Thursday, August 17, 2017

Review of Graham Hutton's Programming in Haskell, 2e

Graham Hutton's Programming in Haskell is a leading textbook on the functional programming language Haskell that first came out in 2007, and it is instructive to see how its second edition reflects what has changed over the last decade. A particular strength of the first edition is its brevity – it comes in at some 150 pages - and the second edition doubles that length. Even taking into account a less compressed layout, this probably reflects some 50% more material, which comes in the form of more examples, more exercises, some sample solutions and coverage of brand new topics.

The book is now structured into two parts: “Basic Concepts" and “Going Further". With a little rearrangement the first part is made up of chapters from the first edition (augmented as noted above) and covers the basics of functional programming in general: function definitions, recursion, types and higher-order functions; as well as more Haskell-specific aspects: list comprehensions, type classes and, of course, the GHC system. The section is rounded off with a capstone example: a “Countdown" program, that shows how Haskell is used to solve a particular concrete problem. Hutton writes very clearly throughout, and illustrates his exposition with a well-chosen set of examples.

Knowledgeable readers may well have spotted that two of the most distinctive aspects of Haskell - monads and lazy evaluation - are not included in the introductory half of the book. Hutton is not alone in doing this: other authors of Haskell texts [2] and (I should declare an interest) myself included [1] have taken the same position in concentrating first on the essential aspects of functional programming, only looking at the particulars of how it works in Haskell after that.

While some parts of the second half – lazy evaluation and reasoning about functional programs - are treated much as before, and there is an entirely new chapter on “calculating compilers" that gives readers a glimpse of Hutton's research interests, the major changes from the first edition - both pedagogical and technical - come in the second half of the book.

The first edition hardly treated monads beyond introducing the type class, and using the do notation in describing interactive programs, whereas the second edition foregrounds monads much more, and presents parsers as an extended example of monads. A change like this would have been entirely foreseeable even at the point when the first edition was written, but other updates reflect much more recent changes in the Haskell ethos. Haskell has taken on a much more explicitly “algebraic" flavour, reflected by a range of new type classes that embody patterns of computation inspired by category theory.

Just as monads can be seen to embody the idea of "effectful" (potentially side-effecting) computation, "applicatives" capture the notion of applying a pure function to effectful arguments. Similarly, "monoids" are abstract structures in which elements can be combined by a binary operation with a unit (think multiplication and 1), and these can then be "folded" over collections of values, just as a set of numbers can be multiplied together. This new material is also very clearly and concisely presented, and gives an approachable introduction to more abstract ideas.

Haskell is "growing old disgracefully": it was first defined in 1990, and for twenty years or so was seen primarily as an academic, research-oriented language. The last decade has seen it grow in popularity as a practical language for program development from startups to financial multinationals. The language has also moved on, and taken up the categorical / algebraic approach more fully, and it is good to see this reflected in Hutton's new edition and HaskellBook [2]; other authors will need to do the same.

To conclude, Hutton's revisions of Programming in Haskell have taken the best aspects of the original – conciseness, clarity and approachability – and built on this foundation to accommodate how Haskell has evolved. I recommend it highly.

[1] Simon Thompson, Haskell, The Craft of Functional Programming (3ed), Addison-Wesley, 2011.

[2] Christopher Allen, Julie Moronuki, Haskell Programming from First Principles, Gumroad, 2017.

[To appear in Computing Reviews.]