Monday, November 9, 2020

Hello Server + Persistent

 The previous post covered setting up a Servant server in Haskell, with automatic codegen for Elm bindings and using the acid-state package for persistence. 

An obvious benefit of the stack is that one only ever needs to think in native Haskell data structures. It's relatively easy to use and achieves the goal of minimizing work across data boundaries (in this case, between the server and persistence layer). It's also self-contained, insofar as it doesn't require an external database server / process / engine.

On the other hand... many external database servers have been developed and refined for years -- if not decades -- yielding stability, tools, and all sorts of best practices and optimizations. 

Yesod's answer to the problem is persistent, which provides a type-safe and unified way to interact with a variety of database backends (primarily Sqlite, MySQL, Postgres, and MongoDB). Although the library was designed for Yesod, it is not tightly coupled and seems to work for Servant.

This file contains the data definitions for a revised version of Hello Server.  persist also uses Template Haskell, so we adapt the data declarations accordingly:

[mkPersist sqlSettings , mkMigrate "migrateAll"]
state String
counter Int
deriving Eq Show Generic

Note that the servant-elm template invocations come later and works as expected -- i.e. first, persistent generates the ServerState data type and associate code it needs; then, servant-elm uses the generated ServerState data type for its purposes. 

This file contains the server instance, following the example in the Servant repo. 

And, with a few small changes on the Elm client side to accommodate API tweaks, it works!

There are a number of fundamental things I still haven't figured out (like... shouldn't the Sqlite connection pool be explicitly closed in the bracket pattern?). Overall, there are more things to think about compared to working with acid-state -- which seems reasonabl, since it corresponds to the relative power of the database engine (or at least, I assume so). Leveraging the "migrations" feature from persistent, though, I've still only written a single data definition in Haskell for the entire stack!

The repo with all examples:

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