Hackage reliability via mirroring

Friday, 30 September 2016, by Duncan Coutts.
Filed under community.

TL;DR: Hackage now has multiple secure mirrors which can be used fully automatically by clients such as cabal.
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Sharing, Space Leaks, and Conduit and friends

Thursday, 29 September 2016, by Edsko de Vries.
Filed under coding.

TL;DR: Sharing conduit values leads to space leaks. Make sure that conduits are completely reconstructed on every call to runConduit; this implies we have to be careful not to create any (potentially large) conduit CAFs (skip to the final section “Avoiding space leaks” for some details on how to do this). Similar considerations apply to other streaming libraries and indeed any Haskell code that uses lazy data structures to drive computation.
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Haskell eXchange, Hackathon, and Courses

Thursday, 01 September 2016, by Andres Löh.
Filed under community, well-typed, training.

In October 2016, we are co-organizing various events in London. Now is the time to register for:

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Announcing MuniHac

Friday, 15 July 2016, by Andres Löh.
Filed under community, well-typed, training.

We are happy to announce


Friday, September 2 – Sunday, September 4, 2016, Munich

This Hackathon is intended for everyone who is interested to write programs in Haskell, whether beginner or expert, whether hobbyist or professional.

In the tradition of other Haskell Hackathons such as ZuriHac, HacBerlin, UHac and many more, the plan is to bring together up to a hundred of Haskell enthusiasts to work together on any Haskell-related projects they like, to share experiences, and to learn new things.

This Hackathon is organized by TNG Technology Consulting GmbH and Well-Typed LLP.

Attendance is free of charge, but there is a limited capacity, so you must register!

We are going to set up a mentor program and special events for Haskell newcomers. So if you are a Haskell beginner, you are very much welcome! And if you’re an expert, we’d appreciate if you’d be willing to spend some of your time during the Hackathon mentoring newcomers. We will ask you about this during the registration process.

We’re also planning to have a number of keynote talks at the Hackathon. We’re going to announce these soon.

We hope to see you in Munich!

Haskell development jobs with Well-Typed

Tuesday, 08 March 2016, by Andres Löh, Duncan Coutts, Adam Gundry.
Filed under well-typed.

tl;dr If you’d like a job with us, send your application as soon as possible.

We are looking for several (probably two) Haskell experts to join our team at Well-Typed. This is a great opportunity for someone who is passionate about Haskell and who is keen to improve and promote Haskell in a professional context.

About Well-Typed

We are a team of top notch Haskell experts. Founded in 2008, we were the first company dedicated to promoting the mainstream commercial use of Haskell. To achieve this aim, we help companies that are using or moving to Haskell by providing a range of services including consulting, development, training, and support and improvement of the Haskell development tools. We work with a wide range of clients, from tiny startups to well-known multinationals. We have established a track record of technical excellence and satisfied customers.

Our company has a strong engineering culture. All our managers and decision makers are themselves Haskell developers. Most of us have an academic background and we are not afraid to apply proper computer science to customers’ problems, particularly the fruits of FP and PL research.

We are a self-funded company so we are not beholden to external investors and can concentrate on the interests of our clients, our staff and the Haskell community.

About the jobs

One of the roles is for a specific project with one of our clients, and requires work on-site in London. The other role is more general and not tied to a single specific project or task, and allows remote work.

Please indicate in your application whether on-site work in London is an option for you.

In general, work for Well-Typed could cover any of the projects and activities that we are involved in as a company. The work may involve:

We try wherever possible to arrange tasks within our team to suit peoples’ preferences and to rotate to provide variety and interest.

Well-Typed has a variety of clients. For some we do proprietary Haskell development and consulting. For others, much of the work involves open-source development and cooperating with the rest of the Haskell community: the commercial, open-source and academic users.

Our ideal candidate has excellent knowledge of Haskell, whether from industry, academia or personal interest. Familiarity with other languages, low-level programming and good software engineering practices are also useful. Good organisation and ability to manage your own time and reliably meet deadlines is important. You should also have good communication skills. Being interested or having experience in teaching Haskell (or other technical topics) is a bonus. Experience of consulting or running a business is also a bonus. You are likely to have a bachelor’s degree or higher in computer science or a related field, although this isn’t a requirement.

Offer details

The offer is initially for one year full time, with the intention of a long term arrangement. For the remote role, living in England is not required. For the on-site role, you have to be allowed to work in England. We may be able to offer either employment or sub-contracting, depending on the jurisdiction in which you live.

If you are interested, please apply via info@well-typed.com. Tell us why you are interested and why you would be a good fit for Well-Typed, and attach your CV. Please indicate whether the on-site work in London is an option for you. Please also indicate how soon you might be able to start.

We are more than happy to answer informal enquiries. Contact Duncan Coutts (duncan@well-typed.com, dcoutts on IRC), Adam Gundry (adam@well-typed.com, agundry on IRC) or Andres Löh (andres@well-typed.com, kosmikus on IRC) for further information.

We will consider applications as soon as we receive them, and will try to fill the positions as soon as possible. In any case, please try to get your application to us by March 27, 2016.

Efficient Amortised and Real-Time Queues in Haskell

Friday, 15 January 2016, by Edsko de Vries.
Filed under coding.

A queue is a datastructure that provides efficient—O(1)—operations to remove an element from the front of the queue and to insert an element at the rear of the queue. In this blog post we will discuss how we can take advantage of laziness to implement such queues in Haskell, both with amortised and with worst-case O(1) bounds.

The results in this blog post are not new, and can be found in Chris Okasaki’s book “Purely Functional Data Structures”. However, the implementation and presentation here is different from Okasaki’s. In particular, the technique we use for real-time datastructures is more explicit and should scale to datastructures other than queues more easily than Okasaki’s.

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Haskell courses in New York City

Tuesday, 12 January 2016, by Adam Gundry.
Filed under community, training, well-typed.

Well-Typed is happy to announce that once again we will be running Haskell training courses in New York alongside

C◦mp◦se conference

Thursday, February 4 – Sunday, February 7, 2016, New York City

This conference is focused on typed functional programming and features a keynote by Eugenia Cheng and an excellent line-up of talks including one by our own Austin Seipp on Cryptography and verification with Cryptol. There’s also an “unconference” with small workshops and tutorials as well as the opportunity to get your hands dirty and try things out yourself.

For several years now, we have been running successful Haskell courses in collaboration in Skills Matter. The New York courses will be taught by Duncan Coutts, co-founder and partner at Well-Typed. He’s an experienced teacher and is involved in lots of commercial Haskell development projects at Well-Typed.

You can participate in our Haskell courses directly before or directly after C◦mp◦se in February, or if that doesn’t suit we are running two of the courses in London this April:

Fast Track to Haskell

Tuesday, February 2 – Wednesday, February 3, 2016, New York City
(and Monday, April 4 – Tuesday, April 5, 2016, London)

Find out more or register here.

This course is for developers who want to learn about functional programming in general or Haskell in particular. It introduces important concepts such as algebraic datatypes, pattern matching, type inference, polymorphism, higher-order functions, explicit effects and, of course, monads and provides a compact tour with lots of hands-on exercises that provide a solid foundation for further adventures into Haskell or functional programming.

Guide to Haskell Performance and Optimization

Monday, February 8 – Tuesday, February 9, 2016, New York City
(and Wednesday, April 6 – Thursday, April 7, 2016, London)

Find out more or register here.

This brand-new course looks under the surface of Haskell at how things are implemented, including how to reason about performance and optimize your programs, so that you can write beautiful programs that scale. It covers the internal representation of data on the heap, what exactly lazy evaluation means and how it works, how the compiler translates Haskell code to a target language via several internal representations, what you can and cannot reasonably expect the compiler to do, and how you can tweak the optimizer behaviour by using compiler pragmas such as inlining annotations and rewrite rules.

Guide to the Haskell Type System

Wednesday, February 10, 2016, New York City

Find out more or register here.

This course is a one-day introduction to various type-system extensions that GHC offers, such as GADTs, rank-n polymorphism, type families and more. It assumes some familiarity with Haskell. It does not make use of any other advanced Haskell concepts except for the ones it introduces, so it is in principle possible to follow this course directly after Fast Track. However, as this course focuses on the extreme aspects of Haskell’s type system, it is particularly recommended for participants who are enthusiastic about static types and perhaps familiar with a strong static type system from another language.

Well-Typed training courses

In general, our courses are very practical, but don’t shy away from theory where necessary. Our teachers are all active Haskell developers with not just training experience, but active development experience as well. In addition to the courses in New York, we regularly offer courses in London.

We also provide on-site training on requests nearly anywhere in the world. If you want to know more about our training or have any feedback or questions, have a look at our dedicated training page or just drop us a mail.

Dependently typed servers in Servant

Wednesday, 02 December 2015, by Edsko de Vries, Andres Löh.
Filed under coding.

Suppose we have a webserver that can perform different kinds of operations on different kinds of values. Perhaps it can reverse or capitalize strings, and increment or negate integers:

# curl http://localhost:8081/example/reverse

# curl http://localhost:8081/example/caps

# curl http://localhost:8081/1234/inc

# curl http://localhost:8081/1234/neg

Moreover, it can echo back any value:

# curl http://localhost:8081/example/echo

# curl http://localhost:8081/1234/echo

However, it does not make sense to try to reverse an integer or increment a string; requesting either of


should result in HTTP error.

This is an example of a dependently typed server: the value that we are given as input determines the type of the rest of the server. If we get a string as input, we expect a string operation as the second argument and the response is also a string; if we get an integer as input, we expect an integer operation as the second argument and the response is also an integer.

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Implementing a minimal version of haskell-servant

Wednesday, 04 November 2015, by Andres Löh.
Filed under coding.

Recently, there was a question on Stack Overflow on how Servant actually works. Others were quick to suggest the Servant paper as a thorough explanation of the approach and the implementation.

As a co-author, I’m obviously happy if the paper is being read, but it’s also 12 pages long in two-column ACM style. And while it explains the implementation, it does not necessarily make it easier to start playing with the code yourself, because it only shows excerpts, and code snippets in a paper are not easily runnable.

At the same time, whenever I want to demonstrate a new concept in the Servant context, or play with new ideas, I find myself not impementing it in the main Servant code base, but rather to create a small library that is “like Servant”, built on the same principles, but much simpler, so that I have less work to do and can evaluate the ideas more quickly. I’ve talked to some other contributors, and at least some of them are doing the same. So I thought it might be useful to develop and present the code of “TinyServant”, which is not exactly tiny, but still small compared to the full Servant code base, strips away a lot of duplication and unessential extras, but is still complete enough so that we can observe how Servant works. Obviously, this still won’t explain everything that one might want to know about the implementation of Servant, but I hope that it will serve as a useful ingredient in that process.

This blog post is a somewhat revised and expanded version of my Stack Overflow answer.

This is not a general tutorial on Servant and using Servant. For learning how to use Servant, the official Servant tutorial or the general documentation section of the Servant home page are better starting points.

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Haskell Hackathon, Haskell eXchange, Haskell courses in London, October 2015

Friday, 11 September 2015, by Andres Löh.
Filed under well-typed, community, training.

Haskell events in London

In the time from 5–13 October, we are (co-)organizing a number of Haskell-related events in London, with Skills Matter.

Here’s the overview:

Haskell infrastructure Hackathon

We’ll co-organize and participate in a two-day Haskell Hackathon, which takes place directly after the Haskell eXchange.

This Hackathon aims at bringing together Haskell developers – both beginners and experts – who want to help improve the Haskell infrastructure, predominantly Hackage and Cabal.

We’ll aim to arrange some introductory overview talks, to e.g. provide an overview over the code bases and the most important open issues.

Participation is free, but please register via Skills Matter.

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