[Haskell-cafe] Collecting MonadError errors generically

Leon Grynszpan leon at grynszpan.com
Tue Jul 13 18:08:33 EDT 2010


This is my first post to the Haskell Café, and I'm hoping the issue
I'm tackling here isn't one that's been thoroughly explored elsewhere.
If that's the case, I'll apologize in advance for my insufficient
google chops. Otherwise, here goes...

I do a lot of work within the error monad. Having read Eric Kidd's
blog on error reporting in Haskell
I'm always a bit paranoid about dealing with errors in the most
generic, flexible way possible. As a result, a lot of the functions I
use that could result in errors have signatures along these lines:

couldThrowError :: (Error e, MonadError e m) => t1 -> t2 -> m t3

The way I figure it, this means that I can use these functions within
any particular MonadError, which keeps me happy if I decide to change
implementations at any particular point during development.

Usually, I use these couldThrowError functions in a standard do block.
Something along the lines of:

couldThrowError :: (Error e, MonadError e m) => t1 -> t2 -> m t3
couldThrowError = do x <- processWithPossibleError t1
                                y <- processWithPossibleError t2
                                return (z :: t3)

Things stay generic at this level. I can still choose any MonadError
implementation I want, and if any of the computations in the do block
throws an Error, the whole function short-circuits.

Often enough, however, I don't want this kind of short-circuiting
behavior. What I want, instead, is to run a whole bunch of
computations that may throw errors. If there are any errors, I want to
collect them all into one big master error. If not, I want a list of
results. Here's an example of usage:

couldThrowError :: (Error e, MonadError e m) => t1 -> m t2
getParams :: (Error e, MonadError e m) => [t1] --> m [t2]
getParams = groupErrors . map couldThrowError

I found it pretty easy to implement groupErrors for Either String:

groupErrors :: (Error e, MonadError e m) => [Either String a] -> m [a]
groupErrors eithers = case partitionEithers eithers of
                                       ([], xs) -> return xs
                                       (es,  _) -> strError $ unlines es

The problem, though, is that running this function now causes type
inference to provide "Either String" as my MonadError implementation.
If any of the potential error sources are dealing with a different
concrete implementation, I'm stuck! It would be nice if I could stay
generic. I made an effort at this that you can see below, but it
strikes me as very awkward.

collectErrors :: (Monoid e, Error e, MonadError e m) => m a -> m [a] -> m [a]
collectErrors m1 m2 =
 do m1r <- (m1 `catchError` (\e1 ->
               (m2 `catchError` (\e2 ->
                   throwError $ mappend e1 e2)) >>
               throwError e1))
    m2r <- m2
    return $ m1r : m2r
groupErrors' :: (Monoid e, Error e, MonadError e m) => [m a] -> m [a]
groupErrors' = foldr collectErrors (return [])

As you can see, I now require an error type that implements monoid. An
example I've tested is:

newtype ErrorString = ES String
    deriving (Read, Show, Eq, Ord, Error)
instance Monoid ErrorString where
    mempty                  = ES ""
    mappend (ES s1) (ES s2) = ES $ s1 `mappend` "\n" `mappend` s2

This seems to work, but something strikes me as being very hackish
here. Maybe it's just that collectErrors looks very ugly, and in
Haskell my intuition tells me that what looks ugly on the surface is
often ugly down below. I suspect that I might really not be
approaching this problem the right way, but after a lot of time spent
wrestling with it, I'm eager to read your suggestions. Is there a
better way to do groupErrors, or should I be looking at an entirely
different philosophy?



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