Add a pretty printer.

src/bin.rs

@@ -5,6 +5,7 @@ use codespan_reporting::term;
 use codespan_reporting::term::termcolor::{ColorChoice, StandardStream};
 use ngr::error::Error;
 use ngr::syntax::Program;
+use pretty::{Arena, Pretty};
 use std::fs;
 
 #[derive(Parser, Debug)]
@@ -34,11 +35,20 @@ fn main() {
     let mut file_database = SimpleFiles::new();
     let initial_file_name = &args.file;
 
-    if let Err(e) = compile_file(&mut file_database, initial_file_name) {
-        let diagnostic = Diagnostic::from(e);
-        let writer = StandardStream::stderr(ColorChoice::Auto);
-        let config = codespan_reporting::term::Config::default();
+    match compile_file(&mut file_database, initial_file_name) {
+        Ok(p) => {
+            let arena = Arena::new();
+            p.pretty(&arena)
+                .into_doc()
+                .render_colored(72, StandardStream::stdout(ColorChoice::Auto))
+                .unwrap()
+        }
+        Err(e) => {
+            let diagnostic = Diagnostic::from(e);
+            let writer = StandardStream::stderr(ColorChoice::Auto);
+            let config = codespan_reporting::term::Config::default();
 
-        term::emit(&mut writer.lock(), &config, &file_database, &diagnostic).unwrap();
+            term::emit(&mut writer.lock(), &config, &file_database, &diagnostic).unwrap();
+        }
     }
 }