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Thanks, Aaron! Following your notes, I corrected the path to:
\path #0.3 #`((moveto ,xoff ,yoff)
(lineto ,(+ 2 xoff) ,(+ 4 yoff)) (moveto ,(+ 1 xoff) ,yoff)
(lineto ,(+ 3 xoff) ,(+ 4 yoff)))
and now it works like a charm. And that Scheme intro book looks
like exactly what I need to help head off similar confusions in
the future. "Relatively new to the language" is spot-on for me
:-)
- Mike
On 12/22/2018 3:58 AM, Aaron Hill
wrote:
On
2018-12-21 8:15 pm, Mike Stickles wrote:
But when I try to implement the numbers, I
get errors no matter what I
do. This (while it doesn't work) shows what I'm trying to get
to:
#(define-markup-command (double-box layout props xoff yoff)
(number? number?)
(interpret-markup layout props
#{
\markup {
\with-dimensions #'(0 . 0) #'(0 . 0)
\path #0.3 #'((moveto xoff yoff) (lineto (+ xoff 2)
(+
yoff 4)) (moveto (+ xoff 1) yoff) (lineto (+ xoff 3) (+ yoff
4)))
}
#}
))
Any ideas on what I'm doing wrong?
You are hitting an common stumbling block in Scheme regarding
quoting. Urs has a great Scheme introduction book[1] online that
would be good reviewing as it sounds like you may be relatively
new to the language.
[1]: https://scheme-book.ursliska.de/
\path needs a list of commands, where an individual command
consists of a symbol (defining the particular command) and then
its arguments, which are typically just numbers.
To construct a suitable \path argument in Scheme, we use the list
function:
(list ((quote moveto) 1 2) ((quote lineto) 3 4))
This is the explicit list construction technique, and we are also
using the explicit invocation of quote. We need quote here
because "moveto" and "lineto" are symbols. We do not want the
value behind the symbols, just the symbols as things on their own.
Scheme (technically LISP) developed a number of shorthands for
common constructions. You can construct a list more succinctly
this way:
'((moveto 1 2) (lineto 3 4))
The leading quote puts us in quote mode so that we can simply type
"moveto" by itself. We also no longer need to say list
explicitly, as we'll end up with a list. The numbers are
technically being quoted here, but a quoted number literal works.
But what if we need a variable? We cannot use the same
construction, because our variables will end up quoted rather than
using the value behind the name. One solution is to go back to
the more explicit invocation:
(list ('moveto a b) ('lineto c d))
Here we are still using the shortcut quote for the symbols, but
everything else will be resolved properly in this form. This is a
perfectly acceptable option, but some folks prefer the shorthand
of quoting. The alternate solution is quasi-quoting:
`((moveto ,a ,b) (lineto ,c ,d))
This one looks funny at first, but the difference is the use of
the grave as opposed to the apostrophe. This sets up a
quasi-quoting mode that behaves nearly identical to normal
quoting, except we can use a comma to "unquote". And here the
variables will be evaluated as expected.
-- Aaron Hill
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