Add quick start section to readme

README.md

@@ -2,14 +2,66 @@
 
 > An opionated Haskell Postgres library.
 
+## Quick Start
+
+We assume that our database contains this table:
+
+```sql
+CREATE TABLE person (
+  name TEXT NOT NULL,
+  age INT NOT NULL,
+  score DOUBLE PRECISION NOT NULL,
+  motto TEXT
+)
+```
+
+We can use `opium` to decode query results into Haskell data types:
+
+```haskell
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+import GHC.Generics (Generic)
+import Database.PostgreSQL.LibPQ (Connection)
+import qualified Database.PostgreSQL.Opium as Opium
+
+data User = User
+  { name  :: String
+  , age   :: Int
+  , score :: Double
+  } deriving (Eq, Generic, Show)
+
+instance Opium.FromRow User where
+
+getUsers :: Connection -> IO (Either Opium.Error [Users])
+getUsers conn = Opium.fetch_ conn "SELECT * FROM user"
+```
+
+The `Opium.FromRow` instance is implemented generically for all product types ("records"). It looks up the field name in the query result and decodes the column value using `Opium.FromField`.
+
+The intended use case for this library is to enable us to write ad-hoc types for query results without having to manually write instances.
+For example, if we wanted to figure out how user age influences their score:
+
+```haskell
+data ScoreByAge = ScoreByAge { t :: Double, m :: Double }
+  deriving (Eq, Generic, Show)
+  
+getScoreByAge :: Connection -> IO ScoreByAge
+getScoreByAge conn = do
+  let query = "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM user"
+  Right [x] <- Opium.fetch_ conn query
+  pure x
+```
+
 ## TO DO
 
 - [x] Implement `String` and `Text` decoding
 - [x] Implement `Int` decoding
 - [x] Implement error reporting i.e. use `Either OpiumError` instead of `Maybe`
 - [x] Implement `Float` and `Double` decoding
+- [x] Clean up and document column table stuff
+- [ ] Implement `fetch` (`fetch_` but with parameter passing)
 - [ ] Implement `UTCTime` and zoned time decoding
 - [ ] Implement JSON decoding
 - [ ] Implement `ByteString` decoding (`bytea`)
   - Can we make the fromField instance choose whether it wants binary or text?
-- [x] Clean up and document column table stuff

test/Database/PostgreSQL/OpiumSpec.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TypeApplications #-}
 
@@ -38,6 +39,13 @@ data ManyFields = ManyFields
 
 instance Opium.FromRow ManyFields where
 
+data ScoreByAge = ScoreByAge
+  { m :: Double
+  , t :: Double
+  } deriving (Eq, Generic, Show)
+
+instance Opium.FromRow ScoreByAge where
+
 isLeft :: Either a b -> Bool
 isLeft (Left _) = True
 isLeft _ = False
@@ -125,3 +133,7 @@ spec = do
     it "Fails for the wrong column type" $ \conn -> do
       rows <- Opium.fetch_ @Person conn "SELECT 'quby' AS name, 'indeterminate' AS age"
       rows `shouldBe` Left (Opium.ErrorInvalidOid "age" $ LibPQ.Oid 25)
+
+    it "Works for the readme regression example" $ \conn -> do
+      rows <- Opium.fetch_ @ScoreByAge conn "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM person"
+      rows `shouldSatisfy` \case { (Right [ScoreByAge _ _]) -> True; _ -> False }

test/SpecHook.hs

@@ -23,8 +23,8 @@ setupConnection = do
   conn <- LibPQ.connectdb $ Encoding.encodeUtf8 $ Text.pack dsn
   _ <- LibPQ.setClientEncoding conn "UTF8"
 
-  _ <- LibPQ.exec conn "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL, motto TEXT)"
+  _ <- LibPQ.exec conn "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL, score DOUBLE PRECISION NOT NULL, motto TEXT)"
-  _ <- LibPQ.exec conn "INSERT INTO person VALUES ('paul', 25), ('albus', 103)"
+  _ <- LibPQ.exec conn "INSERT INTO person VALUES ('paul', 25, 30), ('albus', 103, 50.42)"
 
   pure conn