I would like to thank Owen Stephens, Alexander Batischev, Denis Kasak and Mann mit Hut for the valuable comments! I am going to update the tutorial to fix the indicated issues. Also, I am going to improve description of kinds and singletons. Also, I will improve description of red-black tree example: I think I need to include the whole listing after every big step. Are there any other topics you would like to be described in the context of a practical-oriented introductory-level tutorial on GADTs? PS: Please let me know if I misspelled your names (to make sure that I do not put wrong names in acknowledgements section). You can do it by email to anton dot dergunov dog mail dot ru. -- Best Regards, Anton Dergunov Среда, 9 января 2013, 17:13 +02:00 от Alexander Batischev :

On Tue, Jan 08, 2013 at 11:22:39PM +0400, Anton Dergunov wrote:

...

I have written a draft of an introductory-level tutorial paper about GADTs in Haskell (for submittion to proceedings of the recent LASER summer school) and I would like to seek initial feedback about its content: what information is probably missing? are there any subtle mistakes?

Not exactly the feedback you asked for, but I hope it still can be of some use.

As a person with no prior knowledge about GADTs, I found your paper a good introduction. Following proof of correctness of red-black tree insertions turned out to be a little bit of a challenge as type annotations quickly become tangled (made me wondering how one should prove correctness of the proof).

I particularly liked how you handle things that are not central to tutorial, like phantom and existential types: you give single-sentence explanation that provides good enough intuition to follow you further.

Talking of quick explanations, I would love to see kinds and singleton types explained in the same manner. You tried to explain singleton types at page 13, but I find explanation provided by GHC documentation[1] to be much more clear. As for kinds, it just puzzles me why you use the term but don't explain it.

  1. http://hackage.haskell.org/trac/ghc/wiki/TypeNats/Basics

The same goes for type families - while I was able to quickly look up definition of singleton types, I failed to comprehend basics of type families by reading Haskell Wiki. I ended up pretending that type families are just type-level functions.

I would love to see Yampa optimizations section expanded with more interesting examples. Are there any?

Last but not least, a few minor typos and errors I spotted:

* at page 8, "Than we need to declare type instances..." should be "Then   we need...";

* probably due to excessive editing, parameters to repeatElem at page 13   are in the different order than before;

* at page 14, you state:

  > As in all binary search trees, for a particular node N c l x r   > values greater than x are stored in left sub-tree (in l) and values   > less than x are stored in right sub-tree (in r).

  But later on, your code contradict that statement by recursing to left   sub-tree when x we look for is less than value in the node, and to   right sub-tree when x is greater than value in the node.

Thank you once again for a nice introduction to GADTs!

-- Regards, Alexander Batischev

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