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Build Status Hackage

A sensible starting Prelude for building custom Preludes.

Design points:

  • Banishes String.
  • Banishes partial functions.
  • Compiler warning on bottoms.
  • Polymorphic string IO functions.
  • Polymorphic show.
  • Automatic string conversions.
  • Types for common data structures in scope.
  • Types for all common string types (Text/ByteString) in scope.
  • Banishes impure exception throwing outside of IO.
  • StateT/ReaderT/ExceptT transformers in scope by default.
  • Foldable / Traversable functions in scope by default.
  • Unsafe functions are prefixed with "unsafe" in separate module.
  • Compiler agnostic, GHC internal modules are abstracted out into Base.
  • sum and product are strict by default.
  • Compatibility with GHC 8.0.
  • Includes Semiring for GHC >= 7.6.
  • Includes Bifunctor for GHC >= 7.6.
  • Includes Semigroup for GHC >= 7.6.


  • GHC 7.6.1
  • GHC 7.6.2
  • GHC 7.6.3
  • GHC 7.8.1
  • GHC 7.8.2
  • GHC 7.8.3
  • GHC 7.8.4
  • GHC 7.10.1
  • GHC 7.10.2
  • GHC 7.10.3
  • GHC 8.0.1
  • GHC 8.0.2
  • GHC 8.2.1
  • GHC 8.4.1


To try out standalone prelude at the interactive shell, from the Protolude project directory run.

$ stack exec ghci
> import Protolude

Swapping out the old Prelude

Disable the built-in prelude at the top of your file:

{-# LANGUAGE NoImplicitPrelude #-}

Or directly in your project cabal file:

default-extensions: NoImplicitPrelude

Then in your modules:

import Protolude

Exported Functions

The list of exports is given in the Symbols.md file. Haddock unfortunately breaks in the presence of module reexports and is unable to render documentation.


Protolude tries to be light on dependencies and only pulls in essential libraries that are universally common across most real-world projects. Lower and upper bounds are fully specified and compatible with both vanilla Cabal and tracks Stack LTS resolver.

Dependencies Lower (>=) Upper (<)
array 0.4 0.6
async 2.0 2.3
base 4.6 4.12
bytestring 0.10 0.11
containers 0.5 0.6
deepseq 1.3 1.5
ghc-prim 0.3 0.6
hashable 1.2 1.3
mtl 2.1 2.3
stm 2.4 2.5
text 1.2 1.3
transformers 0.4 0.6


Protolude's main modules are the following:


  • My putStrLn and putStr instances are no longer inferred in the presense of the -XOverloadedStrings extension?

Because the print functions are polymorphic the type of the print functions may require annotations if the type is not fully specified by inference. To force a specific type at the call site use either

putText :: MonadIO m => T.Text -> m ()
putLText :: MonadIO m => TL.Text -> m ()
  • How do I write manual Show instances if show isn't provided?

Generally speaking writing manual instances of Show is a Haskell antipattern because it produces law-violating instances of Show. You probably want to use a pretty printer library for custom printing.

If backwards compatibility is needed then the base library can be imported manually.

import GHC.Show (Show(..))

Automatic deriving of Show for your types is still supported since the class is in scope by default.

  • Partial functions like undefined and error raise compiler warnings on usage.

This is by design. For fatal uncatchable errors use the provided panic function if you intend the program to immediately abort.

panic "Thus I die. Thus, thus, thus. Now I am dead"

If inside of IO simply use throwIO for exception handling, or if in pure business logic use well-typed checked exceptions of the ExceptT variety.

  • Why is id not in scope?

It has been renamed to identity to reserve the id identifier for the more common use case of business logic.


Released under the MIT License. Copyright (c) 2016-2018, Stephen Diehl