Clojurians Log v2

I have some comments to say about &form and &env. Is there a place in the clojure docs wiki Is the best place here? https://clojuredocs.org/clojure.core/defmacro

Sounds like a good place to me

I don’t know the clojure optional argument syntax very well. Is there a way to pass an optional argument to a macro? For example my macro is defined something like this:

(defmacro rte-case [sequence & clauses] ...)

`(rte-case :special-argument {:blah 1 :foo 2} ~sequence ~@clauses)

I’m looking for something like

(defmacro rte-case [sequence &key special-argument & clauses] ...)

There’s something similar in the sh function which special cases the keywords at the end of the argument list.

(println (sh "ls" "-l"))
(println (sh "ls" "-l" "/no-such-thing"))
(println (sh "sed" "s/[aeiou]/oo/g" :in "hello there\n"))
(println (sh "sed" "s/[aeiou]/oo/g" :in (java.io.StringReader. "hello there\n")))
(println (sh "cat" :in "x\u25bax\n"))
(println (sh "echo" "x\u25bax"))
(println (sh "echo" "x\u25bax" :out-enc "ISO-8859-1")) ; reads 4 single-byte chars
(println (sh "cat" "myimage.png" :out-enc :bytes)) ; reads binary file into bytes[]
(println (sh "cmd" "/c dir 1>&2"))

These are typically called kwargs if that helps searching. Use & clauses in the param list. clauses will be a sequence but you can destructure it as if it is a map

will I be able to use the call-site syntax: (foo :key1 100 :key2 200 clause1 clause2 clause3) Or does the call-site have to consist totally of keyword/value pairs after the first keyword is given?

It should consist of key value pairs

I’m trying to figure out how to do the following. I have one macro which in its expansion includes another macro call. in the inner macro I need to emit an error message. I could make a much better error message if I had access to the &body of the parent macro expansion. In this example there is an &body of destructuring case which has line-number information about the call-site. However, -destructuring-fn-many is also a macro, and it would like to emit an error message. I could make a much more human readable error message if I had call-site information from destructuring-case

(defmacro destructuring-case
  [expr & operands]
  (cond
    (not= 0 (mod (count operands) 2))
    (throw (ex-info (cl-format false
                               "destructuring-case expects multiple of 2 number of operands after the first:~
                                not ~A, ~A"
                               (count operands) (apply list 'destructuring-case expr operands))
                    {:error-type :invalid-destructuring-case-call-site
                     :expr expr
                     :operands operands}))

    :else
    (let [pairs (partition 2 operands) ]
      `(apply (-destructuring-fn-many nil ~@pairs) ~expr))))

The vars &form and &env are "special" because you don't do anything to bind them in the macro definition. Macros are invoked with those 2 "extra" parameters which are provided by the compiler at macroexpansion time

a macro does not have access to the surrounding form of it's usage

ok, so I could dynamically bind my own copy of &form (and its metadata) to use in the sub macro expansion which is “private” anyway. is that the correct approach, or am I doing something I will regret later?

or I could even pass it as an explicit parameter to -destructuring-fn-many` since it is private.

they are not dynamic vars, so you can't bind them

you would need to explicitly pass that info down

but don't call it &form or &env! those names really are special