A standard library for Haskell

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The rio library

A standard library for Haskell


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NOTE This code is currently in prerelease status, and has been released as a tech preview. A number of us are actively working on improving the project and getting it to a useful first release. For more information, see the description of goals and the issue tracker for discussions. If you're reading this file anywhere but Github, you should probably read the Github version instead, which will be more up to date.

The goal of the rio library is to make it easier to adopt Haskell for writing production software. It is intended as a cross between:

  • Collection of well designed, trusted libraries
  • Useful Prelude replacement
  • A set of best practices for writing production quality Haskell code

You're free to use any subset of functionality desired in your project. This README will guide you through using rio to its fullest extent.

Standard library

While GHC ships with a base library, as well as a number of other common packages like directory and transformers, there are large gaps in functionality provided by these libraries. This choice for a more minimalistic base is by design, but it leads to some unfortunate consequences:

  • For a given task, it's often unclear which is the right library to use
  • When writing libraries, there is often concern about adding dependencies to any libraries outside of base, due to creating a heavier dependency footprint
  • By avoiding adding dependencies, many libraries end up reimplementing the same functionality, often with incompatible types and type classes, leading to difficulty using libraries together

This library attempts to define a standard library for Haskell. One immediate response may be XKCD #927:

XKCD Standards

To counter that effect, this library takes a specific approach: it reuses existing, commonly used libraries. Instead of defining an incompatible Map type, for instance, we standardize on the commonly used one from the containers library and reexport it from this library.

This library attempts to define a set of libraries as "standard," meaning they are recommended for use, and should be encouraged as dependencies for other libraries. It does this by depending on these libraries itself, and reexporting their types and functions for easy use.

Beyond the ecosystem effects we hope to achieve, this will hopefully make the user story much easier. For a new user or team trying to get started, there is an easy library to depend upon for a large percentage of common functionality.

See the dependencies of this package to see the list of packages considered standard. The primary interfaces of each of these packages is exposed from this library via a RIO.-prefixed module reexporting its interface.

Prelude replacement

The RIO module works as a prelude replacement, providing more functionality and types out of the box than the standard prelude (such as common data types like ByteString and Text), as well as removing common "gotchas", like partial functions and lazy I/O. The guiding principle here is:

  • If something is safe to use in general and has no expected naming conflicts, expose it from RIO
  • If something should not always be used, or has naming conflicts, expose it from another module in the RIO. hierarchy.

Best practices

Below is a set of best practices we recommend following. You're obviously free to take any, all, or none of this. Over time, these will probably develop into much more extensive docs. Some of these design decisions will be catered to by choices in the rio library.

For Haskellers looking for a set of best practices to follow: you've come to the right place!

Import practices

This library is intended to provide a fully loaded set of basic functionality. You should:

  • Enable the NoImplicitPrelude language extension (see below)
  • Add import RIO as your replacement prelude in all modules
  • Use the RIO.-prefixed modules as necessary, imported using the recommended qualified names in the modules themselves. For example, import qualified RIO.ByteString as B. See the module documentation for more information.

TODO In the future, we may have editor integration or external tooling to help with import management. Also, see project template comments below.

Language extensions

Very few projects these days use bare-bones Haskell 98 or 2010. Instead, almost all codebases enable some set of additional language extensions. Below is a list of extensions we recommend as a good default, in that these are:

  • Well accepted in the community
  • Cause little to no code breakage versus leaving them off
  • Are generally considered safe

Our recommended defaults are:


Notes on some surprising choices:

  • RecordWildCards is really up for debate. It's widely used, but rightfully considered by many to be dangerous. Open question about what we do with it.
  • Despite the fact that OverloadedStrings can break existing code, we recommend its usage to encourage avoidance of the String data type. Also, for new code, the risk of breakage is much lower.

TODO Do we recommend setting in package.yaml or in the source files themselves? Need to discuss and come to a conclusion on this point

There are other language extensions which are perfectly fine to use as well, but are not recommended to be turned on by default:


GHC Options

We recommend using these GHC complier warning flags on all projects, to catch problems that might otherwise go overlooked:

  • -Wall
  • -Wcompat
  • -Wincomplete-record-updates
  • -Wincomplete-uni-patterns
  • -Wredundant-constraints

You may add them per file, or to your package.yaml, or pass them on the command line when running ghc. We plan to add these to the package.yaml of our project template, once its ready.

For code targeting production use, you should also use the flag that turns all warnings into errors, to force you to resolve the warnings before you ship your code:

  • -Werror

Further reading: Alexis King explains why these are a good idea in her blog post which was the original inspiration for this section.


A primary design choice you'll need to make in your code is how to structure your monads. There are many options out there, with various trade-offs. Instead of going through all of the debates, we're going to point to an existing blog post, and here just give recommendations.

  • If your code is going to perform I/O: it should live in the RIO monad. RIO is "reader IO." It's the same as ReaderT env IO, but includes some helper functions in this library and leads to nicer type signatures and error messages.

  • If you need to provide access to specific data to a function, do it via a typeclass constraint on the env, not via a concrete env. For example, this is bad:

    myFunction :: RIO Config Foo

    This is good:

    class HasConfig env where
      configL :: Lens' env Config -- more on this in a moment
    myFunction :: HasConfig env => RIO env Foo

    Reason: by using typeclass constraints on the environment, we can easily compose multiple functions together and collect up the constraints, which wouldn't be possible with concrete environments. We could go more general with mtl-style typeclasses, like MonadReader or MonadHasConfig, but RIO is a perfect balance point in the composability/concreteness space (see blog post above for more details).

  • When defining Has-style typeclasses for the environments, we use lenses (which are exposed by RIO) because it provides for easy composability. We also leverage superclasses wherever possible. As an example of how this works in practice:

    -- Defined in RIO.Logger
    class HasLogFunc env where
      logFuncL :: Lens' env LogFunc
    class HasConfig env where
      configL :: Lens' env Config
    instance HasConfig Config where
      configL = id
    data Env = Env { envLogFunc :: !LogFunc, envConfig :: !Config }
    class (HasLogFunc env, HasConfig env) => HasEnv Env where
      envL :: Lens' env Env
    instance HasLogFunc Env where
      logFuncL = lens envLogFunc (\x y -> x { envLogFunc = y })
    instance HasConfig Env where
      configL = lens envConfig (\x y -> x { envConfig = y })
    instance HasEnv Env where
      envL = id
    -- And then, at some other part of the code
    data SuperEnv = SuperEnv { seEnv :: !Env, seOtherStuff :: !OtherStuff }
    instance HasLogFunc SuperEnv where
      logFuncL = envL.logFuncL
    instance HasConfig SuperEnv where
      configL = envL.configL
    instance HasEnv SuperEnv where
      envL = lens seEnv (\x y -> x { seEnv = y })

    TODO Open question: how do we decide when we use a Lens' versus just a SimpleGetter in these Has typeclasses?

  • If you're writing code that you want to be usable outside of RIO for some reason, you should stick to the good mtl-style typeclasses: MonadReader, MonadIO, MonadUnliftIO, MonadThrow, and PrimMonad. It's better to use MonadReader+Has than to create new typeclasses like MonadLogger, though usually just sticking with the simpler RIO env is fine (and can easily be converted to the more general form with liftRIO). You should avoid using the following typeclasses (intentionally not exposed from this library): MonadBase, MonadBaseControl, MonadCatch, and MonadMask.


For in-depth discussion, see exceptions best practices. The basic idea is:

  • If something can fail, and you want people to deal with that failure every time (e.g., lookup), then return a Maybe or Either value.
  • If the use will sometimes not want to deal with it, then use exceptions. In the case of pure code, use a MonadThrow constraint. In the case of IO code: use runtime exceptions via throwIO (works in the RIO monad too).
  • You'll be upset and frustrated that you don't know exactly how some IO action can fail. Accept that pain, live with it, internalize it, use tryAny, and move on. It's the price we pay for async exceptions.
  • Do all resource allocations with functions like bracket and finally.

Strict data fields

Make data fields strict by default, unless you have a good reason to do otherwise.

Project template

TODO In the future, we'll add a new Stack template for using this library. We'll use hpack, not cabal files, and rely on automatic exposed-module discovery.

Safety first

This library intentionally puts safety first, and therefore avoids promoting partial functions and lazy I/O. If you think you need lazy I/O: you need a streaming data library like conduit instead.

TODO Decide if we include a streaming data solution out of the box.

When to generalize

A common question in Haskell code is when should you generalize. Here are some simple guidelines. For parametric polymorphism: almost always generalize, it makes your type signatures more informative and functions more useful. In other words, reverse :: [a] -> [a] is far better than reverse :: [Int] -> [Int].

When it comes to typeclasses: the story is more nuanced. For typeclasses provided by RIO, like Foldable or Traversable, it's generally a good thing to generalize to them when possible. The real question is defining your own typeclasses. As a general rule: avoid doing so as long as possible. And if you define a typeclass: make sure its usage can't lead to accidental bugs by allowing you to swap in types you didn't expect.

TODO Expand, clarify, examples.

Coding style

TODO Point to coding style guidelines, and discuss hindent.

Module hierarchy

The RIO.Prelude. module hierarchy contains identifiers which are reexported by the RIO module. The reason for this is to make it easier to view the generated Haddocks. The RIO module itself is intended to be imported unqualified, with NoImplicitPrelude enabled. All other modules are not reexported by the RIO module, and will document inside of them whether they should be imported qualified or unqualified.



NOTE All releases beginning with 0.0 are considered experimental. Caveat emptor!

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