Generalize ReadTemplate

byg/src/Evaluation/FunctionIO.hs

@@ -24,10 +24,9 @@ evalFunctionIO f x = case (f, x) of
   (IsDirectory, String (StringWrapper s)) ->
     Bool <$> doesDirectoryExist s
 
-  (ReadTemplate, String (StringWrapper s)) -> do
+  (ReadTemplate, Tuple (String (StringWrapper s), Text c)) -> do
     t <- T.readFile s
-    let c = "CONTENT"
-        (beforeContent, after) = T.breakOn c t
+    let (beforeContent, after) = T.breakOn c t
         afterContent = T.drop (T.length c) after
     pure $ Template $ TemplateParts beforeContent afterContent
 
@@ -62,4 +61,4 @@ evalFunctionIO f x = case (f, x) of
     pure $ Text $ TL.toStrict $ B.renderHtml html
 
   _ ->
-    error "unexpected combination of function and argument type"
+    error ("unexpected combination of function and argument type; got function " ++ show f ++ " with argument " ++ show x)