[Haskell-cafe] Adding a DB Connection Pool to a Web Application

Will Yager will.yager at gmail.com
Thu Oct 26 13:33:34 UTC 2017

You want e.g. https://hackage.haskell.org/package/monad-logger-

You can mentally replace “m” with “IO”. 

It takes some monadic action that requires logging (like setting up a database pool) and provides a concrete implementation of how logging should be done (like printing messages to stdout), thus discharging the MonadLogger constraint. See “MTL style”. 

I think MonadLogger is as good an introduction to monad transformers as any, since it’s fairly straightforward. Most of the instances are like the reader monad, where they just pass around some function that takes the log details and does something with them. 

> On Oct 26, 2017, at 8:43 AM, Steven Leiva <leiva.steven at gmail.com> wrote:
>  I thought implementing an instance of MonadLogger for IO that would simply typecheck (and not log), but that would lead to an orphaned instance, correct? What's the easiest next step to make forward progress here? 
> P.S. I am aware that there are a lot of advanced (for me anyway) Haskell that can be done via monad transformers, natural transformations, etc., in order to wire up a DB with Servant, but I am happy with the "next simplest step" until I learn some of those concepts better. (Then Haskell makes refactoring a breeze!).
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