Give parse more useful output, add more useful errors

src/Expr.elm

@@ -16,7 +16,8 @@ type Expr
 
 num : Parser () Expr
 num =
-    Num <$> Num.float <|> Num << toFloat <$> Num.int
+    (Num <$> Num.float <|> Num << toFloat <$> Num.int)
+    <?> "expected a number (int or float)"
 
 stringIs : String -> a -> Parser s a
 stringIs str val =
@@ -33,12 +34,15 @@ op =
 group : Parser () Expr
 group =
     between (string "(") (string ")") (sepBy1 whitespace (lazy <| \_ -> parser)) -- Avoid bad recursion issues using lazy parser evaluation
+    <?> "expected a group (whitespace-separated expressions between brackets)"
     |> map Group
 
 parser : Parser () Expr
 parser =
     (lazy <| \_ -> group) <|> op <|> num
 
-parse : String -> Result (ParseErr ()) (ParseOk () Expr)
+parse : String -> Result (List String) Expr
 parse =
     Combine.parse parser
+    >> Result.mapError (\(_, _, errorList) -> errorList)
+    >> Result.map (\(_, _, expr) -> expr) -- Convert errors/results to nicer format