/wiki/haskell/libraries

Good Haskell Libraries

The Haskell library ecosystem these days is incredibly rich, and I think many good libraries are overlooked. This is my list of favourites, either because they’re my favourite solution to a class of problem (e.g., streaming, effect systems), or because they are elegant and overlooked.

Alternative Preludes

• If writing a library, I never use an alternative prelude. Alternative preludes reorganise instead of providing new features, almost by definition, so the risk/reward payoff for the additional dependency just isn’t there. Every dependency your library takes on is forced on all downstream users, and is a potential bottleneck if anything happens to its maintainer.

• For applications, I still generally prefer base. Of the alternative preludes, I think that relude is the best. It pulls in no additional non-boot dependencies of its own, and provides quite a few things to make the boring stuff a little nicer. Some examples:

  • Imports of common type names like Map and HashMap;
  • Partial functions are hidden;
  • Helpers for non-empty data types, like viaNonEmpty :: (NonEmpty a -> b) -> [a] -> Maybe b;
  • (<<$>>) = fmap . fmap; and
  • usingReaderT = flip runReaderT

  I disagree with the recommendation to fiddle with mixins; I’d rather set default-extensions: NoImplicitPrelude and explicitly import Relude in each file to keep things clear.

Developer Ergonomics

• placeholder is a well-designed TODO library, which provides “placeholders” that raise warnings for the programmer to come back and fill out. Unlike the todo provided by relude, it also provides a pattern synonym which can be handy.

• safe-wild-cards uses Template Haskell to provide an alternative to RecordWildCards-style pattern matches, that generate warnings if any bindings are unused.

Data Structures

• semialign provides a function align :: Semialign f => f a -> f b -> f (These a b), where These a b is an “a or b or both” data type. This is useful in a lot of places: zipping lists but keeping the tail of the longer list, zipping Maps together, checking for coincidnet FRP Events, and more.

• witherable generalises mapMaybe from lists to a large class of data structues.

Enumerating Complex Structures

• finitary is “class Enum done right”. It provides a Finitary a typeclass that exposes the cardinality of a at the type level, witnesses the bijection between a and Finite (Cardinality a), and provides safe next/previous/enumeration functions for a type a.

• If finitary’s GPL-3.0-or-later licence is unacceptable, its bounds are not up-to-date, or you need to enumerate a data type with infinitely many members, the universe can be a handy lightweight alternative.

Error Handling

• hoist-error is a small combinator library for lifting the “failure case” of Maybe and Either values into the “error part” of a MonadError e m. This tends to make the “happy path” through a function much clearer, cut down on “boring case matching”, and reduce >>= either (throwError . MyError) pure noise.

• There are a few libraries that provide a Validation type that’s isomorphic to Either, but gives up the Monad instance to provide an Applicative instance that “accumulates the errors”. I recommend validation-selective for its excellent documentation and a good Semigroup instance. instance (Semigroup e, Semigroup a) => Semigroup (Validation e a) will (<>) failures with failures and successes with successes.

• If you want to accumulate errors but also need a Monad instance, consider monad-chronicle. However, some things to be cautious about:

  • When used as an applicative, ChronicleT accumulates fewer errors than a Validation applicative, because ChronicleT’s Applicative instance must agree with its Monad instance.

  • ChronicleT shares many of the same laziness concerns as naive WriterT, and can build up large thunks in its first type parameter (the “error” one).

Text Manipulation

• formatting provides an expressive DSL for formatting text. Unlike Text.Printf, formatting is type-safe.

• safe-coloured-text is a cross-platform text-colouring library.

Stateful Stuff

• ref-tf lets you write functions over any monad which provides mutable references, which means you can provide an interface that works in IO as well as ST s.

• StateVar provides a typeclass for “reference-like” values, backed by arbitrary I/O. This is occasionally extremely useful for providing interfaces to fundamentally stateful things, like the OpenGL 1.1 API.

Graph Stuff

• algebraic-graphs is an extremely elegant graph library built atop a Functional Pearl.

• search-algorithms is a delightful little package which provides classic algorithms like BFS, DFS, Dijkstra’s, and A*. It doesn’t care what data structure you use for your graph, as it takes the next state relation and edge cost information as function parameters.

Streaming

• streaming is my favourite streaming library. You might imagine an effectful stream of a over a Monad m as:

  data Stream a m r
    = Step !a (Stream a m r)
    | Effect (m (Stream a m r))
    | Return r

  streaming generalises the a to an arbitrary functor, and actually uses the following two main types:

  data Of a b = !a :> b deriving Functor
  data Stream f m r
    = Step !(f (Stream f m r))
    | Effect (m (Stream f m r))
    | Return r

  Most of the time, you work with Stream (Of a) m r, but the functor parameter enables things that seem much harder to write in conduit or pipes. If I’m connecting a source directly to a sink, I usually stick with whatever streaming library underlies other libraries I’m using, which often means using conduit.