Project setup and implementation sketch

.gitignore

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+dist-newstyle/
+*.swp

LICENSE

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+Copyright (c) 2023 Paul Brinkmeier
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1681753173,
+        "narHash": "sha256-MrGmzZWLUqh2VstoikKLFFIELXm/lsf/G9U9zR96VD4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "0a4206a51b386e5cda731e8ac78d76ad924c7125",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "An opionated Postgres library";
+
+  outputs = { self, nixpkgs }:
+    let
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in {
+      apps.x86_64-linux.cabal = {
+        type = "app";
+        program = "${nixpkgs.legacyPackages.x86_64-linux.cabal-install}/bin/cabal";
+      };
+      devShells.x86_64-linux.default = pkgs.mkShell {
+        packages = [
+          pkgs.cabal-install
+          pkgs.haskellPackages.implicit-hie
+          (pkgs.ghc.withPackages (hp: with hp; [
+            containers
+            bytestring
+            postgresql-libpq
+            text
+          ]))
+
+          pkgs.haskell-language-server
+        ];
+        shellHook = ''
+          PS1="<opium> ''${PS1}"
+        '';
+      };
+    };
+}

hie.yaml

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+cradle:
+  cabal:
+    - path: "lib"
+      component: "lib:opium"
+
+    - path: "test"
+      component: "opium:test:opium-test"

lib/Database/PostgreSQL/Opium.hs

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+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeOperators #-}
+
+module Database.PostgreSQL.Opium where
+
+import Data.ByteString (ByteString)
+import Data.Proxy (Proxy (Proxy))
+import Database.PostgreSQL.LibPQ
+  (Result
+  , Row
+  )
+import GHC.Generics (C, D, Generic, K1 (..), M1 (..), Meta (..), Rec0, Rep, S, to, (:*:) (..))
+import GHC.TypeLits (KnownSymbol, symbolVal)
+import Text.Printf (printf)
+
+import qualified Data.Text as Text
+import qualified Data.Text.Encoding as Encoding
+import qualified Database.PostgreSQL.LibPQ as LibPQ
+
+class FromRow a where
+  fromRow :: Row -> Result -> IO (Maybe a)
+  default fromRow :: (Generic a, FromRow' (Rep a)) => Row -> Result -> IO (Maybe a)
+  fromRow row result = fmap to <$> fromRow' row result
+
+class FromRow' f where
+  fromRow' :: Row -> Result -> IO (Maybe (f p))
+
+instance FromRow' f => FromRow' (M1 D c f) where
+  fromRow' row result = fmap M1 <$> fromRow' row result
+
+instance FromRow' f => FromRow' (M1 C c f) where
+  fromRow' row result = fmap M1 <$> fromRow' row result
+
+instance (FromRow' f, FromRow' g) => FromRow' (f :*: g) where
+  fromRow' row result = do
+    y <- fromRow' row result
+    z <- fromRow' row result
+    pure $ (:*:) <$> y <*> z
+
+instance (KnownSymbol nameSym, FromField t) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
+  fromRow' row result = do
+    mbColumn <- LibPQ.fnumber result name
+    case mbColumn of
+      Nothing -> pure Nothing
+      Just column -> do
+        mbField <- LibPQ.getvalue result row column
+        printf "%s: %s" (show name) (show mbField)
+        pure $ M1 . K1 <$> fromField mbField
+    where
+      name = Encoding.encodeUtf8 $ Text.pack $ symbolVal (Proxy :: Proxy nameSym)
+
+class FromField a where
+  fromField :: Maybe ByteString -> Maybe a
+
+instance FromField String where
+  fromField = fmap (Text.unpack . Encoding.decodeUtf8)
+
+instance FromField Int where
+  fromField = fmap (read . Text.unpack . Encoding.decodeUtf8)

opium.cabal

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+cabal-version:      3.0
+-- The cabal-version field refers to the version of the .cabal specification,
+-- and can be different from the cabal-install (the tool) version and the
+-- Cabal (the library) version you are using. As such, the Cabal (the library)
+-- version used must be equal or greater than the version stated in this field.
+-- Starting from the specification version 2.2, the cabal-version field must be
+-- the first thing in the cabal file.
+
+-- Initial package description 'opium' generated by
+-- 'cabal init'. For further documentation, see:
+--   http://haskell.org/cabal/users-guide/
+--
+-- The name of the package.
+name:               opium
+
+-- The package version.
+-- See the Haskell package versioning policy (PVP) for standards
+-- guiding when and how versions should be incremented.
+-- https://pvp.haskell.org
+-- PVP summary:     +-+------- breaking API changes
+--                  | | +----- non-breaking API additions
+--                  | | | +--- code changes with no API change
+version:            0.1.0.0
+
+-- A short (one-line) description of the package.
+-- synopsis:
+
+-- A longer description of the package.
+-- description:
+
+-- The license under which the package is released.
+license:            MIT
+
+-- The file containing the license text.
+license-file:       LICENSE
+
+-- The package author(s).
+author:             Paul Brinkmeier
+
+-- An email address to which users can send suggestions, bug reports, and patches.
+maintainer:         hallo@pbrinkmeier.de
+
+-- A copyright notice.
+-- copyright:
+category:           Database
+build-type:         Simple
+
+-- Extra doc files to be distributed with the package, such as a CHANGELOG or a README.
+-- extra-doc-files:    CHANGELOG.md
+
+-- Extra source files to be distributed with the package, such as examples, or a tutorial module.
+-- extra-source-files:
+
+common warnings
+    ghc-options: -Wall
+
+library
+    -- Import common warning flags.
+    import:           warnings
+
+    -- Modules exported by the library.
+    exposed-modules:
+        Database.PostgreSQL.Opium
+
+    -- Modules included in this library but not exported.
+    -- other-modules:
+
+    -- LANGUAGE extensions used by modules in this package.
+    -- other-extensions:
+
+    -- Other library packages from which modules are imported.
+    build-depends:
+        base,
+        bytestring,
+        containers,
+        postgresql-libpq,
+        text
+
+    -- Directories containing source files.
+    hs-source-dirs:   lib
+
+    -- Base language which the package is written in.
+    default-language: Haskell2010
+
+test-suite opium-test
+    -- Import common warning flags.
+    import:           warnings
+
+    -- Base language which the package is written in.
+    default-language: Haskell2010
+
+    -- Modules included in this executable, other than Main.
+    -- other-modules:
+
+    -- LANGUAGE extensions used by modules in this package.
+    -- other-extensions:
+
+    -- The interface type and version of the test suite.
+    type:             exitcode-stdio-1.0
+
+    -- Directories containing source files.
+    hs-source-dirs:   test
+
+    -- The entrypoint to the test suite.
+    main-is:          Main.hs
+
+    -- Test dependencies.
+    build-depends:
+        base,
+        opium

test/Main.hs

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+{-# LANGUAGE DeriveGeneric #-}
+
+module Main (main) where
+
+import GHC.Generics (Generic)
+
+import Database.PostgreSQL.Opium (FromRow)
+
+data Person = Person
+  { name :: String
+  , age :: Int
+--  , lovesCats :: Bool
+  } deriving (Generic)
+
+instance FromRow Person where
+
+main :: IO ()
+main = putStrLn "TBD"