Upgrade 0.5.1.0 → 0.9.1.3: Query (`Statement`) with generic parameter (formerly using `ToParam`)

[gasi]

Hi Eitan,

I’m currently doing a big migration from Squeal 0.5.1.0 → 0.9.1.3 and I love it overall, especially how EncodeParams + DecodeRows gets rid of so much boilerplate.

Problem

I was able to figure out 95% of the work, but I get stuck on turning this query that used to take an arbitrary (generic?) parameter (using ToParam), that can convert to a PostgreSQL type, to a Statement with EncodeParams + DecodeRow. I had to revert and hard-code it to PGtext + Text which broke some of my queries that need PGtimestampz + UTCTime:

Statement: selectContentBy

Usage: getBy

---

I tried the following without luck:

selectContentBy ::
  (ToParam Schemas pgty hsty, SOP.Generic hsty) =>
  ( TableExpression 'Ungrouped '[] '[] Schemas '[pgty] from ->
    TableExpression 'Ungrouped '[] '[] Schemas '[pgty] from
  ) ->
  Statement Schemas hsty Content
selectContentBy clauses = Query encode decode sql
  where
    encode :: EncodeParams Schemas pgty hsty
    encode = aParam
    decode = decodeContentWithImage
    sql = -- ...

as I get this error ❌:

zoomhub/src/ZoomHub/Storage/PostgreSQL/Internal.hs:180:33: error: [GHC-18872]
    • Couldn't match kind ‘NullType’ with ‘*’
      When matching types
        pgty0 :: [*]
        '[pgty] :: [NullType]
      Expected: EncodeParams
                  '["public"
                    ::: [ZoomHub.Storage.PostgreSQL.Schema.Schema0.ConfigTable0,
                         ZoomHub.Storage.PostgreSQL.Schema.Schema3.ContentTable3,
                         ZoomHub.Storage.PostgreSQL.Schema.Schema0.ImageTable0,
                         ZoomHub.Storage.PostgreSQL.Schema.Schema0.FlickrTable0]]
                  '[pgty]
                  hsty
        Actual: EncodeParams Schemas pgty0 hsty
    • In the first argument of ‘Query’, namely ‘encode’
      In the expression: Query encode decode sql
      In an equation for ‘selectContentBy’:
          selectContentBy clauses
            = Query encode decode sql
            where
                encode :: EncodeParams Schemas pgty hsty
                encode = aParam
                decode = decodeContentWithImage
                sql = -- ...

---

I was thinking it might be something obvious you could spot right away. If not, I’m happy to go back and do the work to create a minimally reproducing example.

I appreciate your help 😄

[echatav]

Hmmm. Looks like GHC is inferring the kind of pgty0 too monomorphically, too soon. Can you try turning on PolyKinds? See if that helps. That's what the error poetry tells me:

    • Couldn't match kind ‘NullType’ with ‘*’
      When matching types
        pgty0 :: [*]
        '[pgty] :: [NullType]

[echatav]

Or perhaps there is a mistake in the type annotation of your encode term.

-    encode :: EncodeParams Schemas pgty hsty
+    encode :: EncodeParams Schemas '[pgty] hsty

Turning on ScopedTypeVariables should let GHC identify this pgty with the one in the type annotation of selectContentBy.

[echatav]

It might typecheck if you just removed the type signature for encode entirely and let GHC infer it.

[gasi]

@echatav Thank you so much for taking the time to look into this. I tried your suggestions, but unfortunately, I wasn’t able to resolve the error.

To make it easier for you and potentially contribute to Squeal with an example of a parametrized query, I created this draft PR with an extension of the existing Example.hs with a parametrized query: #354

Does this help you repro and potentially resolve the issue?

Thanks 🙇‍♂️

---

My dream is that one day a GitHub search for EncodeParams, DecodeRow, consRow, appendRows, etc. will show lots of examples of Squeal in action helping others enjoy this wonderful library. Hopefully figuring out the upgrade in zoomhub/zoomhub#235 will be a first such example ✨

[gasi]

P.S. I let GPT-4 loose on this, but Squeal type errors are too powerful for our AI overlords 😜

[echatav]

I've kind of been stuck working on Squeal because every time I try I run into GHC/Postgres installation location problems.

squeal-postgresql          > build (lib + exe)
squeal-postgresql          > Preprocessing library for squeal-postgresql-0.9.1.3..
squeal-postgresql          > Building library for squeal-postgresql-0.9.1.3..
squeal-postgresql          > ld: warning: -single_module is obsolete
squeal-postgresql          > ld: warning: search path '/opt/homebrew/opt/libpq/lib' not found
squeal-postgresql          > ld: library 'pq' not found
squeal-postgresql          > clang: error: linker command failed with exit code 1 (use -v to see invocation)
squeal-postgresql          > `gcc' failed in phase `Linker'. (Exit code: 1)

Googling hasn't helped me. I hate computers. If you help me figure out how I can get GHC to find LibPQ then I can help with Squeal again.

[gasi]

@echatav Re: libpq. Ouch, I’m sorry! Are you on macOS / Apple Silicon? I had similar issues and it took me a while to resolve. In fact, that’s what that whole stack LTS upgrade is about (I got a new MacBook laptop last year.) If you want, I’d be happy to hop on a Zoom call with you and try and help out. Shoot me an email to daniel@gasienica.ch if you are interested 😄

Re: generic SELECT. I partially figured it out!

1. First, it was through discovering ~ for type equality in your docs (but I had to enable AllowAmbiguousTypes 😢): 3a1851b
2. Then I found out I can drop pgty and infer it from hsty: 168813c
3. Bonus: Added encoding/decoding custom type to example (however, I haven’t looked into how to make it type-safe yet): 21057aa

---

I actually was able to get it working first in my own code (without even needing AllowAmbiguousTypes 🤷‍♂️):
zoomhub/zoomhub@c297109#diff-d0b2f0351a2e5459c9b6306ed066f9e76873900c6bfc62fbc24dc5c2cad2f036R219-R232

I’m generally pretty happy about it except for this long from type signature and the need for OidOfNull Schemas. I originally had from inferred using _ + PartialTypeSignatures, but that started causing problems. Suggestions on how to improve it are welcome: zoomhub/zoomhub@c297109#diff-d0b2f0351a2e5459c9b6306ed066f9e76873900c6bfc62fbc24dc5c2cad2f036R182-R217

[echatav]

@gasi Nice job! Yes, I'm on a macbook. I'll send you an email. I've been wanting to get back to Squeal a bit.

[echatav]

Re: type safety. I guess you mean for OrganizationType on the Haskell side having a corresponding Postgres type so the type column isn't stringly-typed. For that you can use Postgres enum types. Squeal even has special support for creating Postgres enums from a Haskell type like so:

>>> :{
data Schwarma = Beef | Lamb | Chicken
  deriving stock GHC.Generic
  deriving anyclass (SOP.Generic, SOP.HasDatatypeInfo)
  deriving (IsPG, FromPG) via Enumerated Schwarma
:}
>>> :{
let
  createSchwarma :: Definition (Public '[]) '["public" ::: '["schwarma" ::: 'Typedef (PG Schwarma)]]
  createSchwarma = createTypeEnumFrom @Schwarma #schwarma
in
  printSQL createSchwarma
:}
CREATE TYPE "schwarma" AS ENUM ('Beef', 'Lamb', 'Chicken');