opaleye VS product-profunctors

If you ever have any questions about Opaleye I'm happy to help. Feel free to open an issue to ask about anything any time.

Sounds like Opaleye isn't on your list of choices, but if it is then feel free to ask me any questions, any time by filing an issue (I'm the Opaleye maintainer).

Opaleye is Posgres-only, and Postgres does such a good job of optimizing queries that performance issues basically don't arise. I have a long-standing invitation to improve Opaleye's query generation as soon as anyone can produce a repeatable example of a poorly-performing query. In Opaleye's eight years, no one ever has. There's a thread where two reports have come close, but it's still not clear that that's simply due to using a six year old version of Postgres.

Nice! Well done. If you have any more questions about product-profunctors or Opaleye then please let me know. It's best to ask by [opening an issue](https://github.com/tomjaguarpaw/haskell-opaleye/issues/new).

The only way out that I can see is to design embedded domain specific languages (EDSLs) that inherit the expressiveness, composability and type safety from the host language. That's what Opaleye and Rel8 (Postgres EDSLs for Haskell do. Haskell is particularly good for this. The query language can be just a monad and therefore users can carry all of their knowledge of monadic programming to writing database queries.

This approach doesn't resolve all of the author's complaints but it does solve many.

Disclaimer: I'm the author of Opaleye. Rel8 is built on Opaleye. Other relational query EDSLs are available.

[1] https://github.com/tomjaguarpaw/haskell-opaleye/

For an example of how this plays out in practice observe Opaleye's MaybeFields (generously contributed by Shane and /u/ocharles at Circuithub). The definition is essentially identical to Optional from the paper. Instead of a specialised typeclass Inhabited we use the ProductProfunctor NullSpec (which happens to conjure up an SQL NULL, but it could be any other witness).

If you're really interested in solving it then you could download the git repo, run cabal build, see if you get the same error, and if so progressively remove parts of the package (by commenting them out in the source files) until you get something to compile. That way you may be able to pinpoint the exact problem.

I use it in test suites, where the tests are simply to check that something compiles, not its behaviour, for example https://github.com/tomjaguarpaw/product-profunctors/blob/faf...

I had also thought that there was a reason one could need that pattern to import instances even in the absence of orphans, but on reflection I think I was mistaken.

Here is link number 1 - Previous text "PR"