Working on refining my #eventsourcing design/template for #haskell applications. Using GADTs we can turn on or off optimistic concurrency check using "global" journal versions. Depending on the on/off setting, the "concurrent change detected" outcome becomes impossible and need not be handled.

Beyond basic programs, HM-like type systems can be quite mysterious and awesome. The definitions and types of the #purescript functions below look strange at the first sight. I wonder how IWSAMs will fare when ported to #fsharp.