pretty-ghci

Functionality for beautifying GHCi.

Provides a library and an executable for parsing and pretty-printing the output of derived Show instances as well as Haddock docstrings. The idea is to provide functionality that can be easily plugged into GHCi's -interactive-print option, making for a better REPL experience.

pretty-ghci

This library will make your GHCi experience colourful in 3 steps:

1. Install the executable globally with cabal v2-install pretty-ghci

2. Modify your ~/.ghc/ghci.conf

   :set prompt      "λ> "
   :set prompt-cont "|> "
   
   -- Typing `:pretty` will turn on the pretty-printing
   :set -package process
   :{
   :def pretty \_ -> pure $ unlines $
     [ ":{"
     , "let pprint x = System.Process.withCreateProcess cp' $ \\(Just i) _ _ ph -> do"
     , "        System.IO.hPutStrLn i (show x)"
     , "        System.IO.hClose i"
     , "        _ <- System.Process.waitForProcess ph"
     , "        pure ()"
     , "      where cp = System.Process.proc \"pp-ghci\" [\"--value\", \"--smarter-layout\"]"
     , "            cp' = cp{ System.Process.std_out = System.Process.Inherit"
     , "                    , System.Process.std_err = System.Process.Inherit"
     , "                    , System.Process.std_in  = System.Process.CreatePipe }"
     , ":}"
     , ":set -interactive-print pprint"
     ]
   :}
   
   -- Typing `:no-pretty` will turn off the pretty-printing
   :def no-pretty \_ -> pure (":set -interactive-print System.IO.print")
   
   -- Make things pretty by default!
   :pretty
   

3. Enjoy!

Advantages over existing alternatives

• One stop-solution for formatting and coloring with a small dependency graph
• Takes your terminal width into account during the layout step
• Works for values whose Show instance don't produce valid Haskell (ex: Show (->))
• Handles unboxed literals (ex: MyTriple 1# 2.0# "hello"#)
• Your output will be coloured according to its lexical structure even if parsing fails
• Install one global executable, not one library per GHC version
• Works in cabal repl (although you need to call :pretty once at the start)