I've been working with core.match some this week and finding it pretty
nice. However, I have a common case (for me) that is not well handled
right now via core.match: matching symbols. Say that I wrote a match
like this:
;; translate (+ x (+ y z)) to (+ x y z)
(let [e '(+ 1 (+ 2 3))]
(match [e]
[([+ x ([+ y z] :seq)] :seq)] (+ x y z)))
You will see this error:
Pattern row 1: Pattern row reuses wildcards in [([+ x ([+ y
z] :seq)] :seq)]. The following wildcards are ambiguous: +. There's
no guarantee that the matched values will be same. Rename the
occurrences uniquely.
Any symbol inside a pattern row is treated as a bind variable. + is a
symbol. You can achieve this with guards:
(defn +? [s] (= '+ s))
(let [e '(+ 1 (+ 2 3))]
(match ['(+ 1 (+ 2 3))]
[([(_o1 :when +?) x ([(_o2 :when +?) y z] :seq)] :seq)] (list
'+ x y z)))
but, yuck. I can imagine using the reserved ()'s with additional keys
(:symbol or :sym) to do symbol matching like (:symbol +) but also,
yuck. The simplest idea I came up with was:
(let [e '(+ 1 (+ 2 3))]
(match [e]
[(['+ x (['+ y z] :seq)] :seq)] ('+ x y z)))
These come through as (quote x) although the error reporting goes a
little off the rails:
Pattern row 1: Pattern row reuses wildcards in [([(quote +) x ([(quote
+) y z] :seq)] :seq)]. The following wildcards are ambiguous: quote.
There's no guarantee that the matched values will be same. Rename the
occurrences uniquely.
However, that seems fixable and you could then use (quote x) as a
signal to do symbol matching. If I can figure out what the hell I'm
doing in core.match then I'd be happy to work on a patch.
Thoughts?
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