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+# Rel8
+
+Welcome to Rel8! Rel8 is an API built on top of the
+fantastic [Opaleye](https://hackage.haskell.org/package/opaleye) library to
+provide an easy and type-safe way to interact with relational databases.
+
+## Differences With Opaleye
+
+### Table Definition
+
+Opaleye doesn't really prescribe much in the way of table definition. Perhaps
+the most idiomatic approach is to create a record where each field of the record
+is parameterized. Then you provide type aliases that either use concrete Haskell
+types or the `Column` type. The Opaleye tutorial demonstrates this as
+
+```haskell
+data Birthday' a b = Birthday { bdName :: a, bdDay :: b }
+type Birthday = Birthday' String Day
+type BirthdayColumn = Birthday' (Column PGText) (Column PGDate)
+```
+
+In Rel8, the idiomatic approach is to create a record per table, but we use a
+single type parameter to denote "where" this data is, and a type family to
+interpret each column. The above example would be written as
+
+```haskell
+data Birthday f = Birthday { bdName :: Anon f Text , bdName :: Anon f Day}
+instance Table (Birthday Expr) (Birthday QueryResult)
+```
+
+### Schema Declaration
+
+In Opaleye, the schema for a table is a value that you need to provide
+explicitly. For the `Birthday'` type above, in Opaleye you would write
+
+```haskell
+birthdayTable :: Table BirthdayColumn BirthdayColumn
+birthdayTable = Table "birthdayTable"
+                       (pBirthday Birthday { bdName = required "name"
+                                           , bdDay  = required "birthday" })
+```
+
+In Rel8, the schema is directly specified in the type. If `birthday` is a table,
+then our `Birthday` record would be written as:
+
+```haskell
+data Birthday f = Birthday 
+  { bdName :: Col f "name" 'NoDefault Text
+  , bdName :: Col f "birthday" 'NoDefault Day
+  } deriving (Generic)
+
+instance Table (Birthday Expr) (Birthday QueryResult)
+instance BaseTable Birthday where tableName = "birthdayTable"
+```
+
+### Column Types
+
+In Opaleye, column types form a distinct universe from ordinary Haskell types.
+When we define tables, we use types such as `PGText` and `PGDate`.
+
+In Rel8, you work entirely with the "result" types - the result of actually
+querying data from the database. Haskell types map to exactly one type in the
+database - `Text` is `text`, `Int64` is `bigint`, and so on. This mapping is
+captured by the `DBType` type class.
+
+### Aggregation
+
+In Opaleye, aggregation is performed by using the `aggregate` function which
+requires an `Aggregator`. Due to
+the
+[particularities of SQL](https://github.com/tomjaguarpaw/haskell-opaleye/issues/282),
+`Aggregators` are not `Arrow`s, nor are they functions. This leaves us with
+little option to build `Aggregator`s, though with `ProductProfunctor` (and some
+template Haskell), the pain is somewhat eased. From the basic tutorial:
+
+```haskell
+aggregateWidgets :: Query (Widget (Column PGText) (Column PGText) (Column PGInt8)
+                                  (Column PGInt4) (Column PGFloat8))
+aggregateWidgets = aggregate (pWidget (Widget { style    = groupBy
+                                              , color    = groupBy
+                                              , location = count
+                                              , quantity = sum
+                                              , radius   = avg }))
+                             (queryTable widgetTable)
+```
+
+This same approach is compatible with Rel8, but Rel8 has an alternative way to
+perform aggregating. In Rel8, we have
+
+```haskell
+aggregate :: AggregateTable exprsIn exprsOut => Query exprsIn -> Query exprsOut
+```
+
+This means that we can `aggregate` any `Query`, provided the result of that
+query is "valid" for aggregation. In Rel8, the above could be written as:
+
+```haskell
+aggregateWidgets :: Query (Widget Expr)
+aggregateWidgets = aggregate $ proc _ -> do
+  widget <- queryTable -< ()
+  returnA -< Widget { style = groupBy
+                    , color = groupBy
+                    , location = count
+                    , quantity = sum
+                    , radius = avg
+                    }
+
+instance AggregateTable (Widget Aggregate) (Widget Expr)
+```
+
+While the two seem similar, I have found the latter to be a little easier to
+work with, though the former is arguably closer to normal Haskell code
+(`aggregate` being similar to `foldMap`). Personally, I'm not entirely happy
+with either, so this space may change!
+
+### `Column` vs `Expr`
+
+The `Column` and `Expr` types are fundamentally same, but in order to avoid
+orphan instances I currently need to provide a `Expr` `newtype`. While `Column`
+should (morally) scope over `PG` types (`PGText`, `PGBool`, etc), `Expr` scopes
+over Haskell types.
+
+### Aggregation Functions
+
+The built in aggregation functions in Rel8 are a little bit more honest to how
+things work in PostgreSQL, at the expense of being less idiomatic Haskell. As
+PostgreSQL has overloaded functions, the aggregations are also overloaded
+functions provided by type classes. For example, we have
+
+```haskell
+sum :: Expr Int16 -> Aggreagte Int64
+sum :: Expr Int64 -> Aggregate Scientific
+sum :: Expr Double -> Aggreagte Double
+```
+
+### Outer Joins
+
+Rel8 contains a row transforming type `MaybeTable` to capture the result of
+outer joins. Opaleye deals with this by the use of `NullMaker`s. `MaybeTable`s,
+when selected, will return `Maybe` of the actual row itself. You can project
+columns out of a `MaybeTable` with the `$?` operator (function application on a
+possibly-`null` row).