FP Complete

Imperative programming is in my bloodstream. I’ve been a C++ programmer for most of my life. I wrote a book about C++. I helped Andrei and Walter design an imperative language D. If I dabbled in functional programming, it was mostly to make my imperative programs better.

Over the years I also developed a real passion for concurrent programming. I wrote blogs and recorded tutorials about concurrency support in C++11. I went to conferences and read academic papers. I proposed an ownership system for D, which didn’t fly because it required far reaching extensions to the type system and looked painful to use.

There is no doubt in my mind, and most experts agree, that concurrency and parallelism are the future of programming. The key word here is “future.” The hardware is already there — even phones use multicore processors with powerful GPUs. Hardware vendors are desperate for programmers to harness this power. Yet very little is happening in the world of software.

I have recently attended the Multicore DevCon in San Jose, which was part of a big DesignWest Expo. I witnessed tremendous progress in hardware since previous years. Hardware is forging ahead without any signs of slowdown. Year after year more cores and more GPUs are packed into single chips. This is in stark contrast with the situation in software. One could argue that software engineering is actually moving backwards in that area.

Remember the times when progress in software was driven by adding abstraction layers between programming languages and the details of processor architectures? From flipping binary switches to assembly, to FORTRAN and C. Then there was the object-oriented revolution. Programs were written in terms of objects, often with domain-specific names like Student or Account. Java even let go of pointers and explicit memory management. These were the good times. Enter parallel programming…

I may surprise you that the state of the art in parallel programming is OpenMP and OpenCL. At least this is the consensus among panelists who discuss the future of multicore programming at various conferences I attend. To remind you, OpenMP is a system of pragmas that you insert into a program to make parts of it parallel. OpenCL is a low-level language to communicate with GPUs (there’s also CUDA, in case OpenCL is not low enough for you).

But maybe this is just a temporary state of affairs and there is ongoing work to ratchet the levels of abstraction back to where they’d been. Sure, there’s the Chapel language that nicely separates concerns about how to parallelize or distribute a computation from the algorithms required by the problem domain. There are also higher level libraries like Intel TBB (Threading Building Blocks) and Microsoft PPL (Parallel Pattern Library). GPU programming is being addressed by Microsoft C++AMP extensions.

They are all failing because of one problem — data races.

Programmers are scared of concurrency, and rightly so. Managers are scared of concurrency even more. If programmers were electricians, parallel programmers would be bomb disposal experts. Both cut wires. Except that, when the latter cut the wrong wire, mayhem ensues. We make mistakes in coding and we have systems for debugging and testing programs — there is no such system for concurrency bugs. I should know because I worked for a company that made state of the art data race detector. But this detector couldn’t prove that a program was 100% data-race free. There was always a chance that a data race could sneak into a released product and strike at the most inconvenient moment. The company’s web site had a list of major disasters caused by data races. Among them was the famous Therac-25 disaster that killed three patients and injured several others, and the Northeast Blackout of 2003 that affected 55 million people. The fears are well founded!

So why are data races so much harder to detect and fix than regular bugs? It all has to do with non-determinism. Every time a multi-threaded program is run, its threads may interleave differently. The number of possible interleavings is astronomical. If your program has a data race, it usually manifests itself only in a very small subset of possible interleavings. To discover a bug, two threads must access the same memory location at exactly the same time, and one of the threads must modify that location. I have written programs with deliberate data races and I couldn’t trigger buggy behavior even with extensive testing.

Here’s the key insight:

Imperative programs will always be vulnerable to data races because they contain mutable variables.

Did you notice that in the definition of a data race there’s always talk of mutation? Any number of threads may read a memory location without synchronization but if even one of them mutates it, you have a race. That’s why the majority of imperative programmers with their mutable variables will always be reluctant to embrace concurrent programming. And that is the downfall of imperative programming.

So what’s the solution? As Sherlock Holmes famously said, “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” Enter functional programming.

Here’s the second key insight:

There are no data races in purely functional languages because they don’t have mutable variables.

All data is immutable. All functions are pure. You might think this is crazy — how can you program with such stifling restrictions? It turns out that people have been doing just that for a long time. In fact the most popular language for parallel and distributed programming is Erlang — a functional language. An even better candidate for parallel programming is Haskell, which supports a large variety of parallel paradigms.

If you think you can stay away from functional programming, you’re mistaken. Imperative languages are incorporating more and more of the functional style, exactly because of the pressure from the multicore world. In C++ you already have lambdas and higher order functions in the guise of STL algorithms. C# has delegates, Java is introducing closures, D supports immutable objects and pure functions; the list goes on and on. This is all good, but not enough. If you are just mixing functional elements into a predominantly imperative program, the specter of data races will always be lurking in the shadows. You may be a very disciplined expert concurrent programmer and still, a less knowledgeable member of your team may break your code and nothing in the language is going to prevent it. No whistles will blow, no bells will ring.

So here’s the situation: Sooner or later you’ll have to face the multicore reality. You will be forced to learn functional methods to the extent to which your imperative language supports them. Despite that, data races will infest your code and leak into released products. So you might as well take a shortcut and embrace a full blown functional language now.

How hard is functional programming? It definitely requires some getting used to. You’ll have to learn to replace loops with recursion, to map and fold your lists, traverse data structures without iterators, do input/output using monads, and many other exciting things. All these techniques have value on their own. As programmers we constantly learn new tricks, and functional programming is just another bag of tricks.

Originally there wasn’t much interest in FP outside of academic circles because there was no functional killer app. Now we know that this killer app is concurrency. For a functional programmer there is no barrier to concurrency, parallelism, or GPU programming.

After dedicating many years of my life to C++ and imperative languages, I decided to switch to functional programming and try to convince as many programmers as I can to make the same decision.


  1. Miran Lipovača, Learn You a Haskell for Great Good! — an excellent Haskell tutorial
  2. Manuel Chakravarty, Data Parallelism in Haskell — a video presentation.
  3. Data Parallel Haskell — a collection of papers.
  4. Repa — REgular PArallel arrays.

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