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Tutorial: 14:30-16:00 (English)

Andres Löh

A Tutorial on Liquid Haskell

Liquid Haskell is a preprocessor for GHC (the primary Haskell compiler) that allows the programmer to equip a program with more sophisticated type annotations. An automatic external theorem prover such as Z3 is then used to prove the program type-correct with respect to these extra annotations.

The language of types supported by Liquid Haskell is based on refinement types, and allows types to be restricted to certain values or properties of their values. For example, we can define the type of integers between 0 and 10, or the type of lists that have a particular length. We can establish pre- and postconditions of functions using refinement types, potentially proving rather sophisticated properties.

In this tutorial, I will introduce Liquid Haskell, explain how it works, and what can (and cannot) be expected of it. We will look at a number of examples and see how they can be equipped with more precise types than Haskell would normally allow.

Required knowledge

Familiarity with basic Haskell will be very helpful. But the Haskell code we will look at in the tutorial will all be very straight-forward.

In particular, the tutorial does not expect any previous knowledge of:

  • type system extensions of Haskell (such as GADTs or type families),
  • refinement types or Liquid Haskell itself,
  • theorem provers or proof assistants.

Software requirements

We will need GHC as a Haskell compiler, as well as Liquid Haskell, which in turn requires a theorem prover (we will use Z3). We will probably use version 8.2.2 of GHC. As Liquid Haskell itself is in very active development, I will communicate the version of Liquid Haskell that we will use about a week before the conference.

Andres Löh

Andres Löh is a Haskell consultant and co-owner of Well-Typed LLP. He is based in Regensburg, Germany. He started using Haskell in 1997, when being an undergraduate student of mathematics in Konstanz, and has been an enthusiastic functional programmer ever since. Andres obtained a PhD in Computer Science from Utrecht University in 2004, on extending the Haskell language with capabilities for datatype-generic programming. After having been a university lecturer for several years, he joined Well-Typed in 2010.

Andres is very interested in applying functional programming to real-world problems, and in particular in datatype-generic programming, domain-specific languages, (dependent) type systems, parallel and concurrent programming, and the theory of version control.