Research, create, improve and maintain programming languages and their tooling to enhance developer productivity and to deliver reliable, maintainable, correct and performant software with minimum effort.

Internship report: Extensions to the reflection mechanism of Liquid Haskell to handle functions from dependencies.

A summary of the open-source work done by the Programming Languages & Compilers team at Tweag in the second quarter of 2024.

A tale of optimising GHC by improving sharing in its intermediate representation.

An experience report on plugin upgrades

This blog post presents a safe API for destination-passing style programming using Linear Haskell against arbitrary functional data types.

How GHC's design informs linear types

Use monad-bayes and rhine in your interactive machine learning application

A GHC pattern for improving the user experience of type-level programs

A revamp of the mechanism for introducing assumptions

A simple trick to fuzz Haskell programs with libFuzzer

halide-haskell is a Haskell wrapper for Halide that allows you write and just-in-time compile high-performance array and image processing kernels.

Translations of control flow can be tested before the rest of the code generator is ready. The tests examine sequences of abstract state transformations and observations.

Embark with the High Assurance Software Group for a guided tour through the stages of a smart-contract audit, secret weapon included.

Using type-level dependency tags to verify a Haskell pipeline.

How to build state machines in a composable way, controlling which transitions can be executed

How linear constraints get rid of scope functions and why it matters.

Announcement of smtlib-backends, a Haskell library providing a generic interface for interacting with SMT solvers using SMT-LIB

An experience report on improving HLS during my internship at Tweag.

How a linear constraint arrow relates to freezing arrays in Haskell.

Learning and adopting Haskell is not hard with the right resources at hand.

How we gained 2–3% improvements in compile times by making GHC better at printing code.

A practical introduction to using Sparkle to write Haskell programs which interface with Delta Lake.

Announcing the GHC WebAssembly backend – present and future

Introduction to staged programming in Haskell with typeclasses

How to tap into the power of reinforcement learning while specifying and executing open games in Haskell.

Understanding the basic ideas of functional programming by writing a program using only sets and functions

Bazel avoids superfluous recompilation for free, but we can make it even better.

Exploration of a regression in GHC's bytecode generator performance

Painlessly switch your Haskell build from Cabal to Bazel

My experience improving monad-bayes, the probabilistic programming language package, as a Tweag fellow.

On a previous post, we explained how to write tests for stateful systems using traces — sequences of stateful actions — that can be combined and modified to write complex test cases easily and transparently. This post elaborates on the combinators used to generate new traces from…

Limitations of the GHC profiler and announcement of the timestats library

New features to understand old verification failures

Announcing the new haskell_module rule in rules_haskell

Integrating Haskell Stack and Nix in a smooth yet non-invasive way, with an explanation, example and template.

Arnaud takes a step back from the slogans and examines the trade-offs between lazy and strict languages.

A presentation of an alternative encoding for optics such as lenses and prisms using existential types.

Announcement of a WebAuthn Haskell library, and a description of how the protocol works

Use records to structure your Servant APIs

A tutorial on how to write a type-checking plugin for GHC

Why formatting chains of infix operators is hard in Haskell. And how I addressed it for Ormolu.

How to automatically inject attacks and faults in safety tests using random traces.

On the relevance of Liquid Haskell in programming languages

How to transparently separate resource allocation from business logic for scoped effects in Polysemy

In part two of this series on type-checking plugins, we will see how GHC's constraint solver works.

A collection of ways various Tweagers develop in Haskell

Small tutorial on using valgrind to find hot spots in ghc and other Haskell programs

An experience report on polishing Ormolu, the Haskell code formatter, during my internship at Tweag.

Why would you want to write a type-checking plugin? Many limitations of the GHC type-checker are exposed here.

Property test a denotational model of a data structure.

Introducing a library for writing data pipelines which compose well and fail early

Notes from the sixth day of ICFP

Notes from the fifth day of ICFP

Notes from the fourth day of ICFP

Notes from the third day of ICFP

Notes from the second day of ICFP

Notes from the first day of ICFP

Announcing the gazelle_cabal tool for automatic Bazel rules generation for Cabal projects

What should the type of linear traverse be exactly?

Deconstructing the various uses of type classes.

Announcing asciidoc-hs, a new AsciiDoc parser and converter in Haskell, adapted for use with Pandoc.

Tweag is looking for interns who want to contribute to Ormolu.

How to use lenses and other optics within an Arrow-based framework.

Define effect interpreters dynamically using reflection

Incubating the Haskell Foundation during 2020.

Discussion on using Bazel with inline-java

Announcing linear-base, a standard library for linearly typed Haskell programs.

A call to action to improve GHC's performance via a paid internship

How to break encapsulation and import a hidden value in Haskell.

An applicative functor to define shrinkers

Why Template Haskell is hard for cross compilation and how Asterius does it

Destinations give you control over your memory allocations, and linear types make them pure.

Just a few days ago, Cloudflare and Stack builders published a excellent blog post where they demonstrate how to deploy a Haskell program to Cloudflare Worker with Asterius. In this post we go one step further and show that you can deploy full-fledged servant applications just as…

Summary of the Tweag open source fellowship work related to ghcide

Bazel gains support for creating fully statically linked Haskell binaries with rules_haskell and rules_nixpkgs.

Announcement of the upcoming QualifiedDo language extension.

How we ensured that the implementation of random v1.2 produces higher quality random numbers than v1.1.

Looking back at the journey which brought us there, and forward to what still lies ahead.

Benchmarks of inline-java and a discussion of its performance trade-offs.

How to build a complete Haskell project with Bazel.

How to derive instances via arbitrary isomorphisms.

Imprecise, synchronous, asynchronous exceptions: what do they mean, and what to do with them.

This blog post describes the tradeoffs of the choice between eager and lazy instantiation of type variables in GHC.

During my internship, I implemented the explicit specificity proposal to make GHC type signatures complete.

Peirce divides reasoning into three complementary processes: deduction, abduction and induction. In this post, I will show how these logical processes relate to software.

In this blog post series, we're going to lead you through Bayesian modeling in Haskell with the monad-bayes library. In the third part of the series, we setup a simple Bayesian neural network.

A primer on the interaction between linear types and exceptions in Haskell.

In this post about inline-java I aim to walk you through the upcoming safe interface of the library, which allows detecting memory management mistakes at compile time using linear types.

This post describes a profiling technique for Haskell codebases that yields faithful results and is well adapted to large repositories.

Haskell's Data and Control module hierarchies have always bugged me. Now, I understand that the intuition behind the Data/Control separation is rooted in a deep technical justification.

Asterius compiles Haskell code into WebAssembly code to be executed in a browser or in Node.js. It has reached a new milestone by being able to compile the diagrams library and its dependencies.

Here's Part 2 in Tweag's Series about Bayesian modeling in Haskell with the monad-bayes library.

We're happy to announce the first release of Porcupine, an open source framework to express portable and customizable data pipelines.

We're happy to announce the first release of Ormolu, a formatter for Haskell source code.

Bazel gets native support for third-party Haskell libraries and building Cabal packages since the 0.10 release of rules_haskell.

I got the opportunity to work on Bazel's Persistent Worker Mode for GHC during my internship at Tweag. My goal was to improve the mode of communication between Bazel and the Haskell GHC compiler.

In this blog post series, we're going to lead you through Bayesian modeling in Haskell with the monad-bayes library. In the first part of the series, we introduce two fundamental concepts of `monad-bayes`: `sampling` and `scoring`.

I got the opportunity to work on Asterius, a new Haskell to WebAssembly compiler, during my internship at Tweag. My task was to get anything numerics-related stabilized in its compiled code.

People keep asking me, “What's the point of Haskell, why do you folks even use it?”. How can you answer this?

Edsger Dijkstra took issue with the “unbridled” use of the programming construct in 1968. He noted that our already limited ability to reason about the dynamic behaviour of imperative programs was nigh impossible in the presence of -jumps to arbitrary labels. With , control flow…

If we think of the average Haskeller, who do we see? A poor, broken person. Tired head falls into the open palms, sobbing, deprived of the basic human right for automated source code formatting. Is it at all conceivable that so many attempts were made and none quite succeeded…

Tweag.io has a bit of a history with language interop. By this point, we created or collaborated with others in the community on HaskellR, inline-c, inline-java, and now inline-js. The original idea for this style of interop was realized in language-c-inline by Manuel Chakravarty…

There has been recent online discussions about impredicative encoding (aka final encoding) of free monads. Capabilities-as-type-classes, as embodied by the capability library, are essentially the same thing.

Haskell is in a way a unique language. It is, to be clear, a language people and businesses use for building “serious stuff”, yet it is also a language which remains a platform for experimentation with functional programming. This means that the teams which develop in Haskell…

Introduction Haskell and data science - on first sight a great match: native function composition, lazy evaluation, fast execution times, and lots of code checks. These sound like ingredients for scalable, production-ready data transformation pipelines. What is missing then? Why…

Getting your browser to print “Hello World!” is fun, and a milestone for compiler writers targeting the web. Soon expectations change. Can you call existing code? Can you write a moderately complex web app? How fast does it run? Previously, we showed how Haskell can call…

About a year ago, Michael Snoyman made a blog post about the ReaderT pattern. Ostensibly, it’s a blog post about preferring to encode state as -s when you can. But that’s not how we read it. What we saw first and foremost is a story about using extensional type classes describing…

Note: since Mar 19, 2020, we’ve changed the JavaScript import syntax: the -th argument is now instead of . The code snippets in this post have been adjusted accordingly. Previously, we announced the Asterius compiler, a new GHC-backend that translates Haskell to WebAssembly…

Funflow is a workflow management tool. It turns out that workflow management tools and build tools are closely related. So if you’re more familiar with the latter, this post might be of interest to you. We’ll be illustrating some of Funflow’s features by implementing Make on top…

In an earlier blog post, we discussed how streaming libraries help writing composable programs without lazy I/O. We showed, for a simple program, how using a streaming library helps reducing the amount of unaided bookkeeping that the programmer needs to do in order to make it…

Asterius is an experimental WebAssembly backend for GHC. We aim to support most GHC features while making the emitted WebAssembly code as small and fast as possible. The project recently reached an exciting milestone: for the first time, simple Haskell functions can be compiled…

Funflow is a system for building and running workflows. It’s a system we’ve been working on for the last few months with a small number of clients. In this blog post we’ll talk about what workflows are, why we built Funflow, and what we’d like to do with it in the future. What is…

We are happy to announce that Stackage HEAD is now functional. For those who had missed the original blog post, or already forgot it, let’s refresh the idea behind Stackage HEAD. Each Stackage Nightly is a build plan that includes a large set of real-world packages that are known…

In the “all about reflection” post, we introduced the data type. Any sorting function should have the type . However, this doesn’t actually guarantee that is indeed a sorting function: it only says that is a sorted list, but not that it is actually a sort of . For example is…

Publishing code to the world is easy these days: take your code, tack some extra metadata onto it, call that a package and upload it to npmjs.com / crates.io / Hackage / etc. It’s also easy for other developers to include your code as part of their project: build tools are…

The utility of free monads can show up in surprising places. One of the projects I’m working on is an AI, and part of the strategy that it uses for responding to user input is quite simple: it generates many possible responses, and then evaluates them. Most of the computations it…

An important device in the tool belt I carry around everyday is type class reflection. I don’t reach for it often, but it can be very useful. Reflection is a little known device. And for some reason it is often spoken of with a hint of fear. In this post, I want to convince you…

Foreign function interfaces (FFI) allow fast interop between languages. Unlike other approaches, like performing RPC calls between different components written in different languages, using the FFI allows for all manner of data to be shared between each language runtime, in the…

This is the fifth post in a series about array programming in Haskell — you might be interested in the first, second, third, and fourth, too. A recurring theme in array programming is performance. After all, many algorithms in numerical computing and data science are…

If Haskell was a god, often would he be depicted with the ravens Modularity and Abstraction flying above him, hovering the world and reporting to him every detail of our whereabouts. Haskell would sit on the Throne of Purity and look upon the world with an eye full of wisdom. And…

A recent development in Haskell land is the formation of the GHC DevOps Group, which was the topic of last week’s blog post. The group is a community of parties committed to the future of GHC. Tweag I/O is one such party. We are helping the group achieve its goals with concrete…

As Haskell is increasingly used in production environments, the Haskell toolchain is of critical importance to a growing number of people and organisations. At the heart of this toolchain is GHC (The Glasgow Haskell Compiler). Conceived nearly thirty years ago as a research…

This is the fourth post in a series about array programming in Haskell — you might be interested in the first, second, and third, too. In the previous post of this series, we explored the basic, down-to-earth, index-based array interfaces that have their roots in the Haskell…

In our first blog post about streaming, we discussed how streaming libraries strengthen types to catch more errors. However, when we strengthen types, we need to be careful to not hinder program composition. After all, precise type information can make it more onerous to convince…

This is the third post in a series about array programming in Haskell — you might be interested in the first and second, too. In the previous post of this series, we discussed commonly used vector and matrix routines, which are available in highly-optimised implementations in…

Previously, we discussed how to use inline-java to call any Java function from Haskell. The reverse is also possible, though that will be a topic for a future post. In this post, we’ll peek underneath the hood to talk a little about how inline-java does its deed. You might find…

Our introductory post for inline-java showed that we could call methods written in Java (or indeed any JVM function) from Haskell. Much in the style of other packages, it is moreover possible to do using java syntax, so examples from Java API documentation can be reused as-is. In…

This is the second post in a series about array programming in Haskell — you might be interested in the previous post, too. Matrices are the bread and butter of most scientific and numeric computing. It is not surprising then that there is a range of standard libraries and…

We saw last time that with linear types, we could precisely capture the state of sockets in their types. In this post, I want to use the same idea of tracking states in types, but applied to a more unusual example from our paper: sending rich structured data types across the…

Haskell is an awesome language, but we need to remember that it is not very useful in isolation. In almost any realistic application, Haskell has to coexist with other languages, even if only to call existing C libraries or to make use of operating system services. In actual…

Array programming in Haskell is attractive, but it is also somewhat confusing. Haskell’s functional syntax is close to the mathematical notation that is often used to develop and explain the algorithms on which array programs are based, which leads to great clarity of expression…

At the time of our first post on linear types, we were fresh out of the design phase to extend GHC with linear types. We had a prototype implementation, but it was just a proof of concept: there was precious little you could do with it. A few months down the line, we are now in a…

In school, we’re taught that I/O is a simple affair: read data in, write data out. Rinse, repeat. But then as practitioners we realize matters are often more complicated. For one, I/O is slow, so we probably want operations to overlap (i.e. be processed asynchronously…

A guest post by Sigrid Keydana telling us the backstory behind the very cool trading app notebook she put together… Keras, ggplot2, Haskell and R all in one Jupyter notebook! Post originally appeared here. Earlier today, I presented at UseR! 2017 about HaskellR: a great piece of…

We’re extending GHC with linear types. Ever since Jean-Yves Girard discovered linear logic in 1986, researchers around the world have been going “wow! resource tracking, this must be useful for programming languages”. After all, any real computation on a real machine takes…

By now, Haskell has first class support for seamlessly embedding foreign code into source files and casually call anything in C (via inline-c) or R (via inline-r), let alone that whole programs can also be compiled down to in-browser JavaScript, thanks to GHCJS. Today the…

Maintaining a compute cluster for batch or stream data processing is hard work. Connecting it up to storage facilities and time sharing resources across multiple demands even more so. Fortunately cloud service providers these days typically upscale their offering to not just…

Large scale distributed applications are complex: there are effects at scale that matter far more than when your application is basked in the warmth of a single machine. Messages between any two processes may or may not make it to their final destination. If reading from a memory…

Stack and Cabal are great tools for incremental development. But before even starting to think about what your first commit will be about on a freshly checked out project, what you have to do is get a working development environment, by which we mean: see if it builds! Stack…

Most good data stories start with a interesting question. If the average request latency went down by a further 100ms, by how much could we expect user engagement to increase? How can we detect evidence of corruption of government officials given a list of all bids nationwide for…