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Doctest v. HSpec v. HTF v. other?

Haskell on Reddit - Sun, 12/07/2014 - 10:11pm

What is your test system of choice and for what reasons? Bonus points i you can point to an "ELI5"-like tutorial on your system of choice. ;-)

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Categories: Incoming News

What is an intermediate haskell programmer?

Haskell on Reddit - Sun, 12/07/2014 - 8:01pm

What do you think and intermediate haskell programmer knows? If you are beyond intermediate, what did you know at that moment? Could you provide examples of intermediate haskell code?

Thanks in advance.

Categories: Incoming News

Oliver Charles: 24 Days of GHC Extensions: Type Operators

Planet Haskell - Sun, 12/07/2014 - 6:00pm

As GHC gradually adopts more and more extensions that allow us to do dependently typed programming in Haskell, it’s natural that we’d like be more expressive at the type level. Today, we’ll look at the type operators, which allows us to use more natural syntax when we write our types.

The extension itself is straight forward. As the documentation mentions, ordinarily an operator doesn’t mean anything special when used in types - GHC would interpret operators as type variables, not particularly useful! However, now that we’ve got some support for type level literals, it’s desirable to concisely type out computations using infix operators (for example, addition and multiplication of natural numbers).

Recently, I was working with a little templating engine using a variant of data types a la carte. I wanted to be able to write templated emails, where I can compose emails from text fragments and variables that need to be filled in later. However, not all emails have the same variables - a registration email doesn’t have the same variables as a booking confirmation email. To work with this, I decided to use data types a la carte to represent open data types.

If you’re new to this method of programming, we’re essentially breaking apart a data type into a combination of functors. For my email templating, I use the following:

data I a = I { unI :: a } data Var a x = Var { unK :: a }

I is the identity functor - we use this to insert text literals. Var is a constant functor - it ignores the data under the functor, instead choosing a variable as a placeholder. The last remaining piece of the puzzle is the ability to combine literal text with placeholders. We can do this with one more functor - the sum functor:

data Sum f g a = InL (f a) | InR (g a)

Armed with this, we are free to write our emails:

data UserVar = UserName | UserEmail email :: [Sum (Var UserVar) I Text] email = [ InR (I "Dear "), InL (Var UserName), ", thank you for your recent email to Santa & Santa Inc."]

However, the type of email templates becomes messier when we have multiple variables:

data ChristmasVar = ChristmasPresent email :: [Sum (Sum (Var UserVar) (Var ChristmasVar)) I Text] email = ...

What is… Lisp?! Instead, we can define an infix operator, and the type becomes a little more readable:

email :: [(Var UserVar + Var ChristmasVar + I) String] email = [ "Dear " , var UserName , ", thank you for your recent email to Santa & Santa Inc." , "You have asked for a: " , var ChristmasPresent ]

There’s more work that’s needed to complete the email templating, and a full code listing can be found on Github. However, hopefully you can see how a fairly obvious extension (opening up the restrictions on how we write our type signatures) can find its way into some interesting applications.

This post is part of 24 Days of GHC Extensions - for more posts like this, check out the calendar.

Categories: Offsite Blogs

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News