Singpolyma

Error Handling in Haskell

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When I first started learning Haskell, I learned about the Monad instance for Either and immediately got excited. Here, at long last, was a good solution to the error handling problem. When you want exception-like semantics, you can have them, and the rest of the time it’s just a normal value. Later, I learned that the Haskell standard also includes an exception mechanism for the IO type. I was horrified, and confused, but nothing could have prepared me for what I discovered next.

While the standard Haskell exception mechanism infects all of IO it at least has a single, well-defined type for possible errors, with a small number of known cases to handle. GHC extends this with a dynamically typed exception system where any IO value may be hiding any number of unknown and unknowable exception types! Additionally, all manner of programmer errors in pure code (such as pattern match failures and integer division by zero) are thrown into IO when they get used in that context. On top of everything, so-called exceptions can appear that were not thrown by any code you can see but are external to you and your dependencies entirely. There are two classes of these: asynchronous exceptions thrown by the runtime due to a failure in the runtime itself (such as a HeapOverflow) and exceptions thrown due to some impossible-to-meet condition the runtime detects (such as detectable non-termination). Oh, I almost forgot, manually killing a thread or telling the process to exit are also modeled as “exceptions” by GHC.

Once the initial decision to have a dynamically typed exception system was made, everything that could make use of an exception-like semantic in any case was bolted on. What am I going to do, though? Write my own ecosystem and runtime that works how I would prefer? No, I’m going to find a way to make the best of the world I’m in.

When dealing with this situation, there are two separate and equally important things to consider: exception safety, and error handling. Exception safety describes the situation when you are, for example, acquiring and releasing resources (such as file handles). You want to be sure you release the resource, even if the exception system is going to abort your computation unceremoniously. You can never know if this will happen or not, since the runtime can just throw things at you, you always need to wrap resource acquisition/release paths in some exception safety. There are a lot of complex issues here, but it’s not the subject of this post so suffice to say the main pattern of interest for dealing with this is bracket.

Error handling is totally different. This is where you want to be able to recover from possible recoverable errors and do something sensible. Retry, save the task for later, alert the user that their request failed, read from cache when the network is down, whatever.

The first move in this area that I saw that I liked, was the errors package. Many helpers for dealing with error values, and in earlier versions a helper that would exclude unrecoverable errors and convert the rest to error values. I liked this pattern, but wanted more. This is Haskell! I wanted to know, at a type level, when recoverable errors had already been handled. Of course, programmer errors in pure code and unrecoverable errors from the runtime are always possible, so we can’t say anything about them at the type level, but recoverable errors we could know something about. So I wrote a package, iterated a few times, and eventually became a dependency for the helper in the errors package that I had based my whole idea on. Until very recently, errors and unexceptionalio were the two ways I was aware of to handle recoverable errors (and only recoverable errors) reliably, and know at a type level that you had done so. Recently errors decided to change the semantic to fit the previously-misleading documentation of the helper and so unexceptionalio now stands alone (to my knowledge) in this area.

In light of this new reality, I updated the package to make it much more clear (both in documentation and at a type level) what hole in the ecosystem this fills. I exposed the semantic in a few more ways so it can be useful even to people who don’t care about type-level error information. I also named the unrecoverable errors. Things you might want to be safe from, or maybe log and terminate a thread because of, but never recover from. For now, I call these PseudoException.

UnexceptionalIO (when used on GHC) now exposes four instances of Exception that you can use even if you have no use for the rest of the package: ExternalError (for things the runtime throws at you, asynchronously or not), ProgrammerError (for things raised from mistakes in pure code), PseudoException (includes the above and also requests for the process to exit), and SomeNonPseudoException (the negation of the above). All of these will work with the normal GHC catch mechanisms to allow you to easily separate these different uses for the exception system.

From there, the package builds up a type and typeclass with entry and exit points that ensure that values in this type contain no SomeNonPseudoException. The type is only ever used in argument position, all return types are polymorphic in the typeclass (implemented for both UIO and IO, as well as for all monad transformers in the unexceptional-trans package) so that you can use them without having to commit to UIO for your code. If you use the helpers in a program that is all based on IO still, it will catch the exceptions and continue just fine in that context.

Finally, the latest version of the package also exposes some helpers for those cases where you do want to do something with PseudoException. For exception safety, of course, there is bracket, specialized to UIO. For knowing when a thread has terminated (for any reason, success or failure) there is forkFinally, specialized to UIO and PseudoException. Finally, to make sure you don’t accidentally swallow any PseudoException when running a thread, there is fork which will ignore ThreadKilled (you assumedly did that on purpose) but otherwise rethrow PseudoException that terminate a thread to the parent thread.

This is hardly the final word in error handling, but for me, this provides enough sanity that I can handle what I need to in different applications and express my errors at a type level when I want to.

Haskell2010 Dynamic Cast

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This post is not meant to be a suggestion that you should use this code for anything. I found the exploration educational and I’m sharing because I find the results interesting. This post is a Literate Haskell file.

Programmers often mean different things when they say “cast”. One thing they sometimes mean is to be able to use a value of one type as another type, converting as possible.

We’ll use dynamic typing to allow us to check the conversions at runtime.

> module DynamicCast (DynamicCastable(..), dynamicCast, Opaque) where
> import Data.Dynamic
> import Data.Void
> import Control.Applicative
> import Control.Monad
> import Data.Typeable
> import Text.Read (readMaybe)
> import Data.Traversable (sequenceA)

But we don’t want to expose the dynamic typing outside of this module, in case people become confused and try to use a Dynamic they got from elsewhere. Really we’re just using the Dynamic as an opaque intermediate step.

> newtype Opaque = Opaque Dynamic

Types can define how they both enter and exit the intermediate representation. This both allows casting existing types to new types, but also can allow casting new types to existing types without changing the instances for those existing types.

> class (Typeable a) => DynamicCastable a where
> 	toOpaque :: a -> Opaque
> 	toOpaque = Opaque . toDyn
>
> 	fromOpaque :: Opaque -> Maybe a
> 	fromOpaque (Opaque dyn) = fromDynamic dyn

And finally the cast itself.

> dynamicCast :: (DynamicCastable a, DynamicCastable b) => a -> Maybe b
> dynamicCast = fromOpaque . toOpaque

Let’s see some examples.

We’ll say that Integer and simple lists (including String) represent themselves and define no specific conversions.

> instance DynamicCastable Integer
> instance (Typeable a) => DynamicCastable [a]

Int is represented however Integer represents itself.

Anything that can convert to Integer can convert to Int.

Any String that parses using read as an Int can also convert to an Int.

> instance DynamicCastable Int where
> 	toOpaque = toOpaque . toInteger
> 	fromOpaque o = fromInteger <$> fromOpaque o <|> (readMaybe =<< fromOpaque o)

And now

dynamicCast (1 :: Int) :: Maybe Integer
Just 1

dynamicCast (1 :: Integer) :: Maybe Int
Just 1

This is pretty obvious and boring, but perhaps it gives us confidence that this is going to work at all. Let’s try something fancier.

Void is the type with no inhabitants, so it can never be converted to.

> instance DynamicCastable Void where
> 	fromOpaque _ = Nothing

Either is represented as just the item it contains, and any item can be contained in an Either.

> instance (DynamicCastable a, DynamicCastable b) => DynamicCastable (Either a b) where
> 	toOpaque (Left x) = toOpaque x
> 	toOpaque (Right x) = toOpaque x
>
> 	fromOpaque o = Left <$> fromOpaque o <|> Right <$> fromOpaque o

And now

dynamicCast 1 :: Maybe (Either Int Void)
Just (Left 1)

dynamicCast 1 :: Maybe (Either Void Int)
Just (Right 1)

dynamicCast (Left 1 :: Either Int Void) :: Maybe Int
Just 1

Maybe is very similar, store the Just as the unwrapped value, and store Nothing as Void.

> instance (DynamicCastable a) => DynamicCastable (Maybe a) where
> 	toOpaque (Just x) = toOpaque x
> 	toOpaque Nothing = toOpaque (undefined :: Void)
>
> 	fromOpaque = fmap Just . fromOpaque

dynamicCast (Left 1 :: Either Int Void) :: Maybe (Maybe Int)
Just (Just 1)

To be able to cast the contents of a Functor, the possible failure also lives in the Functor, so we need a wrapper.

> newtype FunctorCast f a = FunctorCast (f (Maybe a))

> mkFunctorCast :: (Functor f) => f a -> FunctorCast f a
> mkFunctorCast = FunctorCast . fmap Just

> runFunctorCast :: FunctorCast f a -> f (Maybe a)
> runFunctorCast (FunctorCast x) = x

> runTraversableFunctorCast :: (Traversable f) => Maybe (FunctorCast f a) -> Maybe (f a)
> runTraversableFunctorCast = join . fmap (sequenceA . runFunctorCast)

> instance (Functor f, DynamicCastable a, Typeable f) => DynamicCastable (FunctorCast f a) where
> 	toOpaque = Opaque . toDyn . fmap toOpaque . runFunctorCast
> 	fromOpaque (Opaque dyn) = FunctorCast . fmap fromOpaque <$> fromDynamic dyn

runTraversableFunctorCast $ dynamicCast (mkFunctorCast ["1"]) :: Maybe [Either Void Integer]
Just [Right 1]

Thoughts After #ccsummit

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This year I again attended the Creative Commons Global Summit. There were many great sessions and participants both last year and this year, but I’ve had this growing feeling I can’t shake.

My emotions were best summed up by a conversation I had with a friend just after the conference had ended. “How was your conference?” he asked. “Oh, it was good.” I replied. “Lots of Free Culture?” he asked. There was an awkward pause, “Uh… not really.” “Yeah,” he said, “Free Culture is dead. We lost.”

That’s not to say the the causes of Libraries, Museums, Open Access, Open Science, and Open Data are not good. It’s not to say the conference wasn’t full of many noble things that I support. But Free Culture is just not on anyone’s agenda.

I’ve been saying for awhile that it seems like the Free Culture community has lost leadership and momentum, that there is no rally point, no meeting place. But the truth may be that this is just the symptom of having moved on.

I think there are a couple of reasons for this. One is that the Free Culture movement was always born out of a desire to access *existing* cultural resources, and not to replace them. This means that Free Culture advocates are more likely to spend effort advocating for Fair Dealing / Fair Use, exceptions to copyright, and “balance” rather than promoting Free Culture artists and works. There has also just been more success with education. No work of Free Culture (unless you stretch to include Wikipedia) has had any mainstream cultural impact. The Open Access and OER movements by contrast are changing the face of education around the world. It’s also just a function of students and others who were heavily involved maturing and losing the free time they had to spend on movement activities.

But what could it mean for Free Culture to be “dead”? According to last year’s State of the Commons report, there are on the order of 780 million Free Culture works on the Internet, and the number is growing daily. The licenses are alive and well, and while this report may over-count some things (due to user error, etc, when marking the license of a work) it seems like there is a lot of Free Culture out there. I think, however, that the movement has stalled. Free Culture is just an option in a dropdown on a hosting platform now.

Some might see this as a sign of success. At one point, no one had heard of Free Culture or Creative Commons, now many major hosting platforms offer the licenses as an option. I think it depends on what you see as the goal, and what you think of as success.

For me, right now, short-term success is getting to the point where at least one body of Free Cultural work (needs to be more than a standalone work, probably, to have staying power) achieves mainstream cultural impact. Where I could mention this body of work to someone totally unrelated to the movement and they would have at least a halfway chance of having heard of it.

Awhile ago now the main Vlogbrothers channel went CC-BY. This is certainly a body of work with a reasonable amount of success, and so is very good progress in this area.

However, my favourite to support for this remains Pepper & Carrot a born-free, community-supported webcomic by David Revoy. Revoy is a Free Culture and Free Software supported, and he really seems to understand why this sort of thing is necessary, and how to go about it. He not only embraces, but encourages and promotes all kinds of derivative works both non-commercial and commercial in nature. I think this may be a real shot to have a somewhat-well-known, ongoing franchise with only loose central management. I will continue to do what I can, and continue to remain hopeful.

If you want to connect with like-minded Free Culture enthusiasts, I recommend joining the WIFO Forum.

Awesome Voicemail for Android

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Most phone carriers these days have pretty poor voicemail capabilities. You have to call in to “check” your voicemail, your voicemail box can fill up, and voicemails just play in order without any good way to see what you want.

There have been some advancements in so-called “visual voicemail”, but they are not available to all of us. In this post I will show you how to set up an amazing voicemail for your Android-powered device that only requires your carrier support call forwarding (a pretty standard feature, these days). You will be able to get your voicemails instantly (or, if you don’t have cell data, whenever you have wifi), see immediately who they are from and keep them sorted by the sender, and also read a text approximation of the content so that you don’t always have to listen to it!

All services and technologies referenced in this post are Free Software or Open standards.

You’ll need a Jabber (also called XMPP) account

If you already know you have one, and know what your Jabber ID is, you can skip to the Movim login below. For everyone else, I suggest you register with Movim. Do this part from your computer (not from your phone or tablet). If you click that link, you’ll see a screen like this:

Register with Movim

Fill it in. Your Jabber ID will be whatever username you choose, plus “@movim.eu”. Don’t forget your Jabber ID or password! Push “Create” and you’ll see a page like this:

After Registering with Movim

Click the link that I have circled in red to be taken to the Movim login page, which looks like this:

Movim Login

Enter your full Jabber ID (username you selected followed by “@movim.eu”) and password. Congratulations, you are a part of the Jabber network!

Leave the logged-in Movim tab open while you do the next part.

Get a phone number with the voicemail service

The way this voicemail is going to work, you’re going to set call forwarding to another number. So you’ll get a number with the voicemail service that is in your local area so that the forwarding is a “local call”. The numbers cost $2.99 USD/month right now, but the first 30 days is free so you can try it without risk or handing over your credit card or anything like that.

Head to cheogram.com and you’ll see something like this:

Cheogram

If one of the displayed phone numbers is in your local area, you can click that. Otherwise click the “…” and you can search by area code for a local number. Write down the number you select, you will need it later. Once you have selected a number, you will see a screen like this:

JMP Register: fill out JID

Fill in your Jabber ID as shown and click “Submit” to continue. The registration process will send you a verification code as a message. So head back to your logged-in Movim tab.

Movim Chats Screen

If necessary, click the icon I’ve circled in red to go to the “Chats” screen. Then, select the conversation I’ve circled in blue to get your verification code. It will look like this:

Movim chat showing JMP verification code

That part I’ve circled in red is your verification code. Cut-and-paste that back into the registration process in the other tab, like so:

Entering the verification code for JMP registration

Press submit, and scroll all the way to the bottom of the next page. Here you will fill out and verify your real phone number (the one people use to call your cell phone already). We will use this later to record your voicemail greeting. The filled in page should look like this:

Set JMP forwarding number

When you press “Submit” you will receive a phone call at the phone number you entered. A voice will read you a verification code, which you must type into the form that will look like this:

JMP verification code from phone call

Press submit, and you’re done this part!

Set up the voicemail

Head back to your logged-in Movim tab:

Back in Movim, Cheogram wants to talk

If necessary, click the icon I’ve circled in red to go to your “Contacts” screen, then click the green checkmark I’ve circled in blue to allow cheogram.com to talk to you.

Click the cheogram contact

Then, click your newly-added Cheogram contact, which I have circled in red.

Select the "configure calls" action

Now we want to configure the call behaviour of your new number to always go to voicemail. So select the “Configure Calls” action, which I have circled above in red.

Set it to go to voicemail after 0 seconds

We want it to go to voicemail right away, so set it to 0 seconds and press “Submit”.

Message showing configuration has been saved

You should see a message indicating the configuration has been saved, as above. You can just click “Close” on this message.

Select the "record voicemail greeting" action

Your voicemail is working now, but the greeting on the mailbox will be a robot voice. Probably you want to record a greeting in your own voice, so click the “Record Voicemail Greeting” action, which I’ve circled in red above. You will receive a call on your phone, and a message will tell you to say your desired greeting at the beep. When you are done with your greeting, hang up, and it will be set automatically.

You can just close this one

Once you’ve recorded a greeting, just press “Close” to get rid of the notice on your screen. If you want to change your greeting at any time, just select the action again.

Setting up call forwarding

Of course, you need to actually set up your phone so that your carrier will forward calls you don’t answer to the voicemail service. Every Android version is slightly different, but I’ll walk you through a generic process and it should be fairly similar on your device.

Go to Android call settings

Go to your dialler app, tap the three-dots menu in the top right, and select “Settings”. This may also be under “Call Settings” in your global device settings menu.

Android dialler settings screen

If necessary, tap “Calls” to go into call settings.

Tap "Call forwarding"

Tap “Call forwarding” to go to forwarding-specific settings.

Call forwarding settings

There are often various call forwarding settings available, set everything (except for “Always forward”, you don’t want that) to the phone number you selected from the voicemail service, which you wrote down earlier.

Getting Voicemails on the Go

Your voicemail is all set up and working now! But, probably you want to be notified of new voicemails on your cell phone, and not through the Movim web interface on your computer. So you’ll need a Jabber app for your phone. I suggest getting Conversations which you can get for a couple bucks from Google Play, Amazon Apps, or F-Droid.

Conversations first launch screen

When you first start Conversations, it may ask if you want to create a new account. You already have an account, so choose “Use my own provider” which I have circled in red.

Convensations Login

You will then see a login screen, very similar to the Movim login screen. Enter your full Jabber ID (remember: your username plus “@movim.eu”) and password, then tap “Next”.

That’s it! You’re all set up to receive your amazing new voicemails directly to your Android phone!

Holiday Greeting Freedom

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This year I’ve been working on a new project for cultural freedom. I have adapted several freely-licensed characters and visual art for use on greeting cards. Starting with greeting cards targeting Christmas and the winter holidays generally my plan is to take orders throughout October so that cards can be printed and shipped to people in November (in plenty of time to mail them out to the intended recipients before December 25th!)

I have cards representing art from several different artists in the commons, including David Revoy’s Pepper & Carrot comic, Nina Paley’s Mimi and Eunice, Piti Yindee’s Wuffle, and the Blender Institute’s Caminandes (specifically the most recent wintery episode). This is a unique opportunity to support these artists and help Free Culture get more awareness.

Because this art is all found in the commons, I can produce these cards without needing any special contract with the original creators. No extra legal work or permission needed, I can produce these on the strength of the Creative Commons Attribution and Zero licenses of the sources. However, because I believe in supporting the work that enabled me, all revenue above expenses will be donated directly to the original creators (I’m not even keeping a cut for myself).

So hurry to get in your order, either with a greeting you like, or you can order a blank version if you prefer to write everything yourself.