Documentation on db types to haskell types convertion

[matsumonkie]

First thing first, thanks a lot for this awesome project ! I'm in the process the try to understand the source as I find it really interresting.

As a Haskell beginner, I sometime found a bit hard to use Opaleye. Issue #197 is something I have also ran into. I thought it could be nice to add some documentation on how to handle convertions.

What do you think ?

[saurabhnanda]

Why does one need to derive FromField AND QueryRunnerColumnDefault? Looking at the API surface, it seems that the latter has all the information required to teach Opaleye how to convert a PGText to Gender:

instance QueryRunnerColumnDefault PGText Gender where

Also, what about the other way round? How does Opaleye know how to convert a Gender to PGText (or whatever else raw SQL is going to accept)?

[saurabhnanda]

Also, I'm not clear how PGText came into the picture? In case of an ENUM datatype, wouldn't Postgres send it as a completely new type, altogether?

[saurabhnanda]

Also, a link to https://hackage.haskell.org/package/postgresql-simple-0.5.2.1/docs/Database-PostgreSQL-Simple-FromField.html in the documentation might be a good idea. It contains a more detailed example of how to handle parsing failures as well.

[matsumonkie]

Also, I'm not clear how PGText came into the picture? In case of an ENUM datatype, wouldn't Postgres send it as a completely new type, altogether?

I think Postgres handle an enum type internally as an Int but present it as a string. Your enum type should probably as a PGString, not sure though.

Also, a link to https://hackage.haskell.org/package/postgresql-simple-0.5.2.1/docs/Database PostgreSQL-Simple-FromField.html in the documentation might be a good idea. It contains a more detailed example of how to handle parsing failures as well.

Yeah you're right, I have changed the PR. Thanks

Also, what about the other way round? How does Opaleye know how to convert a Gender to PGText (or whatever else raw SQL is going to accept)?

I don't know yet, still learning Opaleye :-)

[tomjaguarpaw]

Thanks very much for this. I don't really have time at the moment to handle all the inbound Opaleye requests I'm getting but I will get to this sooner or later!