Refactor pagination SQL queries to use generic paramaters

[rvarun11]

Changes examples to use [] to match the expected [Action] parameter type.

[mpscholten]

Will the empty list not generate a type error?

[rvarun11]

No, using the () you get

 Couldn't match expected type ‘[Database.PostgreSQL.Simple.ToField.Action]’

It will get rid of that error but you may need to annotate it with the type of model if you get the following:

 The type variable ‘model0’ is ambiguous

to fix that, you can:

(users, pagination) <- paginatedSqlQuery @User "SELECT * FROM users" []

Should I add this as well?

[amitaibu]

Yes, I think it's best to have a fully working example.

[mpscholten]

The type of paginatedSqlQuery is wrong. It needs to be a type parameter. E.g. sqlExec or sqlQuery in IHP use a generic query type argument with a ToRow q constraint:

ihp/IHP/ModelSupport.hs

Line 416 in 17f17b5

sqlExec :: (?modelContext :: ModelContext, PG.ToRow q) => Query -> q -> IO Int64

[CSchank]

@rvarun11 We played with this exact problem recently and ran into lots of weird stuff with the constraints. I believe the FromRow constraint is unnecessary and will constrain things more than needed. For instance, sqlQuery doesn't require it: https://ihp.digitallyinduced.com/api-docs/IHP-ModelSupport.html#v:sqlQuery. It would mean you can't use certain types of values as inputs, if they don't have FromRow instances, at least that's what we ran into.

I think it seems to be needed here because of the subquery to do the count query. We actually implemented this exact function in our project a few months ago and were able to avoid the need for FromRow. Our code isn't open source, but I'll provide you a link to our version in case it's useful.

[CSchank]

Update: sorry, I got confused. FromRow is on the output result, not the input. So I think this matches what we came up with. I think for a while our type signature was more complex and I remember us running into some other weird things but we both arrived at the same type signature in the end. 😊

And sqlQuery does have the FromRow constraint on the output too, I must have misread it yesterday!