ezyang’s blog

the arc of software bends towards understanding

Groom: human readable Show for Haskell

Tapping away at a complex datastructure, I find myself facing a veritable wall of Babel.


“Zounds!” I exclaim, “The GHC gods have cursed me once again with a derived Show instance with no whitespace!” I mutter discontently to myself, and begin pairing up parentheses and brackets, scanning the sheet of text for some discernible feature that may tell me of the data I am looking for.

But then, a thought comes to me: “Show is specified to be a valid Haskell expression without whitespace. What if I parsed it and then pretty-printed the resulting AST?”

Four lines of code later (with the help of Language.Haskell)...


Ah, much better!

How to use it. In your shell:

cabal install groom

and in your program:

import Text.Groom
main = putStrLn . groom $ yourDataStructureHere

Update. Gleb writes in to mention ipprint which does essentially the same thing but also has a function for putStrLn . show and has some tweaked defaults including knowledge of your terminal size.

Update 2. Don mentions to me the pretty-show package by Iavor S. Diatchki which also does similar functionality, and comes with an executable that lets you prettify output offline!