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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.
  • Includes Semiring for GHC >= 7.6.
  • Includes Bifunctor for GHC >= 7.6.
  • Includes Semigroup for GHC >= 7.6.


  • GHC 7.6.3
  • GHC 7.8.4
  • GHC 7.10.3
  • GHC 8.0.2
  • GHC 8.2.2
  • GHC 8.4.1
  • GHC 8.4.4
  • GHC 8.6.1
  • GHC 8.6.4
  • GHC 8.6.5
  • GHC 8.8.1
  • GHC 8.10.1
  • GHC 9.0.1
  • GHC 9.2.1

Stack LTS:

  • lts-6.x
  • lts-7.x
  • lts-8.x
  • lts-9.x
  • lts-10.x
  • lts-11.x
  • lts-12.x
  • lts-13.x
  • lts-14.x
  • lts-15.x
  • lts-16.x
  • lts-17.x
  • lts-18.x
  • lts-19.14 and higher


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

$ stack repl
> 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


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 4.16
bytestring 0.10 0.11
containers 0.5 0.7
deepseq 1.3 1.5
ghc-prim 0.3 0.7
hashable 1.2 1.4
mtl 2.1 2.3
stm 2.4 2.6
text 1.2 1.3
transformers 0.4 0.6
fail 4.9 4.10


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 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.

  • But what if I want the partial functions?

You if you need partial functions for backwards compatibility you can use the Protolude.Partial module and mask the safe definitions as needed.

import Protolude hiding (head)
import Protolude.Partial (head)

Development Tools

GHC Magic

To build the exports management tool use:

$ cabal new-build exports --flag dev
$ cabal run exports

This tool uses GHC’s internal compile symbol table to generate a list of exports and keep the export list of protolude stable across different versions of GHC and base.

Continious Integration

There is a massive test suite that tests all versions of GHC 7.6 - GHC HEAD alongside all Stack resolvers to ensure no regressions. Any pull requests or patch has to pass the 47 integrity checks before being considered. Any pull request must keep the export list consistent across GHC and Base version and not have any accidental symbol dropping or drift without updating the export golden tests.


Released under the MIT License. Copyright (c) 2016-2022, Stephen Diehl



  • GHC 9.4.4 support


  • GHC 9.2.2 support
  • Drop export executable


  • GHC 9.0.1 and 9.2.1 support
  • Add HasCallStack to unsafe* functions.
  • Banish String on readMaybe and readEither.


  • GHC 8.10.1 support
  • Use Protolude.ConvertText as the default string conversion class. This removes partial functions when converting to/from ByteStrings.
  • Provide Protolude.Conv as a compatibility layer for old string conversion interface.
  • Migrated Debug and Unsafe to Protolude.Debug and Protolude.Unsafe.
  • Export Unicode functions:
    • intToDigit
    • isAlpha
    • isAlphaNum
    • isAscii
    • isControl
    • isDigit
    • isHexDigit
    • isLetter
    • isLower
    • isPrint
    • isSpace
    • isUpper
  • Export MonadFail class.
  • Export gcast from Data.Typeable.
  • Export typeOf from Data.Typeable.
  • Export Handler from Control.Exception.
  • Export yield from Control.Concurrency.
  • Provide compatibility module Protolude.Partial as single export for unsafe partial functions with the same naming conventions as Prelude.


  • GHC 8.8.1 support


  • GHC 8.6.1 support
  • Export fromLeft and fromRight.
  • Mask always and alwaysSucceeds from STM export for stm-2.5.


  • Add explicit witness function for use as type witness without warnings. Makes undefined semantically distinguishable from type witnesses.
  • Backwards compatible Protolude.Safe module for explicit handling of partial list operations.
  • Export minimumDef, maximumDef.
  • Looser lower-bound on Data.Kind export for GHC 8.0.x.


  • Exposes throwE and catchE.
  • Add transformers-compat for old versions of transformers that require throwE, catchE.
  • Fix safe version bounds for new versions.
  • Add mapExceptT and withExceptT`.
  • Export scanl' and provide shim for backwards compatibility.
  • Add putErrLn.
  • Expose RealFloat.
  • Expose GHC.Records exports for GHC 8.2 and above.


  • Expose Symbol and Nat types from GHC.TypeLits by default.
  • Switch exported (<>) to be from Data.Monoid instead of Semigroup.
  • Expose putByteString and putLByteString monomorphic versions of putStrLn functions
  • Export genericLength and other generic list return functions.
  • Rename msg to fatalErrorMessage.
  • Export ExceptT, ReaderT, and StateT constructors.
  • Mask displayException from default exports.
  • Mask stToIO from default exports.
  • Export NonEmpty type and constructor for Base 4.9 only.
  • Export Data.Semigroup type and functions for Base 4.9 only.
  • Restrict exported symbols from async to set available in 2.0.
  • Add (&&^), (||^), (<&&>), (<||>)
  • Expose unzip.
  • Export maximumMay and minimumMay.
  • Mask Type export from Data.Kind.
  • Wrap die to take Text argument instead of [Char].
  • Export constructors GHC.Generics: (:+:), (:*:), and (:.:).
  • Expose StablePtr, IntPtr and WordPtr types.


  • Make sum and product strict


  • foreach for applicative traversals.
  • hush function for error handling.
  • tryIO function for error handling.
  • pass function for noop applicative branches.
  • Mask Handler typeclass export.
  • Mask yield function export.


  • Exports monadic (>>) operator by default.
  • Adds traceId and traceShowId functions.
  • Exportsreader and state functions by default.
  • Export lifted throwIO and throwTo functions.


  • Adds uncatchable panic exception throwing using Text message.
  • Removes printf
  • Removes string-conv dependency so Stack build works without extra-deps.
  • Brings Callstack machinery in for GHC 8.x.
  • Removes throw and assert from Control.Exception exports.
  • Removes unsafeShiftL and unsafeShiftR from Data.Bits exports.
  • Reexport throw as unsafeThrow via Unsafe module.
  • Hides all Show class functions. Only the Class itself is exported. Forbids custom instances that are not GHC derived.
  • Export encodeUtf8 and decodeUtf8 functions by default.
  • Adds unsnoc function.


  • Initial release.