Building in-memory

[bcc...@gmail.com]

hello,

I want to build an in-memory data-structure that tracks changes in
source files (think of something like an LSP server that tracks
changes in source code, builds some kind of data-structure and can then
answer questions about it). My current approach has shake writing the
data processed from a source file to a target file; when the program
is invoked it reads all target files, joins the data (they have a
Monoid instance), and returns something.

I want to make the program work like a server instead (so that I forgo
all the needless I/O), with shake being called to update an in-memory
data-structure. I can write empty target files to satisfy shake if
needed, and thought of using the State monad to modify the in-memory
data-structure; but I'm struggling with something related to
https://github.com/ndmitchell/shake/issues/572; couldn't shake's (the
function) signature be |Monoid a => ShakeOptions -> Rules a -> IO a|?
But I guess what I really want to know is the best way to do what I
want (i.e., using shake to update an in-memory datastructure)… Should
I be using IORef or …?

thank you,

-- bruno

[Neil Mitchell]

Hi Bruno,

The Ghcide project does something quite similar to what you are describing - see https://github.com/digital-asset/ghcide. There's also a presentation and video me of talking about how it works, including some details on Shake: https://ndmitchell.com/#ghcide_7_sep_2019

You probably want to use the Database module https://hackage.haskell.org/package/shake-0.18.5/docs/Development-Shake-Database.html and specify `/dev/null` as the shakeFiles field - with that, you can stop doing all IO. Using shakeRunDatabase you pass in a set of Action a -> IO a, which should be what you need. The idea is that your rules are fixed, but the actions vary, which is why its not Rules a -> IO a.

Let me know if those things are what you were looking for.

Thanks, Neil

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