Towards defining an inner join between two tables.

[MetaMemoryT]

I want to define an innerJoin between two tables, something like:

innerJoin ::
    (Tabular t1
    ,Tabular t2
    ,Tabular t3
    ,PKT t1 ~ PKT t2
    ,PKT t2 ~ PKT t3
    ,PKT t1 ~ PKT t3) =>
        (t1 -> t2 -> Bool) ->
    Table t1 -> Table t2 -> Table t3

How could the result t3 have a Tabular instance automatically derived for it?

How could the lenses that are used to manipulate Table t1 and Table t2 still be able to manipulate Table t3?

[ekmett]

What determines the type t3 in your signature there? it only occurs in positive position.

[MetaMemoryT]

Hmmm, nothing currently, t3 could be anything.

Would an instance of a typeclass be a sensible way to determine the t3 as in:

class Mergeable t1 t2 t3 | t1 t2 -> t3 where
    merge :: t1 -> t2 -> t3

innerJoin ::
    (Tabular t1
    ,Tabular t2
    ,Tabular t3
    ,Mergeable t1 t2 t3) =>
        (t1 -> t2 -> Bool) ->
    Table t1 -> Table t2 -> Table t3

I think I never need the primary keys to be the same type anymore.

Then I could use template haskell to derive for each table and joined table I want to conduct inner joins on:

• t3 tables for joined table results
• specific instances of the Mergeable class

[ekmett]

An instance seems like a pretty brutal way to make the decision. Not sure how the whole thing would come together. If you can bang something out that works, I'd be curious to see it.