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 7.2. 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.
• System.Environment.Extra.lookupEnv is a functional available in GHC 7.6 and above. On GHC 7.6 and above this package reexports the version from System.Environment while on GHC 7.4 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. 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.