Improve doc about using esqueleto with Yesod

[meteficha]

On Fri, Sep 7, 2012 at 7:42 AM, Arthur Clemens arthurclemens@gmail.com wrote:

This looks promising.

Trying out, I notice that I need to hide ==. from Yesod and
Database.Persist.Sqlite, so my imports become:

import Yesod hiding ((==.))
import Database.Persist.Sqlite (withSqliteConn, withSqlitePool)
import Database.Persist.TH

Yesod exports Yesod.Persist, which in turn exports Database.Persist, which in turns exports Database.Persist.Query, which conflicts with a lot of symbols from esqueleto.

Hmm... I guess I'll have to say on the docs that either you manually import Yesod modules, or you import esqueleto qualified.

[ghost]

How TO HIDE PERSISTENT?

[rehno-lindeque]

Just FYI - what I've done for my project is to split off queries from handlers so that handler can just import Import (which includes import Yesod in the standard scaffold) without worrying about name clashes.

Then module Query.Foo will import only Database.Esqueleto and contain functions like e.g.

findAFromBWithCount :: TableBId -> SqlPersistT Handler [(Value TableAId, Value (Maybe Int))]
findAFromBWithCount = select $ from $ \(ta `InnerJoin` tb) -> -- etc...

The type signatures are unfortunately slightly awkward to maintain (and non-optional) but it helps prevent any overlap with persistent operators. (Perhaps helpers for unboxing Value would help?) I could also imagine there's a chance you might want to compose queries so that it's useful to have them separated out into modules.

[joefiorini]

@rehno-lindeque I tried that for myself but got an error finding "Handler". The only "Handler" I could find is Control.Exception. Is that what you use in your Query module?

[rehno-lindeque]

@joefiorini oh the Handler type is generated by Yesod, which I guess you're probably not using then.

These days I often just add all the type constraints the compiler tells me I need. E.g.

getFoo :: (PersistMonadBackend m ~ SqlBackend, MonadResource m, MonadSqlPersist m) 
       => m (Entity Foo)

The type signatures for esqueleto may have changed now with persistent 2 though and I haven't had a chance to upgrade yet.

You may find the ConstraintKind extension helpful: Have a look at the Persistent 2.1 upgrade guide in this blog post

[meteficha]

BTW, since you guys revived this issue, what should be done about it, if anything? :)

[rehno-lindeque]

Some beginner-friendly documentation on how to write type signatures for standalone queries perhaps? I think the persistent upgrade doc did a reasonable job of it for persistent.

[bburdette]

I'm using this:

import Import hiding ((==.), (!=.), (||.))
import Database.Esqueleto;
import qualified Database.Persist as P

For some reason (^.) doesn't need to be hidden. I'm not sure about the other operators. If there's no more elegant way of hiding persistent, then maybe something like this should be in the docs, as some noob friendly boilerplate.