[Haskell-cafe] Portability of MonadError

[Luke Palmer]

On Mon, Jan 5, 2009 at 2:13 PM, Luke Palmer <lrpalmer at gmail.com> wrote:

> On Mon, Jan 5, 2009 at 1:48 PM, Peter Robinson <thaldyron at gmail.com>wrote:
>
>> Hello,
>>
>> One thing that's been bothering me about MonadError monads is
>> the non-portability of code that uses a custom Error type.  Meaning, if I
>> have libraries A and B that use different error types, I won't be able to
>> write a function func:
>>
>> func = (funcA >> funcB) `catchError` (\e -> ...)
>>
>> funcA :: ErrorT MyErrorA m ()
>>
>> funcB :: ErrorT MyErrorB m ()
>>
>> So I'm wondering whether there's a reason not to introduce a type class
>> hierarchy instead of custom error types to make the code more portable.
>
>
> I think this worry is related to a world view of "large monads", which also
> proliferates claims like "monads are not appropriate for large programs",
> and is related to fat interfaces in OOP.  I claim that, like objects, monads
> are appropriate for little pieces of computations, and monadic computations in
> different monads deserve to be composed just as much so as those in the
> same monad.
>
> The complex type-directed approach gives the feel of exceptions from
> mainstream languages, but will run into problems when eg. two computations
> both use ErrorT String, but you want to differentiate the errors.  All that
> is necessary is a simple function:
>
> mapError :: (e -> e') -> ErrorT e a -> ErrorT e' a
> mapError f = ErrorT . liftM (left f) . runErrorT
>

Modulo obvious errors, as usual.  Haskell type inference knows better than I
do:

mapError :: Monad m => (e -> e') -> ErrorT e m a -> ErrorT e' m a

Luke


> (Where "left" is from Control.Arrow, and is a "semantic editor")
>
> Then your example can become::
>
> func = (mapError Left funcA >> mapError Right funcB) `catchError` (\e ->
> ...)
>
> Luke
>
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