The printer Haskell deserves

Friday night project ahoy!


I was working with haskell-names the other day. Its data types are nice enough, but are rather unweildly to read in the REPL when debugging and inspecting. This got me thinking about inspection and printers for Haskell data structures again.

I’ve made several approaches for to haskell-mode in the past.

Today I implement yet another one, but this one I like best. I’ve always wanted to have a Haskell printer that can evaluate on demand, piece-wise, taking care not to evaluate the whole structure too eagerly. I should be able to type [1..] into my REPL and not be spammed by numbers, but rather to expand it at my leisure.


My plan was to use the Data.Data API to traverse data structures breadth-first, display to the user something like Just … and then allow the user to continue evaluating on request by clicking the slot.

I chatted with Michael Sloan about it and we came up with a simple experimental design and thought it would be a nice idea. We hypothesized a nice class-based way to provide custom presenters for your types, so that e.g. a Diagram could be rendered as a bitmap inline with the rest of the data structure, but that needs more thinking about.

I’ve implemented a basic version of it in the present package (a la “presentations” in CLIM) and implemented a usable front-end for it in Emacs. There’s some information about the implementation in the README which you can read on Github.


Yes! It works. Here is a demonstration video. Concept proven. This is definitely my favourite way so far. I will probably write a simple algorithm in Emacs to format things on separate lines, which would make it much easier to read, and I want to make strings expand to fill the screen width, but no further. But this is already an improvement.

I’ll trial it for a while, if I end up using it more often than not, I’ll make the option to make :present implicit for all REPL evaluations.


For kicks, here’s the output for

loeb (map (\i l -> Node i (map (fmap (+1)) l)) [1..3])

Normally you would get:

[Node {rootLabel = 1, subForest = [Node {rootLabel = 2, subForest =
 [Node {rootLabel = 3, subForest = [Node {rootLabel = 4, subForest =
  [Node {rootLabel = 5, subForest = [Node {rootLabel = 6, subForest =
   [Node {rootLabel = 7, subForest = [Node {rootLabel = 8, subForest =

Ad infinitum! With presentation, you get:

λ> :present loeb (map (\i l -> Node i (map (fmap (+1)) l)) [1..3])
Tree Integer:[Tree Integer]

If you click Tree Integer on the left, you get:

(Node 1 [Tree Integer]):[Tree Integer]

Click the new one on the left:

(Node 1 (Tree Integer:[Tree Integer])):[Tree Integer]

Et cetera:

(Node 1 ((Node 2 [Tree Integer]):[Tree Integer])):
((Node 2 [Tree Integer]):[Tree Integer])

In other words, every [Tree Integer] is a placeholder that you can click to get more output.