Types and Tailcalls

Why I love Haskell

published on July 19, 2013

I really like Haskell. But why? In this post I try to articulate why I like this language so much and why I think it would be worth most programmers’ while to learn it. None of this is new, but I want to express my own view in my own words. I write this in part to clarify my thinking and in part to improve my ability to articulate these points.

Many of my arguments apply to other strongly typed functional programming languages. I just happen to know Haskell much better than any of the other ones and thus I am focussing on Haskell.

Purity is good for you1

When I first started to learn Haskell purity2 seemed like a crazy concept. Programming by modifying some kind of state and interacting with the ‘real world’ is so ingrained in mainstream programming languages that it is hard to imagine how you can get anything done without it. Ironically, after having programmed in Haskell for a while, impure languages now make my toenails curl. What do you mean, I have to look at the function body to find out if it writes to a file, makes calls over the network, or talks to the database? That is insane!

The beauty of Haskell’s way of dealing with pure and impure functions is that you can tell from the type signature what a function is and isn’t allowed to do. You can still do all those impure calls-over-the-network and array-update-in-place things that regular languages allow you to do, but you have to tell everyone about it in your function’s type signature.

This, and the Haskell community, pushes you to make your program as pure as possible. I have found that this has huge benefits: Pure functions are easy to understand: just by the name and type signature it is usually obvious what a function does. When you do inspect a function’s body you don’t have to keep track of what state the world is in right now (or all the states that it could theoretically be in), its behavior only depends on your input parameters. As a result, this makes programs (large parts of which are now pure functions) much easier to understand. In fact, I find that one of the hardest things about understanding programs is to keep track of state. This burden is hugely reduced in the pure part of your code.

Purity makes testing a breeze. Most of the time the hard part about writing tests is to construct the state where the tests should run, verify it was changed in the intended ways, and clean up afterwards. Worse, the code you want to test could depend on a global state that you can’t modify easily, such as the system time or the random number generator. If you have pure functions, you just pass in the arguments and check the result. Testing becomes much easier.

Pure functions simplify conceptualizing and designing programs. In functional programming you have two concepts: functions and data. The two are orthogonal in their purpose. The functions modify your data until you get to the result. I find that conceptualizing and reasoning about data that gets successively modified by a number of (pure) functions is much easier than doing the equivalent with an object oriented approach. It is just very difficult to mentally enumerate all the possible states that a number of different objects could be in at any given time. But understanding all possible states is necessary to draw conclusions about the behavior of a program.

Have you ever tried to reuse some piece of code and found to your dismay that it reads or writes stuff from or to disk, uses the system time function in a way that is now inappropriate, talks over the network, prints to the console or pops up dialogs? These things are hugely inconvenient and often inhibit reuse. State just does not compose well and having the ability to modify state tends to create implicit dependencies. If you have a pure function, you can call it anywhere you want!

You can, of course, try to reap most of those benefits by discipline, e.g. by trying to minimize state whenever possible and to make as many functions pure as possible. In fact, doing that is probably the way in which Haskell has most changed how I write code in other languages. I now strive to minimize the amount of code that causes side effects wherever possible. Alas, I find that the type system really makes a difference here: when you have to inspect each and every function to find out (or recall) if it is pure or not, the benefit of writing pure functions is significantly reduced. I also find that other languages are lacking the cultural aspect of seeing state as bad and striving for purity. So you have ‘time()’, ‘random()’ and calls to the outside world all over the place. It might seem easier at first, but more often than not, it will come back to bite you. Purity has had such a profound impact on me that I cannot imagine that I will ever call a language that does not have control over effects in the type system my favorite programming language.

Terse syntax

Haskell is a pretty terse language. When you compare the amount of code you have to write to get typical tasks done it is usually right up there with Python, Ruby, Clojure and the likes. And this is the case even though Haskell has a strong static type system which in mainstream languages is often associated with verbosity. I do not consider syntax to be all that important, but not having to read or write huge amounts of boilerplate is definitely a plus.

Believe the Type!3

Haskell is often lauded for its strong type system and rightly so. But what does having a strong type system mean? Why is Haskell’s type system stronger than Java’s, C++’s or that of other languages that are considered to have a strong static type system?

More precision

Haskell’s type system is stronger than that of many other languages because it is more precise: you can distinguish more things based on their type. A prime example of this is the null reference or pointer present in many languages. A null pointer is fundamentally type-unsafe: If the pointer is null most operations that you could usually do with the pointer will be invalid, and the compiler is going to do zilch about you doing it in error. Many people, including its inventor,4 consider null references a mistake, yet they persist in all mainstream languages.

This is an example where more type safety actively eliminates a class of errors which are hard to get rid off in other languages. In general, the more things that can be expressed and checked at compile time, the more errors you will be able to rule out before ever running your program.

Sum types

The previous example with null pointers is actually part of a bigger concept which is usually referred to as sum types or tagged unions. The idea is that values of a certain type can have several forms, based on what state they are in. Most languages don’t have support for sum types, yet they are a very powerful concept. They allow you to rule out many things that should be impossible at the type level, giving you more precise types.

As an example, imagine you want to keep track of the state of a financial transaction. If the transaction has been initialized you have the init time, the amount, and a currency. If the transaction has succeeded we furthermore have a succeeded time stamp, and an authorization code. If it has failed it has a failed time stamp, and a failure reason, but no auth code. In a language without sum types this might be represented like this:

struct Amount
  int amount;
  string currency;

struct Transaction
  TransactionState state;
  Timestamp initTime;
  Amount amount;
  Timestamp failedTime;
  string failedReason;
  Timestamp succeededTime;
  string authCode;

It is of course possible to infer from the field names, to some degree, which field should be used when, but it is nonetheless not entirely clear. Does a failed transaction have an amount? Or an auth code? Instead consider the following definition in Haskell:

data Amount = Amount { amount :: Int, currency :: String }

data Transaction =
     InitialTransaction    { itInitTime :: Timestamp
                           , itAmount :: Amount }
   | FailedTransaction     { ftInitTime :: Timestamp
                           , ftAmount :: Amount
                           , failedReason :: String }
   | SuccessfulTransaction { stInitTime :: Timestamp
                           , stAmount :: Amount
                           , authCode :: String }

The Haskell representation of the transaction can have one of three states each of which carries the data relevant for this state (the example also reveals one of Haskell’s weak points, namely that the accessors of a record cannot have the same name). This clearly communicates to any reader of the code what state must be present in each case and what is not going to be present. What is more, in a function using this data type, the compiler will check that you handled all the cases and you can’t even write a program that accesses data that is not available in a case. If you later have to add a case, the compiler will inform you of all the places where this case is not handled yet. This hugely helps you avoiding bugs.

Compiler checked documentation

More precise types also communicate more to other programmers. Have you ever worked with code where you and your cocoders had the mutual understanding that a certain pointer or reference can never be null? That certain keys must be present in a map or that some string field in a struct will always be null if conditions X and Y hold? The problem with this knowledge is that it is implicit. Implicit knowledge is not enforced in interfaces, at best a comment will document it. Unfortunately comments are notorious for not being kept up to date with the code. Since it is not verified and does not affect the resulting program, it is all too easy to let comments and code diverge. Then the comments become actively misleading and therefore many have argued to use comments as sparingly as possible.5

Yet conveying this kind of information is critically important when programming in teams. If certain invariants should hold then it is important that everyone understands this and thus it should be documented somewhere and checked somehow. What better place to do this than in the types? It clearly conveys the required information to the programmers and ’ye old compiler assures you that these invariants actually hold. If they are not, your code won’t compile and there are few incentives that work better on programmers.

Encode your invariants in the type system

All the above points really harp on the same idea: it is better to encode your invariants in the type system. Doing so will move more knowledge into an area of the code that is easily seen and the compiler will be your alley in maintaining those invariants. A more powerful type system will allow you to do more of this, which is, all other things being equal, good.

Of course, there is always the question of tradeoffs. Dynamically typed languages are popular for some reason after all: a type system can get in the way. Yet I find that (after a learning period) Haskell’s type system does so surprisingly little and that its power is far too large to give it up for the occasional inconvenience.

The right kind of abstractions

Another magical thing about Haskell is that its creators and its community just come up with the right, often mathematically inspired, abstractions. That can mean that things are a bit abstract at times but that is part of the beauty.

What is a good abstraction? To me it is a small, useful concept that abstracts an underlying pattern from a number of concrete contexts. By that measure we can judge an abstraction by (a) how simple the concept is, and (b) the number of different contexts to which it applies. Alas (a) is often not clear until you fully grasp the abstraction, which, given its abstract nature, can be hard.

Let’s check out Haskell’s most famous abstraction the monad. A monad is an abstraction for doing things sequentially where the second thing can depend on the result of the first. It is unbelievable in how many situations it applies, for example optional values, lists, things that carry state around (e.g. random number generators), parsers, IO, software transactional memory, and lots and lots of other concepts. Note that these are quite unrelated, or did you ever think that an optional value was basically the same thing as a parser? When you understand this abstraction then you will have a really good intuition on what a certain kind of monad is supposed to do even if you have no idea how it is implemented. You’re going to think: this is a monad, so it should do this in this situation and you’re going to be right. This is the right kind of abstraction and I would kill to get more of them.

This is just one example and Haskell’s abstractions don’t stop there. In fact, many abstractions are so deceptively simple it is easy to forget about them until we compare to how this would be done in a language that doesn’t offer them. One such concept is ‘higher-order-functions’, that is functions that take other functions as arguments. This is called the ‘strategy pattern’ in OO design patterns. But in Haskell this concept is so simple that you will never think “hm, should I use the strategy pattern here?,” instead you’re breathing these things without wasting the first thought on it. That these powerful patterns become so intuitive is really important. If you have to grab the design pattern book to recall how exactly that abstract factory pattern works then you’re unnecessarily spending precious brain cycles whne you should, instead, be spending them on solving your problem. Worse, you’re not nearly as likely to come up with solutions that make heavy use of these patterns because it is much more difficult to get an intuitive understanding of them. This is necessary, however, to creatively solve problems. If you’re still struggeling with the concepts then it’s unlikely that you’ll use them to their full potential. This is why good abstractions are so imporant and Haskell has loads of them.


In his 1977 Turing Award lecture, John Backus famously asked “Can Programming Be Liberated from the van Neumann Style?”. He bemoaned the lack of abstractions inherent in imperative programming languages and layed out the vision for a functional language in which operations would combine programs to form new programs, thus raising the bar of abstraction. 36 years later, we’re not quite there yet, but I think languages like Haskell are the closest we can get at the moment. They certainly allow you to abstract from the “word-at-a-time” programming most of the time and think on a higher level.

Freeing your mind from unnecessary details lets you focus on higher order abstractions. I think the sheer number of powerful ideas that have come out of Haskell and similar languages (e.g. purity, type classes, monads, STM) indicate that Haskell makes a useful tradeoff.

The future is already here …

… it is just not very evenly distributed. Haskell has driven a lot of innovations that have slowly shown up in other languages and it is still going strong. Of course, I am not going to claim that all programming language innovation comes from Haskell. However, I think that it is fair to say that Haskell is responsible for a good chunk of new ideas and that the current bleeding edge is made up of languages that were heavily inspired by either Haskell or a member of the ML family. So if you’re the kind of person that likes to find out about the new possibilities sooner rather than later, I think you owe it to yourself to check out Haskell.

Some gripes

Is it all love and roses then? Of course not. There are some gripes I have with Haskell and eventually I think it is going to be replaced by an even better language. But Haskell has driven the state of the art forward, has allowed me to evolve my thinking, and is currently the most practical language combining a number of very innovative features. Additionally, I think of Haskell more as a set of ideas than as a specific language implementation. The languages that will eventually supersede Haskell have so much in common with it that to me they all fall into the Haskell camp. If you’re comming from the traditional imperative-OO ship then learning Haskell is probably the biggest jump you’ll have to make.

The end

So that is my super long Haskell love letter. If your interest is piqued, I encourage you to check it out. The learning curve can be a bit steep at times, but hey, that means you’re actually learning something new instead of the same old stuff dressed up slightly differently!

To get started, I think Learn you a Haskell is a great resource. The School of Haskell is also quite nice, especially if you like to experiment with things interactively. If you have difficulty understanding monads I recommend You could have invented monads (and maybe you already have). All about monads is also quite good, but more demanding. Just keep going, you will understand them eventually and then wonder why you found them difficult in the first place ;). The Haskell irc channel and the super-friendly haskell-beginners and haskell-cafe mailing lists are also worth checking out.

  1. Simon Peyton Jones: Escape from the Ivory Tower: The Haskell Journey from 1990 to 2011.

  2. In Haskell, purity means that a function must be free from side effects such as printing to the console or modifying variables.

  3. Proudly stolen from Learn you a Haskell.

  4. Tony Hoare created ALGOL W which introduced null references. He has called this his billion dollar mistake.

  5. Whatever you think of Robert Martin and Clean Code, I think this is a point that he makes persuasively.

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