Large-scale design in Haskell?

Don gave you most of the details above, but here's my two cents from doing really nitty-gritty stateful programs like system daemons in Haskell.

1. In the end, you live in a monad transformer stack. At the bottom is IO. Above that, every major module (in the abstract sense, not the module-in-a-file sense) maps its necessary state into a layer in that stack. So if you have your database connection code hidden in a module, you write it all to be over a type MonadReader Connection m => ... -> m ... and then your database functions can always get their connection without functions from other modules having to be aware of its existence. You might end up with one layer carrying your database connection, another your configuration, a third your various semaphores and mvars for the resolution of parallelism and synchronization, another your log file handles, etc.

2. Figure out your error handling first. The greatest weakness at the moment for Haskell in larger systems is the plethora of error handling methods, including lousy ones like Maybe (which is wrong because you can't return any information on what went wrong; always use Either instead of Maybe unless you really just mean missing values). Figure out how you're going to do it first, and set up adapters from the various error handling mechanisms your libraries and other code uses into your final one. This will save you a world of grief later.

Addendum (extracted from comments; thanks to Lii & liminalisht) —
more discussion about different ways to slice a large program into monads in a stack:

Ben Kolera gives a great practical intro to this topic, and Brian Hurt discusses solutions to the problem of lifting monadic actions into your custom monad. George Wilson shows how to use mtl to write code that works with any monad that implements the required typeclasses, rather than your custom monad kind. Carlo Hamalainen has written some short, useful notes summarizing George's talk.

I talk a bit about this in Engineering Large Projects in Haskell and in the Design and Implementation of XMonad. Engineering in the large is about managing complexity. The primary code structuring mechanisms in Haskell for managing complexity are:

The type system

• Use the type system to enforce abstractions, simplifying interactions.
• Enforce key invariants via types
  • (e.g. that certain values cannot escape some scope)
  • That certain code does no IO, does not touch the disk
• Enforce safety: checked exceptions (Maybe/Either), avoid mixing concepts (Word, Int, Address)
• Good data structures (like zippers) can make some classes of testing needless, as they rule out e.g. out of bounds errors statically.

The profiler

• Provide objective evidence of your program's heap and time profiles.
• Heap profiling, in particular, is the best way to ensure no unnecessary memory use.

Purity

• Reduce complexity dramatically by removing state. Purely functional code scales, because it is compositional. All you need is the type to determine how to use some code -- it won't mysteriously break when you change some other part of the program.
• Use lots of "model/view/controller" style programming: parse external data as soon as possible into purely functional data structures, operate on those structures, then once all work is done, render/flush/serialize out. Keeps most of your code pure

Testing

• QuickCheck + Haskell Code Coverage, to ensure you are testing the things you can't check with types.
• GHC + RTS is great for seeing if you're spending too much time doing GC.
• QuickCheck can also help you identify clean, orthogonal APIs for your modules. If the properties of your code are difficult to state, they're probably too complex. Keep refactoring until you have a clean set of properties that can test your code, that compose well. Then the code is probably well designed too.

Monads for Structuring

• Monads capture key architectural designs in types (this code accesses hardware, this code is a single-user session, etc.)
• E.g. the X monad in xmonad, captures precisely the design for what state is visible to what components of the system.

Type classes and existential types

• Use type classes to provide abstraction: hide implementations behind polymorphic interfaces.

Concurrency and parallelism

• Sneak par into your program to beat the competition with easy, composable parallelism.

Refactor

• You can refactor in Haskell a lot. The types ensure your large scale changes will be safe, if you're using types wisely. This will help your codebase scale. Make sure that your refactorings will cause type errors until complete.

Use the FFI wisely

• The FFI makes it easier to play with foreign code, but that foreign code can be dangerous.
• Be very careful in assumptions about the shape of data returned.

Meta programming

• A bit of Template Haskell or generics can remove boilerplate.

Packaging and distribution

• Use Cabal. Don't roll your own build system. (EDIT: Actually you probably want to use Stack now for getting started.).
• Use Haddock for good API docs
• Tools like graphmod can show your module structures.
• Rely on the Haskell Platform versions of libraries and tools, if at all possible. It is a stable base. (EDIT: Again, these days you likely want to use Stack for getting a stable base up and running.)

Warnings

• Use -Wall to keep your code clean of smells. You might also look at Agda, Isabelle or Catch for more assurance. For lint-like checking, see the great hlint, which will suggest improvements.

With all these tools you can keep a handle on complexity, removing as many interactions between components as possible. Ideally, you have a very large base of pure code, which is really easy to maintain, since it is compositional. That's not always possible, but it is worth aiming for.

In general: decompose the logical units of your system into the smallest referentially transparent components possible, then implement them in modules. Global or local environments for sets of components (or inside components) might be mapped to monads. Use algebraic data types to describe core data structures. Share those definitions widely.

Designing large programs in Haskell is not that different from doing it in other languages. Programming in the large is about breaking your problem into manageable pieces, and how to fit those together; the implementation language is less important.

That said, in a large design it's nice to try and leverage the type system to make sure you can only fit your pieces together in a way that is correct. This might involve newtype or phantom types to make things that appear to have the same type be different.

When it comes to refactoring the code as you go along, purity is a great boon, so try to keep as much of the code as possible pure. Pure code is easy to refactor, because it has no hidden interaction with other parts of your program.

I did learn structured functional programming the first time with this book. It may not be exactly what you are looking for, but for beginners in functional programming, this may be one of the best first steps to learn to structure functional programs - independant of the scale. On all abstraction levels, the design should always have clearly arranged structures.

The Craft of Functional Programming

http://www.cs.kent.ac.uk/people/staff/sjt/craft2e/