Don't need to nest ifs like that

src/moss.ts

 // };
 
 const rawArgs = minimist(process.argv.slice(2));
-
 const args = {
   eval: rawArgs.e || rawArgs.eval,
   file: rawArgs.f || rawArgs.file,
 ): LexerError | ParserError | EnvError | string => {
   const lexer = new Lexer(source, file);
   const tokens = lexer.scan();
-  if (tokens instanceof LexerError) {
-    return tokens;
-  } else {
-    const parser = new Parser(tokens);
-    const tree = parser.parse();
-    // print(tree);
-    if (tree instanceof ParserError) {
-      return tree;
-    } else {
-      const compiler = new Compiler(tree, { prettyPrint: args.pretty });
-      return compiler.compile();
-    }
-  }
+  if (tokens instanceof LexerError) return tokens;
+  const parser = new Parser(tokens);
+  const tree = parser.parse();
+  // print(tree);
+  if (tree instanceof ParserError) return tree;
+  const compiler = new Compiler(tree, { prettyPrint: args.pretty });
+  return compiler.compile();
 };
 
 const run = () => {