optparse-applicative (hackage): optparse-applicative is a haskell library for parsing options on the command line, and providing a powerful applicative interface for composing them.  optparse-applicative takes care of reading and validating the arguments passed to the command line, handling and reporting errors, generating a usage line, a comprehensive help screen, and enabling context-sensitive bash, zsh, and fish completions.  See the included README for detailed instructions and examples, which is also available on github https://github.com/pcapriotti/optparse-applicative.
Keywords: bsd3|cli|library|options|parsing|system|applicative|haskell|option-parser