new package watch-outputs and example of its use

pkgs/watch-outputs/output-template.fnl

@@ -0,0 +1,44 @@
+(local svc (require :anoia.svc))
+
+(fn json-escape [s]
+  ;; All Unicode characters may be placed within the quotation marks,
+  ;; except for the characters that MUST be escaped:
+  ;; quotation mark, reverse solidus, and the control characters (U+0000
+  ;; through U+001F). (RFC 8259)
+  (-> s
+      (string.gsub
+       "[\"\b\f\n\r\t]" {
+                         "\b" "\\b"
+                         "\"" "\\\""
+                         "\f" "\\f"
+                         "\n" "\\n"
+                         "\r" "\\r"
+                         "\t" "\\t"
+                         })
+      (string.gsub
+       "([\x00-\x1b])"
+       (fn [x] (string.format "\\u%04X" (string.byte x))))))
+
+
+(fn substitute [text opening closing]
+  (let [delim (.. opening "(.-)" closing)
+        myenv {
+               : string
+               :output
+               (fn [service-path path]
+                 (let [s (assert (svc.open (.. service-path "/.outputs")))]
+                   (s:output path)))
+               :lua_quote #(string.format "%q" %1)
+               :json_quote (fn [x] (.. "\"" (json-escape x) "\""))
+               }]
+    (string.gsub text delim
+                 (fn [x]
+                   (assert ((load (.. "return " x) x :t myenv))
+                           (string.format "missing value for %q" x))))))
+
+(fn run []
+  (let [[opening closing] arg
+        out (substitute (: (io.input) :read "*a") opening closing)]
+    (io.write out)))
+
+{ : run }