Proposal: add handleError to mtl - Libraries - Haskell.org

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Erik de Castro Lopo

4 Jun 2013 4 Jun '13

11:04 p.m.

Hi all, I would like to propose the following convenience function (with implementation) for the mtl library: handleError :: MonadError e m => (e -> m a) -> m a -> m a handleError = flip catchError which follows the handle function from Control.Exception.Base: handle :: Exception e => (e -> IO a) -> IO a -> IO a Like the handle function, handleError allows the handling of exceptions to be expressed in a more natural way, turning: catchError ( do mumble something mumble ) handler into: handlerError handler $ do mumble something mumble -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Edward Kmett

5 Jun 5 Jun

1:50 p.m.

Sure. I can see merit to consistency with base in this regard. I would like a version of it in transformers monomorphized to the particular ErrorT monad though as well for consistency. Does that mean we'd want a liftHandle to avoid doing a lot of flipping with liftCatch? I maintain mtl though, not transformers, so any action on adding it there would have to go through Ross. Sent from my iPad On Jun 4, 2013, at 11:04 PM, Erik de Castro Lopo wrote:

Hi all,

I would like to propose the following convenience function (with implementation) for the mtl library:

handleError :: MonadError e m => (e -> m a) -> m a -> m a handleError = flip catchError

which follows the handle function from Control.Exception.Base:

handle :: Exception e => (e -> IO a) -> IO a -> IO a

Like the handle function, handleError allows the handling of exceptions to be expressed in a more natural way, turning:

catchError ( do mumble something mumble ) handler

into:

handlerError handler $ do mumble something mumble

-- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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Ross Paterson

4:23 p.m.

On Wed, Jun 05, 2013 at 01:50:01PM -0400, Edward Kmett wrote:

Sure. I can see merit to consistency with base in this regard.

I would like a version of it in transformers monomorphized to the particular ErrorT monad though as well for consistency. Does that mean we'd want a liftHandle to avoid doing a lot of flipping with liftCatch?

For the monomorphic version, it's probably best to keep these lifts to a minimum, and use flip on liftCatch. On the subject of ErrorT, I wonder if it's time to deprecate the Error class.

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Henning Thielemann

6 Jun 6 Jun

6:14 a.m.

New subject: deprecating Error class (Was: Proposal: add handleError to mtl)

On Wed, 5 Jun 2013, Ross Paterson wrote:

On the subject of ErrorT, I wonder if it's time to deprecate the Error class.

Since I think ErrorT gives the wrong association with the 'error' function, I would prefer to give up the Control.Monad.Trans.Error module entirely and start a new module Control.Monad.Trans.Exception. You could deprecate Error class and ErrorT type and stay in the version range 0.3.*.

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Roman Cheplyaka

6:55 a.m.

New subject: deprecating Error class (Was: Proposal: add handleError to mtl)

* Henning Thielemann [2013-06-06 12:14:10+0200]

On Wed, 5 Jun 2013, Ross Paterson wrote:

...
On the subject of ErrorT, I wonder if it's time to deprecate the Error class.

Since I think ErrorT gives the wrong association with the 'error' function, I would prefer to give up the Control.Monad.Trans.Error module entirely and start a new module Control.Monad.Trans.Exception.

... which would give the wrong association with exceptions (in the Control.Exception sense). I don't see how this is an improvement. Roman

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Henning Thielemann

7 a.m.

New subject: deprecating Error class (Was: Proposal: add handleError to mtl)

On Thu, 6 Jun 2013, Roman Cheplyaka wrote:

* Henning Thielemann [2013-06-06 12:14:10+0200]

...
On Wed, 5 Jun 2013, Ross Paterson wrote:

...
On the subject of ErrorT, I wonder if it's time to deprecate the Error class.

Since I think ErrorT gives the wrong association with the 'error' function, I would prefer to give up the Control.Monad.Trans.Error module entirely and start a new module Control.Monad.Trans.Exception.

... which would give the wrong association with exceptions (in the Control.Exception sense). I don't see how this is an improvement.

It's the better association and it is already mentioned in the module description of Control.Monad.Trans.Error: "This monad transformer adds the ability to fail or throw exceptions to a monad." Nonetheless, you may suggest another module name. Independent from the name of a new module, I would suggest to deprecate the Error name instead of removing something from the Error module.

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participants (5)

Edward Kmett
Erik de Castro Lopo
Henning Thielemann
Roman Cheplyaka
Ross Paterson