[Haskell-cafe] What features should an (fictitious) IDE for Haskell have?

[Alexandre Moine]

7 décembre 2020 14:25 "Johannes Waldmann" <johannes.waldmann at htwk-leipzig.de> a écrit:

> I often wanted a tool that finds (nearly) duplicate AST sub-trees
> in a large code base, and suggests refactorings.
> 
> Of course, in an IDE, it could alert me on-the-fly
> that I'm typing some code that's already present elsewhere.
> 
> How might one go about implementing this?
> Actual (approximate) sub-tree matching seems the easy part;
> but I have no clear idea about whether this
> should just use syntax, or needs types as well (my guess is: yes)
> what libraries are there to provide the (annotated) ASTs, etc.
> 
> - J.W.

Hi,

I developed last year a small tool for redundancy detection in OCaml called asak [1], which can be easily adapted for Haskell.

The idea is pretty simple: use an intermediate language of the compilation pipeline to normalize the code and remove sugar (for Haskell, one can take Core), inline everything, and hash the tree bottom-up (abstracting constants) keeping intermediates trees. Then you just compare hashs of trees against pre-computed hashs of, let's say, the whole Hackage ecosystem.

The technique is pretty efficient and scalable. I ran asak against the whole OPAM repository and got some results (like the detection of `map_opt` 140 times under 32 different names). There are some drawbacks: the code needs to be compiled down to Core and one has to maintain a database of available hashs, but the first one seems legitimate and the other inherent to any such tool.

I started developing a plugin editor for emacs, but never go further due to a lack of time.

Anyway, the approach is language-agnostic and could be easily adapted to Haskell. Moreover, I have an "inline everything" part which is not possible in OCaml (due to effects), and will be highly valuable in Haskell.

Best,

--
Alexandre Moine

[1]: https://github.com/nobrakal/asak