add fennelc, a script to do what writeFennel does

it's a bit simpler because no need to do quite as much string surgery
to compute package.{c,}path

also it can be used in Makefiles

pkgs/default.nix

@@ -85,6 +85,7 @@ in
   devout = callPackage ./devout { };
   fetch-freebsd = callPackage ./fetch-freebsd { };
   fennel = callPackage ./fennel { };
+  fennelc = callPackage ./fennelc {  };
   fennelrepl = callPackage ./fennelrepl { };
   firewallgen = callPackage ./firewallgen { };
   firmware-utils = callPackage ./firmware-utils { };

pkgs/fennelc/default.nix

@@ -0,0 +1,19 @@
+{
+  lua,
+  runCommand
+}:
+let fennel = lua.pkgs.fennel; in
+runCommand "build-fennelc"
+{
+ nativeBuildInputs = [ fennel  ] ;
+} ''
+LUAPATH=$(ls -d ${fennel}/share/lua/*)
+mkdir -p $out/bin
+(
+exec > $out/bin/fennelc
+echo '#! ${lua}/bin/lua'
+echo "package.path = \"''${LUAPATH}/?.lua;''${LLPATH}/?.lua;\" .. package.path"
+fennel  --compile ${./fennelc.fnl}
+)
+chmod +x $out/bin/fennelc
+''

pkgs/fennelc/fennelc.fnl

@@ -0,0 +1,49 @@
+(local { : view &as fennel } ( require :fennel))
+
+(fn assoc [tbl k v & more]
+  (tset tbl k v)
+  (case more
+    [k v] (assoc tbl k v)
+    _ tbl))
+
+(fn parse-args [args]
+  (case args
+    ["--correlate" & rest]
+    (assoc (parse-args rest)
+           :correlate true)
+
+    ["--main" main-fn & rest]
+    (assoc (parse-args rest)
+           :main main-fn)
+
+    ["-o" output-file & rest]
+    (assoc (parse-args rest)
+           :output-file output-file)
+
+    [source] { :source-file source }))
+
+(fn output-name [opts]
+  (or opts.output-file
+      (string.gsub opts.source-file "%.fnl$" "")))
+
+(let [luapath (os.getenv "LUA_PATH")
+      luacpath (os.getenv "LUA_CPATH")
+      path (os.getenv "PATH")
+      opts (parse-args arg)
+      output-file (output-name opts)]
+  (with-open [o (io.open output-file :w)]
+    (o:write "#!/usr/bin/env lua\n")
+    (and luapath
+         (o:write (string.format "package.path = %q .. \";\" .. package.path\n" luapath)))
+    (and luacpath
+         (o:write (string.format "package.cpath = %q .. \";\" .. package.cpath\n" luacpath)))
+    (let [(ok? msg)
+          (pcall
+           fennel.compile-string
+           (: (io.open opts.source-file :r) :read  "*a")
+           {:filename opts.source-file
+            :correlate opts.correlate})]
+      (when (not ok?)
+        (error (.. "error: " msg)))
+      (o:write msg)
+      (os.execute (string.format "chmod +x %q" output-file)))))