Remove HasField and pattern synonym

[parsonsmatt]

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[jaredramirez]

I'm not super familiar with this, but in the next release of GHC with this merged, could HasField be added back to persistent?

[parsonsmatt]

I think we'd need to gate it on the GHC version, and having a different definition of a foundational type based on which version of GHC you're using feels a little weird to me.

One thing we do at work is generate specific HasField instances for specific table types, so we have instance HasField "fieldName" (Entity Table) FieldType and instance HasField "fieldName" Table FieldType. This is part of our TemplateHaskell code that we apply to every table, so it works out pretty much the same right now.

[jaredramirez]

Cool, I finally got around to trying this out and it's working well. I put the TH code I wrote here: #1381 (comment) in case anyone in the future finds it useful!