in Code

Prompt: README / hackage / github

Have you ever wanted to specify a computation involving some limited form of IO — like querying a database, or asking stdio — but didn’t want a computation in the IO monad, opening the entire can of worms that is arbitrary IO? Have you ever looked at complicated IO a you wrote last week at 4am and prayed that it didn’t launch missiles if you decided to execute it? Do you want to be able to run an effectful computation and explicitly say what IO it can or cannot do?

Introducing the prompt library! It’s a small little lightweight library that allows you to specify and describe computations involving forms of effects where you “ask” with a value and receive a value in return (such as a database query, etc.), but not ever care about how the effects are fulfilled — freeing you from working directly with IO.

data Foo = Foo { fooBar :: String
               , fooBaz :: Int
               } deriving Show

-- ask with a String, receive a String as an answer
promptFoo :: Prompt String String Foo
promptFoo = Foo
        <$> prompt "bar"
        <*> fmap length (prompt "baz")

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mtl is not a monad transformer library — contrary to popular conception. I believe that this commonly spread myth is due in part to some rather peculiar branding choices (the name of the library) and in part to some historical accidents (mtl was, in the distant and pre-historic past, indeed a monad transformer library).

What is mtl? It is a library of interfaces you can provide to your own types, in the form of typeclasses. It abstracts over different design patterns for different types, in the form of typeclasses. Just like Functor abstracts over “things that can be fmapped”. mtl provides typeclasses abstracting over many useful patterns that many types satisfy — patterns involving different sorts of “effects”.

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Update: This post was written by me when I was just starting to learn about type-level things in Haskell, and reflects my own inexperience at the time of writing it. I have released an update, which presents what I hope to be an introduction that is more grounded in modern Haskell and dependent type idioms.

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Nothing too crazy today, just a cute (basic/intermediate) haskell trick as a response to Mark Dominus’s excellent Universe of Discourse post on Easy exhaustive search with the list monad intended for people new or unfamiliar with haskell demonstrating the common “list monad as a constraint solver” approach that is standard fare for learning Haskell. I myself have literally done an entire series of blog posts on this usage.

Mark’s use case however incorporates a bit of an extra pattern not typically discussed. The list monad is good for taking “independent samples” of things (looking at different samples from a list):

ghci> do x <- "abc"
         y <- "abc"
         z <- "abc"
         return [x,y,z]
["aaa","aab","aac","aba","abb" ... ]

However, what if you wanted to instead “draw” from a pool, and represent different drawings? Traditionally, the answer was something like:

ghci> do x <- "abc"
         y <- filter (/= x) "abc"
         z <- filter (/= y) . filter (/= x) $ "abc"
         return [x,y,z]
"abc","acb","bac","bca","cab","cba"]

This is a little bit awkward…and it definitely gets a lot worse (\(O(n^2)\)) when you have more items. Also, it relies on an Eq constraint — what if our thing doesn’t have an Eq instance? And this also falls apart when our list contains duplicate items. If we had used "aabc" instead of "abc", the result would be the same — despite having more 'a's to pick from!

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Continuing along with All About Auto, let’s look at another exciting and useful application of the auto library: GUI’s. We’re going to look at the canonical “hello world” of GUI apps these days — the todo app. We’re going to be using the specs of todoMVC to build a todoMVC “candidate” that follows the specs…and along the way see what auto offers in its tools of managing isolated state components and modeling GUI logic. We’re really going to be focusing on application logic — “control” and “model” — and not looking too close on “views”, which auto doesn’t quite try to offer and where you can really pick your own view rendering system, making this adaptable to really any platform — javascript/web, desktop, command line, etc.

A live version of our end-product is hosted and online.

This post does assume some concepts from the tutorial…if not all, then at least those in the introductory post or the README. If you ever find yourself thinking that these concepts are completely new and crazy, you might want to try looking through the tutorial or docs to refresh your mind. As always, comments are welcome, and I’m also usually on #haskell-auto as jle`, and also on twitter

(Fair warning…this is not quite a “ghcjs tutorial”, if that’s what you’re looking for; it’s an auto tutorial that uses some rudimentary ghcjs. Hopefully you can learn from that too!)

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Today we’re going to continue along with the All About Auto introduction series and look at building a declarative chatbot using the denotational components from the auto library that is modular and has implicit serialization. Most importantly, we’ll look at the “design process”, and principles of architecture that you can apply to your own projects.

This post assumes some concepts from the tutorial, or at least my last post or the README. If some of these ideas seem completely new, than looking through the tutorial or the docs might refresh your mind…feel free to also leave a comment, stop by #haskell-auto on freenode where I go by jle`, or tweet me

All of the code in this tutorial can be downloaded and run using runghc (with the appropriate dependencies installed). Feel free to play along!

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Auto: README (with examples) / hackage / tutorial / examples / github

(Before anything, maybe take a quick look at the detailed description in the README for a quick motivating example and explanation of the library)

Today I’m announcing and beginning promotion of my auto library, a denotative and locally stateful programming DSL and platform, now on hackage. auto is suitable when your program involves an input or output that is a discrete stream of things — events, views, etc., like turn based games, GUI’s, numerical computations…; it allows you to state (possibly cyclic) complex relationships between streams of values by composing simple, primitive ones. You can read the README too for a detailed buzz-word laden exposition with nice well-commented short demos and examples, get started with the tutorial, check out the directory of sample projects, and even see a live running todoMVC (source) example!

Over the next week or two I’m going to be breaking down real-world projects written on auto, and even be talking about the design processes of programs written using auto. You can follow along on the series page, follow me on twitter, or just subscribe to the rss feed feed; expect a post on designing, from start to finish,

1. A fully running chat bot
2. A GUI-based todo app on ghcjs
3. A text-based adventure game a la the classic rogue
4. A numerical computation DSL

But enough of that…what is auto, why does it exist, and what are its design principles?

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Hi! I have to apologize a bit for the long delay; starting grad school and things like that have made me have to scramble to adjust to the new life. But a couple of people have asked me to finish up and wrap up this series, and I think I owe it to them then :) Welcome to the final chapter.

In the last post, we looked deeper into the Auto type, played around with instancing it as familiar typeclasses, saw it as a member of the powerful Category and Arrow typeclasses, and took advantage of this by composing Autos both manually and using proc/do notation, and were freed from the murk and mire of explicit recursion. We observed the special nature of this composition, and saw some neat properties, like local statefulness.

At this point I consider most of the important concepts about working with Auto covered, but now, we are going to push this abstraction further, to the limits of real-world industrial usage. We’re going to be exploring mechanisms for adding effects and, making the plain ol’ Auto into something more rich and featureful. We’ll see how to express denotative and declarative compositions using recursively binded Autos, and what that even means. It’ll be a trip down several avenues to motivate and see practical Auto usage.¹ Basically, it’ll be a “final hurrah”.

A fair bit of warning — if the last post is not fresh in your mind, or you still have some holes, I recommend going back and reading through them again. This one is going to hit hard and fast :) (Also, it’s admittedly kind of long for a single post, but I didn’t want to break things up into two really short parts.)

As always, feel free to leave a comment if you have any questions, drop by freenode’s #haskell, or find me on twitter :)

All of the code in this post is available for download and to load up into ghci for playing along!

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