Text.Pretty.Simple

pretty-simple is a pretty printer for Haskell data types that have a Show instance.

For example, imagine the following Haskell data types and values:

data Foo = Foo { foo1 :: Integer , foo2 :: [String] } deriving Show

foo :: Foo
foo = Foo 3 ["hello", "goodbye"]

data Bar = Bar { bar1 :: Double , bar2 :: [Foo] } deriving Show

bar :: Bar
bar = Bar 10.55 [foo, foo]

If you run this in ghci and type print bar, you’ll get output like this:

Bar {bar1 = 10, bar2 = [Foo {foo1 = 3, foo2 = ["hello","goodbye"], foo3 = 3.3},Foo {foo1 = 3, foo2 = ["hello","goodbye"], foo3 = 3.3}], bar3 = 10.55}

This is pretty hard to read. Imagine if there were more fields or it were even more deeply nested. It would be even more difficult to read.

pretty-simple can be used to print bar in an easy-to-read format:

Usage

pretty-simple can be easily used from ghci when debugging.

When using stack to run ghci, just append append the --package flag to the command line to load pretty-simple.

$ stack ghci --package pretty-simple

Once you get a prompt in ghci, you can use import to get pretty-simple’s pPrint function in scope.

> import Text.Pretty.Simple (pPrint)

You can test out pPrint with simple data types like Maybe or tuples.

> pPrint $ Just ("hello", "goodbye")
Just
    ( "hello"
    , "goodbye"
    )

Features

pretty-simple has these features:

• Easy-to-read
  • Complex data types are simple to understand.
• Color
  • Prints in color using ANSI escape codes.
  • It is possible to print without color by using the pPrintNoColor function.
• Rainbox Parentheses
  • Easy to understand deeply nested data types.
• Configurable Indentation
  • Amount of indentation is configurable with the pPrintOpt function.
• Fast
  • No problem with data types thousands of lines long.
• Works with any data type with a Show instance
  • Some common Haskell data types have a Show instance that produces non-valid Haskell code. pretty-simple will pretty-print even these data types.

Why not (some other package)?

Other pretty-printing packages have some combination of these defects:

• No options for printing in color.
• No options for changing the amount of indentation
• Requires every data type to be an instance of some special typeclass (instead of just Show).
• Requires all Show instances to output valid Haskell code.

Contributions

Feel free to open an issue or PR for any bugs/problems/suggestions/improvements.