Extra

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.concatMapM provides a monadic version of concatMap, in the same way that mapM is a monadic version of map.
• Data.Tuple.Extra.fst3 provides a function to get the first element of a triple.
• Control.Exception.Extra.retry provides a function that retries an IO action a number of times.
• Data.Either.Extra.fromLeft is a function available in GHC 8.0 and above. On GHC 8.0 and above this package reexports the version from Data.Either while on GHC 7.10 and below it defines an equivalent version.

The module 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.

Which functions?

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 (whenJust).
• Most of the functions have trivial implementations that are obvious from their name/signature. 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.

Base versions

A mapping between base versions and GHC compiler versions can be found here.

Contributing

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 runghc Generate.hs or ghci (which uses the .ghci file) and typing :generate. All PR's should contain regenerated versions of those files.