quasiquotes | I 'd just like to interject for a moment

Short answer: slightly modify the embedded language.

The User's Guide on QuasiQuoters explains that no escaping can done for the |]:

The quoted ⟨string⟩ finishes at the first occurrence of the two-character sequence "|]". Absolutely no escaping is performed. If you want to embed that character sequence in the string, you must invent your own escape convention (such as, say, using the string "|~]" instead), and make your quoter function interpret "|~]" as "|]".

Your parser can not decide when the quasiquoters ends, because the substring is passed to the quasiquoter that started after the [quasiquoter|… part and before the …|] part.

You thus should slightly alter your language and thus for example work with a pre-processor that translate |~] (which is not considered the end of the quasiquoted string) to |] instead.

You can ignore all the monadic parts of getBy's type signature. Provided you get your code to type check, counterEntity has type Maybe (Entity Counter), and that's all that's important here.

The counterEntity is Nothing if the query fails (i.e., no record in the table for that counter). Otherwise, it's Just an Entity Counter containing the retrieved record:

This unfortunately is a fundamental flaw of the current implementation of Typed Template Haskell, with no easy workaround available.

Constraints are simply not handled correctly.

We've been running into the same problem when looking at how to implement a staged version of generics-sop. You might want to have a look at Section 5 of the Staged Sums of Products paper. We also have a solution for the handling of constraints in Template Haskell, but this solution needs implementation in GHC, and that at the moment is only available in an experimental branch (see the staged-sop repo for some instructions on how to try it). At the moment, the branch is in limbo, because is needs significant rebasing due to all the linear types changes. Hopefully, we get around to doing that at some point soon, because I would really like to have proper constraint handling in Typed Template Haskell.

I am also a beginner in Haskell. Following the same documentation, I was trying to move the schema definition to another module and import it in main. It simply didn't work with the same error message.

After 2-3 hours of trying out copy/pasting code here and there, and testing it with postgres, I realized something during the process. In the documentation the comments containing Language features appeared on top of file before anything else.

Simply move the comments to the top before everything else

Besides extractor's apply metod accepting a Name

That's ok for a macro code to be difficult to read :) This is why metaprogramming shouldn't be the tool #1.

I can't see how your code can be reduced significantly.

You can replace

It looks like haskell-src-meta doesn’t support OverloadedLabels yet. The haskell-src-exts parser has an OverloadedLabels case, but haskell-src-meta doesn’t have a case for it in the ToExp instance for Exp. I guess the “unsupported” error message from haskell-src-meta is getting swallowed by the error handling in here.

You can't write

Quasiquote