# Evolution of error messages

Published on July 26, 2017

This is the second and final post about Megaparsec 6.0.0 development. This time we are going to walk through re-design of parse error messages.

## Custom error components

Since version 5 we’ve had the ability to use any data type as custom component of error message, e in MonadParsec e s m. So everything that isn’t a combination of expected/unexpected items should be representable as e, which must be an instance of the ErrorComponent type class:

class Ord e => ErrorComponent e where

-- | Represent the message passed to 'fail' in parser monad.

representFail :: String -> e

-- | Represent information about incorrect indentation.

representIndentation
:: Ordering -- ^ Desired ordering between reference level and actual level
-> Pos             -- ^ Reference indentation level
-> Pos             -- ^ Actual indentation level
-> e


Its methods force the programmer to provide a way to encode the two built-in custom error conditions: fail usage and incorrect indentation messages.

By default we had Dec:

-- | “Default error component”. This is our instance of 'ErrorComponent'
-- provided out-of-box.

data Dec
= DecFail String         -- ^ 'fail' has been used in parser monad
| DecIndentation Ordering Pos Pos
-- ^ Incorrect indentation error: desired ordering between reference
-- level and actual level, reference indentation level, actual
-- indentation level
deriving (Show, Read, Eq, Ord, Data, Typeable)


But if you wanted to add something different, there were two options:

• Duplicate the DecFail and DecIndentation constructors and also copy the rendering logic in the instance of ShowErrorComponent for your new data type. For example, you could define a data type with constructors MyFail, MyIndentation, and MyCustomThing.

• Embed Dec into your own custom component and delegate all the work to it in the fail and indentation cases.

After a while I realized that the design sucks because of the boilerplate one had to deal with to support the obligatory fail and indentation errors, so I wanted to finally fix it in version 6.

After some thinking I came up with the idea of the ErrorFancy sum type:

-- | Additional error data, extendable by user. When no custom data is
-- necessary, the type is typically indexed by 'Void' to “cancel” the
-- 'ErrorCustom' constructor.

data ErrorFancy e
= ErrorFail String
-- ^ 'fail' has been used in parser monad
| ErrorIndentation Ordering Pos Pos
-- ^ Incorrect indentation error: desired ordering between reference
-- level and actual level, reference indentation level, actual
-- indentation level
| ErrorCustom e
-- ^ Custom error data, can be conveniently disabled by indexing
-- 'ErrorFancy' by 'Void'
deriving (Show, Read, Eq, Ord, Data, Typeable, Generic, Functor)


It covers the default custom error messages and provides the opportunity to add something custom via the ErrorCustom constructor. By default we don’t want anything custom, so we can just “multiply it by zero” by indexing the type by Void.

Then we have the really straightforward instance definition for ShowErrorComponent:

instance ShowErrorComponent e => ShowErrorComponent (ErrorFancy e) where
showErrorComponent (ErrorFail msg) = msg
showErrorComponent (ErrorIndentation ord ref actual) =
"incorrect indentation (got " <> show (unPos actual) <>
", should be " <> p <> show (unPos ref) <> ")"
where
p = case ord of
LT -> "less than "
EQ -> "equal to "
GT -> "greater than "
showErrorComponent (ErrorCustom a) = showErrorComponent a

instance ShowErrorComponent Void where
showErrorComponent = absurd


The ErrorComponent type class can be dropped now. The user only needs to think about the custom part—the problem is solved.

## Separation of custom and trivial error messages

Roman Cheplyaka opened an issue which made me think about re-organizing the ParseError data type. Previously we had:

data ParseError t e = ParseError
{ errorPos        :: NonEmpty SourcePos -- ^ Stack of source positions
, errorUnexpected :: Set (ErrorItem t)  -- ^ Unexpected items
, errorExpected   :: Set (ErrorItem t)  -- ^ Expected items
, errorCustom     :: Set e              -- ^ Associated data, if any
} deriving (Show, Read, Eq, Data, Typeable, Generic)


The problem with this is that it by default Megaparsec’s machinery generates errors with unexpected/expected items and sometimes it may get mixed with custom messages the user tries to generate. See the linked issue for an example of this, I’ll just state here that it’s rarely useful to mix these two types of error messages together, so we want to have two different constructors: one for trivial errors (unexpected/expected items) and another one for “fancy” stuff (recall the ErrorFancy type from the previous section):

data ParseError t e
= TrivialError
(NonEmpty SourcePos)
(Maybe (ErrorItem t))
(Set (ErrorItem t))
-- ^ Trivial errors, generated by Megaparsec's machinery. The data
-- constructor includes the stack of source positions, unexpected token
-- (if any), and expected tokens.
| FancyError (NonEmpty SourcePos) (Set (ErrorFancy e))
-- ^ Fancy, custom errors.
deriving (Show, Read, Eq, Data, Typeable, Generic)


When merging errors from several branches of parsing fancy errors are preferred. The merging logic is like this:

mergeError :: (Ord t, Ord e)
=> ParseError t e
-> ParseError t e
-> ParseError t e
mergeError e1 e2 =
case errorPos e1 compare errorPos e2 of
LT -> e2
EQ ->
case (e1, e2) of
(TrivialError s1 u1 p1, TrivialError _ u2 p2) ->
TrivialError s1 (E.union u1 u2) (E.union p1 p2)
(FancyError {}, TrivialError {}) -> e1
(TrivialError {}, FancyError {}) -> e2
(FancyError s1 x1, FancyError _ x2) ->
FancyError s1 (E.union x1 x2)
GT -> e1


The change also entailed addition of the fancyFailure method, the old failure method now can only be used to signal trivial parse errors:

class (Stream s, A.Alternative m, MonadPlus m)
=> MonadParsec e s m | m -> e s where

-- | The most general way to stop parsing and report a trivial
-- 'ParseError'.

failure
:: Maybe (ErrorItem (Token s)) -- ^ Unexpected item (if any)
-> Set (ErrorItem (Token s)) -- ^ Expected items
-> m a

-- | The most general way to stop parsing and report a fancy 'ParseError'.

fancyFailure
:: Set (ErrorFancy e) -- ^ Fancy error components
-> m a


I think this is a step forward because we now have a more structured approach where different types of parse errors do not get mixed resulting in something that may make no sense. The new approach is not less powerful because we can plug any type as e, e.g. we can attach context to a trivial parse error and store the whole thing as a fancy error ErrorFancy e, similar to what I have shown in an older post.

## The problem with multiple unexpected items

Another issue Roman drew my attention to is that we can have a set of unexpected items. Indeed, it’s a bit strange to see something like:

1:10:
unexpected "foo" or "bar"
expecting "baz"


How it’s even possible to run into two different unexpected things at the same time? You can build something weird on top of token though and get a message like the one shown above, but is it really useful? Does it make sense?

If you don’t do anything weird, there is only one case (that I’m aware of) when you can get multiple unexpected tokens with Megaparsec 5, looks like this (this is with master as it where when Roman opened the issue):

module Main (main) where

import Data.Void
import Text.Megaparsec -- current master
import Text.Megaparsec.Char

type Parser = Parsec Void String

pScheme :: Parser String
pScheme = choice
[ string "data"
, string "file"
, string "ftp"
, string "https"
, string "http"
, string "irc"
, string "mailto" ]

main :: IO ()
main = parseTest pScheme "dat"


Then:

1:1:
unexpected "dat" or 'd'
expecting "data", "file", "ftp", "http", "https", "irc", or "mailto"


Yep, since there are alternatives that do not start with a d, we could indeed claim that d by itself is unexpected. This is a consequence of error reporting as implemented by old tokens (before we extended the Stream type class, as described here). The old tokens reports unexpected sequence of tokens up to first mismatch. First mismatch is different for different alternatives, so it gets merged into the thing you have just seen.

In principle, I have nothing against having Maybe (ErrorItem t) instead of Set (ErrorItem t) as unexpected item in TrivialError, but then there are problems with merging parse errors from different branches of parsing. Look at the definition of ErrorItem t:

data ErrorItem t
= Tokens (NonEmpty t)      -- ^ Non-empty stream of tokens
| Label (NonEmpty Char)    -- ^ Label (cannot be empty)
| EndOfInput               -- ^ End of input
deriving (Show, Read, Eq, Ord, Data, Typeable, Generic, Functor)


When merging two Maybe (ErrorItem t) from different branches, should labels take precedence over sequence of tokens? How unexpected EndOfInput should merge with the other stuff? How two labels should merge? Any particular logic seemed kind of arbitrary, so at the time I decided to keep the set-based definition.

The change in Stream type class and new, more efficient implementation of tokens forced me to re-think the set-based approach again. For efficiency, the new tokens definition reports unexpected token sequence of the same length as the chunk we match against. With the old merging strategy and sets of unexpected tokens we could easily get (if we fed something like datzzzzz as input to that scheme parser):

1:1:
unexpected "dat", "datz", "datzz", or "datzzz"
expecting "data", "file", "ftp", "http", "https", "irc", or "mailto"


This is definitely much worse than the previous behavior and completely unacceptable. The logic of tokens could not be improved in a way that I would find reasonable. After all, the behavior of individual tokens-based parsers is OK, the problem was in the merging logic—it needed an update.

The first attempt looked like this:

mergeError :: (Ord t, Ord e)
=> ParseError t e
-> ParseError t e
-> ParseError t e
mergeError e1 e2 =
case errorPos e1 compare errorPos e2 of
LT -> e2
EQ ->
case (e1, e2) of
(TrivialError s1 u1 p1, TrivialError _ u2 p2) ->
-- u1 and u2 (unexpected items) are still sets
TrivialError s1 (n u1 u2) (E.union p1 p2)
(FancyError {}, TrivialError {}) -> e1
(TrivialError {}, FancyError {}) -> e2
(FancyError s1 x1, FancyError _ x2) ->
FancyError s1 (E.union x1 x2)
GT -> e1
where
n = …


n, which I’m too lazy to re-implement for the sake of just this post, made sure that from any number of Tokens (it’s a data constructor of ErrorItem, shown above) only single longest Tokens item will survive after merging. The logic is as following: since we only combine parse errors happening exactly at the same position in input, then any different Tokens things are inevitably prefixes of the input stream at that point. If they differ, then they are just prefixes of different length, so we can go ahead and just pick the longest Tokens item and drop the others.

This turned the awkward parse error I showed earlier into this:

1:1:
unexpected "datzzz"
expecting "data", "file", "ftp", "http", "https", "irc", or "mailto"


Normalization worked well, but now we got a new problem: ParseError no longer was a lawful Monoid! Indeed, if we take a ParseError that already contains two Tokens items of different length and mappend it with mempty (which has unexpected and expected items just as empty sets), the normalization kicks in and we get a different thing as output: right/left identity laws for Monoid do not hold.

But really, we would like ParseError to be a Monoid, very much.

If ParseError is just a Semigroup, then we can’t make our parser an instance of Alternative and MonadPlus which are monoids on applicative functors and monads respectively, i.e. they imply that some sort of identity exists, for which we need ParseError‘s mempty. If we went ahead with non-monoid ParseError, we would need to pull in something like semigroupoids, and use something like Alt, which is a path I don’t want to step on with Megaparsec, which should stay close to “popular” abstractions, be reasonably conventional and light in terms of dependencies.

The only way to keep normalization and make ParseError a proper Monoid is to switch from Set (ErrorItem t) to Maybe (ErrorItem t) for the unexpected component of error message—we virtually have no choice!

Returning to the problems with merging, there is some points that should make the arbitrary merging strategy I have chosen less frightening:

• Choose longest Tokens item following the logic I’ve just described.
• Let Labels win over sequences of tokens.
• Let EndOfInput win over anything else.

The final solution:

mergeError :: (Ord t, Ord e)
=> ParseError t e
-> ParseError t e
-> ParseError t e
mergeError e1 e2 =
case errorPos e1 compare errorPos e2 of
LT -> e2
EQ ->
case (e1, e2) of
(TrivialError s1 u1 p1, TrivialError _ u2 p2) ->
TrivialError s1 (n u1 u2) (E.union p1 p2)
(FancyError {}, TrivialError {}) -> e1
(TrivialError {}, FancyError {}) -> e2
(FancyError s1 x1, FancyError _ x2) ->
FancyError s1 (E.union x1 x2)
GT -> e1
where
n (Just x) (Just y) = Just (max x y)
n x        y        = x <|> y


Yep, just like that. In reality there is nothing to worry about. As I said, two “unexpected” components to merge are inevitably located exactly at the same spot, so they are essentially different names for the same thing.

## Displaying of offending line in error messages

Finally, we now have the ability to display offending line in error messages, with problematic position marked with the caret symbol ^. Parse errors now look like this:

1:10:
|
1 | foo = (x $y) * 5 + 7.2 * z | ^ unexpected '$'
expecting ')', operator, or the rest of expression


Use parseErrorPretty' or parseTest' instead of parseErrorPretty and parseTest respectively to get the enhanced print-outs.

I’d like to thank Alex Washburn aka recursion-ninja for working on the feature.

## Conclusion

Right now I’m playing with Megaparsec 6 porting some existing parsers and I’m very pleased with results. I think I’ll upload the library on Hackage very soon (maybe tomorrow, I’ll announce on Twitter when done). When version 6 is out I’ll push updates to tutorials and upgrade some projects that I maintain, such as stache.