there's outputStrLn

app/Frontend.hs

@@ -33,16 +33,16 @@ interpret = (>> return True) . lex
   where
     lex input = do
       case MP.parse P.lexPrlg "-" input of
-        Left bundle -> liftIO $ putStr (MP.errorBundlePretty bundle)
+        Left bundle -> lift . outputStr $ MP.errorBundlePretty bundle
         Right toks -> parse toks
     parse toks = do
       case MP.parse P.parsePrlg "-" toks of
-        Left bundle -> liftIO $ putStr (MP.errorBundlePretty bundle)
+        Left bundle -> lift . outputStr $ MP.errorBundlePretty bundle
         Right asts -> traverse_ shunt asts
     shunt ast = do
       o <- gets ops
       case P.shuntPrlg o ast of
-        Left err -> liftIO $ putStrLn $ "expression parsing: " ++ err
+        Left err -> lift . outputStrLn $ "expression parsing: " ++ err
         Right prlg -> intern prlg
     intern prlgs = do
       prlgi <- withStrTable $ \st -> C.internPrlg st prlgs
@@ -58,14 +58,13 @@ interpret = (>> return True) . lex
       execute code
     execute code = do
       res <- I.prove code
-      case res of
-        Left err -> liftIO $ putStrLn err
-        Right res ->
-          liftIO $
-          putStrLn $
-          if res
-            then "yes."
-            else "no proof."
+      lift . outputStrLn $
+        case res of
+          Left err -> err
+          Right res ->
+            if res
+              then "yes."
+              else "no proof."
 
 interpreterStart :: PrlgEnv ()
 interpreterStart = do