheme, quasiquote and unquote always come together.
>
> Is pmatch syntax in bad taste?  I'm looking for opinions.

It really depends on what you’re going to use the pattern matcher for.
For ‘sxml-match’, it’s more convenient to have literals be the default
because there are usually more literals than variables, as in:

       (sxml-match x
         ((album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...)
          `(ul (li ,t)
               (li (b ,n) (i ,f)) ...)))

In more general cases, I prefer the (ice-9 match) style because patterns
typically have more variables than literals.

In one case, I found myself implementing pmatch-style quoting on top of
(ice-9 match) so I would have the best of both worlds:

  https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/installer/tests.scm#n84

:-)

Ludo’.