Building a Better Custom Haskell Prelude

The Haskell Prelude is the default import into all Haskell modules, it provides an endless number of ways to shoot ourselves in the foot and historical cruft that can’t be removed. While it is difficult to fix upstream, we can however remove the Prelude entirely on a project-level and replace it with a more sensible set of defaults using the -XNoImplicitPrelude language extension.

There are two philosophies on building new Preludes:

I don’t prescribe to the large vehicle approach, a lot of the default Prelude is not ideal but good enough to get the job done. And interoperability with the rest of the ecosystem, which typically uses default Base and Prelude, is incredibly important.

In previous posts I’ve written about how rolling our own small-vehicle Prelude is a good idea for large teams working in industry but I left it somewhat ambiguous about what to include. Obviously everyone’s company is different so I thought I’d expand on what I consider a sensible set of defaults that one could use to then build a custom Prelude. We’ll call this a Protolude.

Git Project

Module Structure

The basic module structure is consists of a main module reexport which we’ll dump everything in called X. And various miscellaneous functions which we’ll reexport from the root module. Off of this we’ll have several utilities modules which have domain specific functions.

{-# LANGUAGE NoImplicitPrelude #-}

module Protolude (
  module X,

) where

import qualified Prelude as P

import qualified List as X
import qualified Show as X
import qualified Bool as X
import qualified Debug as X
import qualified Monad as X
import qualified Applicative as X

Base Types

The Basic types (Int, Integer, Float, Char) we reexport wholesale along with their associated functions.

-- Base types
import Data.Int as X
import Data.Bits as X
import Data.Word as X
import Data.Bool as X hiding (bool)
import Data.Char as X (Char)
import Data.Maybe as X hiding (fromJust)
import Data.Either as X
import Data.Complex as X

import Data.Function as X (
  , const
  , (.)
  , flip
  , fix
  , on

We define the flipped application operator for GHC < 7.10 which does not provide this be default.

(&) :: a -> (a -> b) -> b
x & f = f x

We rename the id function to identity since it’s far more likely we’ll have variable names called id over the identity function. This is a contentious point and your mileage may vary with this change.

identity :: a -> a
identity x = x

The bool function (the combinator analogue of maybe and either) is provided by GHC 7.10, but we reexport it for earlier versions.

bool :: a -> a -> Bool -> a
bool f t b = if b then t else f

The uncons function is not provided by default, but it unpacks a list into a Maybe tuple containing the head in the first tuple parameter if it exists.

uncons :: [a] -> Maybe (a, [a])
uncons []     = Nothing
uncons (x:xs) = Just (x, xs)

The applyN function takes a function and a count and applies it n number of times.

applyN :: Int -> (a -> a) -> a -> a
applyN n f = X.foldr (.) id (X.replicate n f)

And then we reexport various boolean combinators for working with branching monadic logic.

whenM :: Monad m => m Bool -> m () -> m ()
whenM p m =
  p >>= flip when m

unlessM :: Monad m => m Bool -> m () -> m ()
unlessM p m =
  p >>= flip unless m

ifM :: Monad m => m Bool -> m a -> m a -> m a
ifM p x y = p >>= \b -> if b then x else y

guardM :: MonadPlus m => m Bool -> m ()
guardM f = guard =<< f


Safe provides Maybe versions of many of the various partial functions (head, tail) that are shipped by default. Wrapping it up in a Maybe is widely considered the right approach and if Haskell were designed today, they would not be present.

-- Maybe'ized version of partial functions
import Safe as X (


Various debugging and trap commands are still provided since they are useful for partial code and various fatal program logic. However now they have a warning emitted by the compiler if they are left in place, with position information about where they are used. This can be toggled between production code and debugging code.

{-# WARNING undefined "'undefined' remains in code" #-}
undefined :: a
undefined = P.undefined

{-# WARNING error "'error' remains in code" #-}
error :: P.String -> a
error = P.error

{-# WARNING trace "'trace' remains in code" #-}
trace :: P.String -> a -> a
trace = T.trace

{-# WARNING traceShow "'traceShow' remains in code" #-}
traceShow :: P.Show a => a -> a
traceShow a = T.trace ( a) a

{-# WARNING traceShowM "'traceShowM' remains in code" #-}
traceShowM :: (P.Show a, P.Monad m) => a -> m ()
traceShowM a = T.traceM ( a)

{-# WARNING traceM "'traceM' remains in code" #-}
traceM :: P.Monad m => P.String -> m ()
traceM = T.traceM

{-# WARNING traceIO "'traceIO' remains in code" #-}
traceIO :: P.String -> P.IO ()
traceIO = T.traceIO

{-# WARNING notImplemented "'notImplemented' remains in code" #-}
notImplemented :: a
notImplemented = P.error "Not implemented"


Various either combinators to convert between Maybe and Either probably should be provided but are not, there are four various combinations of these. Handling a maybe which happens to have a monoidal instance for mempty in Left is also fairly common task.

leftToMaybe :: Either l r -> Maybe l
leftToMaybe = either Just (const Nothing)

rightToMaybe :: Either l r -> Maybe r
rightToMaybe = either (const Nothing) Just

maybeToRight :: l -> Maybe r -> Either l r
maybeToRight l = maybe (Left l) Right

maybeToLeft :: r -> Maybe l -> Either l r
maybeToLeft r = maybe (Right r) Left

maybeToEither :: Monoid b => (a -> b) -> Maybe a -> b
maybeToEither = maybe mempty


The various list functions are exported wholesale excpet for the ones which are generalized by Foldable and Traversable. head is reexported as a total function written in terms of foldr. The ordNub function is accidentally quadratic in the Prelude is replaced by a logarithmic variant using a hashmap underneath.

head :: (Foldable f) => f a -> Maybe a
head = foldr (\x _ -> return x) Nothing

sortOn :: (Ord o) => (a -> o) -> [a] -> [a]
sortOn = sortBy . comparing

-- O(n * log n)
ordNub :: (Ord a) => [a] -> [a]
ordNub l = go Set.empty l
    go _ []     = []
    go s (x:xs) =
      if x `Set.member` s
      then go s xs
      else x : go (Set.insert x s) xs


The core moand class, functions, and combinators are reexported wholesale. Variants like mapM are provided by Data.Traversable instead of Control.Monad.

module Monad (
  , MonadPlus(..)

  , (=<<)
  , (>=>)
  , (<=<)
  , forever

  , join
  , mfilter
  , filterM
  , mapAndUnzipM
  , zipWithM
  , zipWithM_
  , foldM
  , foldM_
  , replicateM
  , replicateM_
  , concatMapM

  , guard
  , when
  , unless

  , liftM
  , liftM2
  , liftM3
  , liftM4
  , liftM5
  , liftM'
  , liftM2'
  , ap

  , (<$!>)
  ) where

import Prelude (concat, seq)
import Control.Monad

concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b]
concatMapM f xs = liftM concat (mapM f xs)

Strict versions of liftM are also provided by default since this is a common source of unexpected laziness.

liftM' :: Monad m => (a -> b) -> m a -> m b
liftM' = (<$!>)
{-# INLINE liftM' #-}

liftM2' :: (Monad m) => (a -> b -> c) -> m a -> m b -> m c
liftM2' f a b = do
  x <- a
  y <- b
  let z = f x y
  z `seq` return z
{-# INLINE liftM2' #-}


The applicative module core types are reexported wholesale. Unlike in 7.8 we have the Applicative class in scope by default, as it should be in this day and age.

-- Applicatives
import Control.Applicative as X (
  , Alternative(..)
  , Const(..)
  , ZipList(..)
  , (<**>)
  , liftA
  , liftA2
  , liftA3
  , optional


The core GHC typeclasses for ordering and numeric tower are reexported wholesale. We also bring Traversable and Foldable into scope masking a few of the partial functions which generally should be avoided. Semiring is also common enough these days that it should be in scope implicitly.

-- Base typeclasses
import Data.Eq as X
import Data.Ord as X
import Data.Monoid as X
import Data.Traversable as X
import Data.Foldable as X hiding (
  , foldl1
  , maximum
  , maximumBy
  , minimum
  , minimumBy
import Data.Semiring as X
import Data.Functor.Identity as X
import Data.Functor as X (
  , ($>)
  , (<$>)
  , void


Deepseq is usually an important typeclass to derive for various usecases, and so we bring in the deepseq library and it’s various functions.

-- Deepseq
import Control.DeepSeq as X (
  , ($!!)
  , deepseq
  , force

Data Structures

The core data structures provided by GHC are reexported, but with List we make sure to mask the partial functions, and operators are provided in more general interfaces.

-- Data structures
import Data.Tuple as X
import Data.List as X (
  , break
  , intercalate
  , isPrefixOf
  , drop
  , filter
  , reverse
  , replicate

The containers library all provide functions which overlap with each other (lookup, insert, etc) so we just export the types for these structures so we can use them in signatures. On a module level these would then be typically imported using a qualified import like import qualified Data.Map as Map.

import Data.Map as X (Map)
import Data.Set as X (Set)
import Data.Sequence as X (Seq)
import Data.IntMap as X (IntMap)
import Data.IntSet as X (IntSet)

Monad Transformers

The basic mtl transformer stack are usually ubiqitious in modern Haskell, specifically StateT, ReaderT and ExceptT. We don’t export all functions in these modules but just enough that most uses cases of the common transformers are brought into scope.

-- Monad transformers
import Control.Monad.State as X (


import Control.Monad.Reader as X (

import Control.Monad.Except as X (

import Control.Monad.Trans as X (

Wired-In Types

Several basic types (IO, Show) are necessary to do anything so we obviously bring these in. The Exts provides the various pointer types for working with FFI as well as the Constraint type which is somewhat common in the presence of -XConstraintKinds and more advanced type family techniques.

-- Base GHC types
import GHC.IO as X (IO)
import GHC.Num as X
import GHC.Real as X
import GHC.Float as X
import GHC.Show as X
import GHC.Exts as X (
  , Ptr
  , FunPtr
  , the


Generics are also ubiqitious in modern Haskell and the various typeclasses and type synonyms should be provided so that we can -XDeriveGeneric as well as implement default instances for Generic classes.

-- Generics
import GHC.Generics (
  , Rep
  , K1(..)
  , M1(..)
  , U1(..)
  , V1
  , D1
  , C1
  , S1
  , (:+:)
  , (:*:)
  , NoSelector
  , Rec0
  , Par0
  , Constructor(..)
  , Selector(..)
  , Arity(..)
  , Fixity(..)

String Types

Strings in Haskell are a giant fractal of suffering. In a just world, we would all just use Text most of the time, and ByteString when we needed to deal with network but we don’t live in that world an large portions of Hasell ecosystem still use the wildly-inefficient linked-list of Char.

Controversially we just reexport the Lazy text type as a type synonym LText and provide the string-conv library function toS which automatically converts between any two string types using a multiparam typeclass. String heavy programs might also consider reexporting the various Text and ByteString builder classes.

-- ByteString
import qualified Data.ByteString.Lazy
import qualified Data.ByteString as X (ByteString)

-- Text
import Data.Text as X (Text)
import qualified Data.Text.Lazy
import qualified Data.Text.IO
import Text.Printf as X (printf)

import Data.Text.Lazy (
  , fromStrict

import Data.String.Conv as X (
  , toS
  , toSL
  , Leniency(..)

-- Printf
import Text.Printf as Exports (
  , printf
  , hPrintf
type LText = Data.Text.Lazy.Text
type LByteString = Data.ByteString.Lazy.ByteString


IO is essentially obvious, so we reexport the various IO operations and command low-level command line option and file handler functions.

-- IO
import System.Exit as X
import System.Environment as X (getArgs)
import System.IO as X (
  , hClose

In a just world we’d have a generic putStr that didn’t require us to put boilerplate imports in every damn module just to print a string, but alas we don’t live in that world. But we can implement a basic class to do this for the common text representation and then just export the class instead of the four libraries that all provide the same interface.

class Print a where
  putStr :: MonadIO m => a -> m ()
  putStrLn :: MonadIO m => a -> m ()

instance Print T.Text where
  putStr = liftIO . T.putStr
  putStrLn = liftIO . T.putStrLn

instance Print TL.Text where
  putStr = liftIO . TL.putStr
  putStrLn = liftIO . TL.putStrLn

instance Print BS.ByteString where
  putStr = liftIO . BS.putStr
  putStrLn = liftIO . BS.putStrLn

instance Print BL.ByteString where
  putStr = liftIO . BL.putStr
  putStrLn = liftIO . BL.putStrLn

instance Print [Char] where
  putStr = liftIO . Prelude.putStr
  putStrLn = liftIO . Prelude.putStrLn

-- For forcing type inference
putText :: MonadIO m => T.Text -> m ()
putText = putStrLn
{-# SPECIALIZE putText :: T.Text -> IO () #-}

putLText :: MonadIO m => TL.Text -> m ()
putLText = putStrLn
{-# SPECIALIZE putLText :: TL.Text -> IO () #-}

Like the functions above we automatically lift print and the various string IO operations into a generic MonadIO instance so we can embed them in transformer stacks.

print :: (X.MonadIO m, P.Show a) => a -> m ()
print = liftIO . P.print


The basic thread primitives are common enough that they should be brought into scope implicitly. The async library provides the preferred method of interacting with concurrent logic and so we export this wholesale as well.

-- ST
import Control.Monad.ST as ST

-- Concurrency and Parallelism
import Control.Exception as X
import Control.Concurrent as X
import Control.Concurrent.Async as X

Now in our cabal file we can just add:


And then include your sensible Prelude in your modules. You can escape hatch out of this choice on a module-by-module basis by just pulling in Prelude explicitly using import Prelude.

So that’s the basic proto-prelude I use for my large multi-cabal-file projects. Your mileage by vary on some of these choices, but I consider this a pretty sensible and practical foundational set of functions and types that should at least provide a good starting point.