programacao-funcional | discipline Special Topics | Functional Programming library

In Haskell, all functions are pure---so something like selectPosts, which needs to go out and do IO to talk to a database, can't just do that and return the value from the database. Instead, these kinds of functions return something of type IO a, which you can think of as basically a description of how to go out and do IO to get a value of type a. These "IO actions" can be composed together, and one of them can be assigned to main; at runtime, the RTS will execute these IO actions.

However, you aren't composing the IO a value that you get back from selectPosts to be part of the larger IO value that eventually becomes main; you are trying to directly use it by feeding it into json. This won't work, because there's no (good/easy) way to convert a description of how to do IO into a JSON string.

The way that Haskell deals with composing these values is through an abstraction known as a "monad", which is useful in a lot of other cases as well. do notation can be used to write this monadic sequencing in a very natural style. You can't just write posts <- withTestConnection S.selectPosts here because Scotty's get function takes a value of the monadic ActionM type, not IO. However, it turns out that ActionM is basically a bunch of other useful stuff layered on top of IO, so it should be possible to "lift" the IO action from selectPosts into Scotty's ActionM monad: