$DEBUG to see output in stderr

src/config.rs

@@ -18,6 +18,7 @@ pub struct Streams {
 pub struct Stream {
    pub engine: Engine,
    pub format: Option<String>,
+   pub debug: bool,
 }
 
 #[derive(Serialize, Deserialize, Debug)]
@@ -83,6 +84,7 @@ fn build_config_std() -> Config {
    return Config {
       streams: Streams{
          input: Stream {
+            debug: env::var("DEBUG").unwrap_or(String::from("false")) == "true",
             format: match env::var("INPUT_FORMAT") {
                Ok(x) => Some(x),
                Err(_) => None,
@@ -110,6 +112,7 @@ fn build_config_std() -> Config {
             },
          },
          output: Stream {
+            debug: env::var("DEBUG").unwrap_or(String::from("false")) == "true",
             format: match env::var("OUTPUT_FORMAT") {
                Ok(x) => Some(x),
                Err(_) => None,

src/stream.rs

@@ -201,21 +201,27 @@ pub fn build_output_stream(cfg: &Stream) -> Box<dyn OutputStream> {
 pub struct OutputStreamFormatted {
    format: Option<String>,
    stream: Box<dyn OutputStream>,
+   debug: bool,
 }
 
 pub fn build_formatted_output_stream(cfg: &Stream, stream: Box<dyn OutputStream>) -> Box<dyn OutputStream> {
    return Box::new(OutputStreamFormatted{
       format: cfg.format.clone(),
+      debug: cfg.debug,
       stream: stream,
    });
 }
 
 impl OutputStream for OutputStreamFormatted {
    fn put(&mut self, v: Vec<char>) {
-      match self.format.as_ref() {
+      let v2 = match self.format.as_ref() {
-         Some(x) => self.stream.put(sprintf(x, v)),
+         Some(x) => sprintf(x, v),
-         None => self.stream.put(v),
+         None => v,
       };
+      if self.debug {
+         eprintln!("output: {}", v2.iter().collect::<String>());
+      }
+      self.stream.put(v2);
    }
 }