A library of extra functions for the standard Haskell libraries. Most functions are simple additions, filling out missing functionality. A few functions are available in later versions of GHC, but this package makes them available back to GHC 7.10. A few examples:
Control.Monad.Extra.concatMapMprovides a monadic version of
concatMap, in the same way that
mapMis a monadic version of
Data.Tuple.Extra.fst3provides a function to get the first element of a triple.
Control.Exception.Extra.retryprovides a function that retries an
IOaction a number of times.
Data.Either.Extra.fromLeftis a function available in GHC 8.0 and above. On GHC 8.0 and above this package reexports the version from
Data.Eitherwhile on GHC 7.10 and below it defines an equivalent version.
Extra documents all functions provided by this library. Modules such as
Data.List.Extra provide extra functions over
Data.List and also reexport
Data.List. Users are recommended to replace
Data.List imports with
Data.List.Extra if they need the extra functionality.
When producing a library of extra functions I have been guided by a few principles. I encourage others with small useful utility functions contribute them here, perhaps as a temporary stop before proposing they join the standard libraries.
- I have been using most of these functions in my packages - they have proved useful enough to be worth copying/pasting into each project.
- The functions follow the spirit of the original Prelude/base libraries. I am happy to provide partial functions (e.g.
fromRight), and functions which are specialisations of more generic functions (
- Most of the functions have trivial implementations. If a beginner couldn't write the function, it probably doesn't belong here.
- I have defined only a few new data types or type aliases. It's a package for defining new utilities on existing types, not new types or concepts.
A mapping between
base versions and GHC compiler versions can be found here.
My general contribution preferences are available here. In addition, this repo contains some generated code which is checked in, namely src/Extra.hs and test/TestGen.hs. You can generate those files by either running
runhaskell Generate.hs or
ghci (which uses the
.ghci file) and typing
:generate. All PR's should contain regenerated versions of those files.