MonadFail constraint on Entity _ _ <- pattern match

[parsonsmatt]

You may be running into a problem where, upon upgrading to persistent-2.14, you're seeing a ton of MonadFail constraints arise due to pattern matching on Enttiy in a bind arrow.

foo = do
    Entity userId user <- getBy $ UserEmail "foo@bar.com"
    ...

This is a bug in GHC, where the COMPLETE pragma for a PatternSynonym is not taken into account in do notation. Here's the GHC issue: https://gitlab.haskell.org/ghc/ghc/-/issues/15681

[parsonsmatt]

To work around:

1. Add the MonadFail constraint. This kinda sucks, but it's the easiest way for now.
2. Use the Entity' constructor, which is not a synonym. This does not have record fields, though, which sucks.

These work-arounds are both pretty crappy, so I'm tempted to make this a compile-time opt-in flag that's on by default whenever the GHC bug is fixed.

[parsonsmatt]

closed in #1401