pretty-display: typeclass for human-readable display

In Haskell the Show typeclass is used to display a syntactically correct Haskell expression. However, there are times when you want to provide a richer display for a value while still retaining the benefits of having derived Show instances. This can be especially useful when working interactively in ghci. pretty-display provides a tiny registered package with the Display typeclass for just this purpose.

GHCi usage

To use Display instances as the default in ghci create a .ghci file with the following:

import Text.Display

:set -interactive-print=Text.Display.dPrint
:def pp (\_ -> return ":set -interactive-print=Text.Display.dPrint")
:def npp (\_ -> return ":set -interactive-print=print")

Typeclass usage

By default, all instances of Show are also instances of Display. To create a custom instance you will need to use the OVERLAPPING pragma, and define the display method of type a -> DisplayText. For example:

import Numeric

import Text.Display
import Text.PrettyPrint.ANSI.Leijen as PP

data MyRecord = MyRecord
  { numerators :: [Double]
  , denominator :: Double
  } deriving (Show)

record :: MyRecord
record = MyRecord
  { numerators = [0, 5..100]
  , denominator = 1326
  }

instance {-# OVERLAPPING #-} Display MyRecord where
  display a = mkDisplayTextFromStr
      $ show
      $ toCol "MyRecord (percentage): " <> toCol (displayPerc a)
    where
      displayPerc a = showFFloat (Just 2) (toPerc a) "%"
      toPerc a = sum (numerators a) / denominator a * 100
      toCol a = (PP.dullgreen . PP.fill 25 . PP.text) a

main :: IO ()
main = do
  dPrint record
  pPrint record

Using Show when defaulting to Display

If you've set ghci to use dPrint by default you can still print Show instances for debugging. For normal printing use print. For convenience pretty-display re-exports pPrint from the pretty-show package for pretty printing Show instances.