Use cabal2nix for flake expression

README.md

@@ -83,3 +83,4 @@ getScoreByAge conn = do
     - [ ] `FromRow`
     - [ ] Custom `FromField` impls
 - [ ] Improve type errors when trying to `instance` a type that isn't a record (e.g. sum type)
+- [ ] Improve documentation for `fromRow` module

flake.nix

@@ -3,33 +3,24 @@
 
   outputs = { self, nixpkgs }:
     let
-      pkgs = nixpkgs.legacyPackages.x86_64-linux;
-    in {
-      apps.x86_64-linux.cabal = {
-        type = "app";
-        program = "${nixpkgs.legacyPackages.x86_64-linux.cabal-install}/bin/cabal";
-      };
-      devShells.x86_64-linux.default = pkgs.mkShell {
-        packages = [
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+      opium = pkgs.haskellPackages.developPackage {
+        root = ./.;
+        modifier = drv:
+          pkgs.haskell.lib.addBuildTools drv [
             pkgs.cabal-install
             pkgs.haskellPackages.implicit-hie
-          (pkgs.ghc.withPackages (hp: with hp; [
-            attoparsec
-            containers
-            bytestring
-            hspec
-            postgresql-libpq
-            text
-            time
-            transformers
-            vector
-          ]))
-
             pkgs.haskell-language-server
           ];
-        shellHook = ''
-          PS1="<opium> ''${PS1}"
-        '';
       };
+    in {
+      packages.${system}.opium = pkgs.haskell.lib.overrideCabal opium {
+        # Currently the tests require a running Postgres instance.
+        # This is not automated yet, so don't export the tests.
+        doCheck = false;
+      };
+
+      devShells.${system}.default = opium.env;
     };
 }

lib/Database/PostgreSQL/Opium/FromRow.hs

@@ -89,7 +89,7 @@ type family NumberOfMembers f where
   NumberOfMembers (M1 D _ f) = NumberOfMembers f
   -- The constructor has as many members as the type that it contains.
   NumberOfMembers (M1 C _ f) = NumberOfMembers f
-  -- A product type has as many types as its subtypes have together.
+  -- A product type has as many members as its subtypes have together.
   NumberOfMembers (f :*: g) = NumberOfMembers f + NumberOfMembers g
   -- A selector has/is exactly one member.
   NumberOfMembers (M1 S _ f) = 1
@@ -99,19 +99,23 @@ data FromRowCtx = FromRowCtx
   Result -- ^ Obtained from 'LibPQ.execParams'.
   ColumnTable -- ^ 'Vector' of expected columns indices and OIDs.
 
+-- Specialized proxy type to be used instead of `Proxy (n, f)`
 data FRProxy (n :: Nat) (f :: Type -> Type) = FRProxy
 
 class FromRow' (n :: Nat) (f :: Type -> Type) where
   fromRow' :: FRProxy n f -> FromRowCtx -> Row -> ExceptT Error IO (f p)
 
 instance FromRow' n f => FromRow' n (M1 D c f) where
-  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    M1 <$> fromRow' @n FRProxy ctx row
 
 instance FromRow' n f => FromRow' n (M1 C c f) where
-  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    M1 <$> fromRow' @n FRProxy ctx row
 
 instance (FromRow' n f, FromRow' (n + NumberOfMembers f) g) => FromRow' n (f :*: g) where
-  fromRow' FRProxy ctx row = (:*:) <$> fromRow' @n FRProxy ctx row <*> fromRow' @(n + NumberOfMembers f) FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    (:*:) <$> fromRow' @n FRProxy ctx row <*> fromRow' @(n + NumberOfMembers f) FRProxy ctx row
 
 instance {-# OVERLAPPABLE #-} (KnownNat n, KnownSymbol nameSym, FromField t) => FromRow' n (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
   fromRow' FRProxy = decodeField memberIndex nameText $ \row ->
@@ -158,4 +162,3 @@ decodeField memberIndex nameText g (FromRowCtx result columnTable) row = do
       first
         (ErrorInvalidField (ErrorPosition row nameText) oid field)
         (Just <$> fromField field)
-