Persistent TH generates splice so large that GHCI runs out labels

[tysonzero]

This is with around 70 tables, generally with <10 fields each.

The compile times are also quite long.

It seems like the newest GHC's increase the amount of bits used for labels to avoid this problem, but a lot of libraries (and GHCJS) are not up to date enough to use these GHC's.

I was wondering if it would be possible to tweak things so that less code is generated while maintaining the same functionality.

It seems like manually writing out instances is pretty verbose too. Opaleye seems to be more concise in this regard, so it might be a useful reference.

[parsonsmatt]

The work-around here is to split the work into multiple modules. I just did a big refactor on the TH machinery with #1256, so it should be possible to generate even less code hopefully soon.

[tysonzero]

Is it possible to split up the TH splices whilst still generating foreign keys properly? IIRC that didn't work last time I tried (years ago).

[parsonsmatt]

Yes, with persistent-2.13 :)

module Foo where

mkPersist sqlSettings [persistLowerCase| ... |]

module Bar where

import Foo

mkPersistWith sqlSettings $(discoverEntities) [persistLowerCase| ... |]

This should Just Work

[tysonzero]

Oh very cool!

How do migrations work with that approach?

[tysonzero]

These error messages are very confusing. It frequently complains about multiple declarations, and the fix is usually to add a missing import, which seems a bit backwards. I'm having trouble in some cases though, as it complains about multiple declarations of "Foo", where "Foo" is not mentioned anywhere in the module or in any imports, so it's unclear what it wants me to do to solve it.

[tysonzero]

Ah ok you have to import transitive dependencies as well.

[parsonsmatt]

mkMigration is now just running migrateModels on the mkEntityDefList generated by the quasi quoter.

You can do something even fancier by importing all of the persistent models, running discoverEntities, and then calling migrateModels $(discoverEntities). See persistent-discover for a helper to make that easier.

Those UX notes on bad error messages are super welcome thanks!