Shared data in Yesod

Question

I want to share some data across requests in Yesod. In my case that data is a MVar (Data.Map Text ReadWriteLock), but I don't think the format of the data being shared matters too much here.

In Foundation.hs, there is a comment that says I can add fields to App, and every handler will have access to the data there. This seems like an approach I could use to share data between different handlers. I have been looking through the Yesod book, but I could not find any examples of getting data from App.

• How would I access the newly created field from within a handler?

I think this might be a good use case for STM. I could share a TVar (Data.Map Text ReadWriteLock). But creating a TVar wraps the TVar in the STM monad. I might be mistaken, but to me that seems like the entire Yesod "main loop" would need to be run in the STM monad.

• Is using STM a viable option here? Could anyone elaborate on how this might be achieved?

Answer 1

This tutorial for building a file server with Yesod shows quite nicely how you can use STM to access shared data. The relevant part starts from part 2.