Wrote my first Janet macro today: a verbose version of
if, where you have to explicitly specify the then-case and the else-case. I don't know much about macros yet so if I'm doing something weird let me know!(defmacro ifte
``
ifte = if-then-else
Works just like `if`, but you have to supply `:then` and `:else` keywords for
extra verboseness. This can be helpful when the then-expression of
else-expression is huge and it becomes hard to keep track of things. In
general, you should probably consider creating variables to mitigate that
problem, but sometimes you're better off using `ifte`.
Usage: (ifte true :then true :else false)
``
[if-condition then-keyword then-expression else-keyword else-expression]
(with-syms [$if-condition
$then-keyword
$then-expression
$else-keyword
$else-expression]
~(let [,$if-condition ,if-condition
,$then-keyword ,then-keyword
,$then-expression (when ,$if-condition ,then-expression)
,$else-keyword ,else-keyword
,$else-expression (unless ,$if-condition ,else-expression)]
(assert
(= :then ,$then-keyword)
"The second argument of `ifte` should always be the :then keyword.")
(assert
(= :else ,$else-keyword)
"The fourth argument of `ifte` should always be the :else keyword.")
(if ,$if-condition
,$then-expression
,$else-expression))))
1 reply
(Not a macro thing, but you could also probably make it shorter using
match, with literal keywords in the appropriate positions in the matching pattern)
Is this what you meant?
(defmacro ifte
[if-condition then-keyword then-expression else-keyword else-expression]
(assert
(= :then then-keyword)
"The second argument of `ifte` should always be the :then keyword.")
(assert
(= :else else-keyword)
"The fourth argument of `ifte` should always be the :else keyword.")
(with-syms [$if-condition
$then-expression
$else-expression]
~(let [,$if-condition ,if-condition
,$then-expression (when ,$if-condition ,then-expression)
,$else-expression (unless ,$if-condition ,else-expression)]
(if ,$if-condition
,$then-expression
,$else-expression))))
Hmm I guess the if-condition doesn't have to have a generated symbol either, as it will always be evaluated?
So I guess this is better:
(defmacro ifte
[if-condition then-keyword then-expression else-keyword else-expression]
(assert
(= :then then-keyword)
"The second argument of `ifte` should always be the :then keyword.")
(assert
(= :else else-keyword)
"The fourth argument of `ifte` should always be the :else keyword.")
(with-syms [$then-expression
$else-expression]
~(let [,$then-expression (when ,if-condition ,then-expression)
,$else-expression (unless ,if-condition ,else-expression)]
(if ,if-condition
,$then-expression
,$else-expression))))
Like this:
repl:1:> (defmacro ifte
repl:2:(> ""
repl:3:(> [& args]
repl:4:(> (match args
repl:5:((> @[if-cond :then then-exp :else else-exp] ~(if ,if-cond ,then-exp ,else-exp)
repl:6:((> (error "Expected (ifte <if> :then <then> :else <else>")))
<function ifte>
repl:7:> (ifte true :then (print "ok") :else (/ 0 0))
ok
nil
repl:8:> (ifte false :then (print "ok") :else (/ 0 0))
-nan
repl:9:> (ifte true (print "ok") :else (/ 0 0))
repl:9:1: compile error: error: (macro) Expected (ifte <if> :then <then> :else <else>
in ifte [repl] on line 6, column 2
repl:10:>
You don't have to bind to additional symbols, because
if is also a macro (or a special form, don't remember). So you can feel free sticking whatever you like in the then and else positions of a normal if form, and they won't be automatically evaluated
1 reply
Just as, when you hand-write an
if, those don't get evaluated
repl:10:> (if false (error "oh nooooo!"))
nilrepl:11:> (ifte true :then (print "ok") :else (error "oh noooo!"))
ok
nil
Anyway, can I update all jpm packages with one jpm command?
I didn't know about
match yet, btw. I like it, but it does make the macro its signature less descriptive in this case. 🤔
Ah, I can change the signature and the match expression, of course:
(defmacro ifte
[if-cond then-kw then-exp else-kw else-exp]
(match
@[if-cond then-kw then-exp else-kw else-exp]
@[if-cond :then then-exp :else else-exp]
~(if ,if-cond ,then-exp ,else-exp)
(error "Expected (ifte <if> :then <then> :else <else>"))
Whoa, I didn't know that would result in compile time errors if the wrong keywords are supplied. That's awesome!
Will stop spamming now. Thanks again, Zach Smith (subsetpark)
so an error thrown in a macro must be a compile time error
bakpakin: the latest Janet is not showing error message only the stacktrace I guess after janet-lang/janet@e8c7380 Repro:
> janet -e '(empty? 1)'
in empty? [boot.janet] on line 121, column 50
in _thunk [eval-string] (tailcall) on line 1, column 1
in eval1 [boot.janet] on line 2333, column 16
in run-context [boot.janet] on line 2393, column 13
in eval-string [boot.janet] on line 2432, column 3
in e-switch [boot.janet] (tailcall) on line 3618, column 12
in cli-main [boot.janet] on line 3644, column 13Is it by design? Or is it error on my side?
Hmm that is a bug. There were some changes there to handle a missing status line in some cases but I missed a really obvious case.
I will release a 1.19.2 patch today with a fix
speaking of macros, I am playing with these two:
(defmacro vars [& bindings]
~(upscope
,;(seq [[n v] :in (partition 2 bindings)] (tuple 'var n v))))
(defmacro defs [& bindings]
~(upscope
,;(seq [[n v] :in (partition 2 bindings)] (tuple 'def n v))))How would one increase the value of a field in a 3-dimensional array by one (like the ++ macro does) the janet way?
To give an example I want to increase the 1 to a 2:
@(
@(
@(0 0 0)
@(0 1 0)
@(0 0 0))
@(
@(0 0 0)
@(0 0 0)
@(0 0 0))
)
I may have fudged the indexing a bit with the gets but that's sorta it
```
compile error: expected 2 element tuple for l-value to set
compile error: expected 2 element tuple for l-value to set
oh hmm, what does
(get-in wait, you're using@(instead of@[`?
so...
try put-in
Maybe i should look into making my own
++in macro
I guess it's
put and alike for associative datastructures and set for vars
hmmm, no, set works on keyed data structures per the specials doc
but yes, I sorta would have assumed
(++ (variant-amount-of-gets-here)) would have worked
Ok, this does the trick for me:
(defmacro ++in [variable arr] ~(,put-in ,variable ,arr (+ (,get-in ,variable ,arr) 1)))
I think
update-in is probably the best way to do what you want using standard pieces
1 reply
Relevant to the recurring conversation about more powerful version resolution in jpm, this is apparently all the rage: https://nex3.medium.com/pubgrub-2fb6470504f