Add indexed combinator (#117)

Changelog.md

@@ -27,6 +27,8 @@
 
 * The new `Rel8.Tabulate` module has been added, which gives a `Map`-esque interface to writing and composing queries. ([#70](https://github.com/circuithub/rel8/pull/70))
 
+* The new `indexed` `Query -> Query` function was added. This function augments each row in a query with it's 0-based index. ([#117](https://github.com/circuithub/rel8/pull/117))
+
 ## Breaking changes
 
 * `Insert`, `Delete` and `Update` have all changed. In particular, for `Insert` users should now replace `rows = xs` with `rows = values xs`. ([#85](https://github.com/circuithub/rel8/pull/85))

rel8.cabal

@@ -106,6 +106,7 @@ library
     Rel8.Query.Evaluate
     Rel8.Query.Exists
     Rel8.Query.Filter
+    Rel8.Query.Indexed
     Rel8.Query.Limit
     Rel8.Query.List
     Rel8.Query.Maybe

src/Rel8.hs

@@ -244,6 +244,9 @@ module Rel8
   , nullsFirst
   , nullsLast
 
+    -- ** Window functions
+  , indexed
+
     -- ** Bindings
   , rebind
 
@@ -322,6 +325,7 @@ import Rel8.Query.Either
 import Rel8.Query.Evaluate
 import Rel8.Query.Exists
 import Rel8.Query.Filter
+import Rel8.Query.Indexed
 import Rel8.Query.Limit
 import Rel8.Query.List
 import Rel8.Query.Maybe

src/Rel8/Query/Indexed.hs

@@ -0,0 +1,34 @@
+module Rel8.Query.Indexed
+  ( indexed
+  )
+where
+
+-- base
+import Data.Int ( Int64 )
+import Prelude
+
+-- opaleye
+import qualified Opaleye.Internal.HaskellDB.PrimQuery as Opaleye
+import qualified Opaleye.Internal.PackMap as Opaleye
+import qualified Opaleye.Internal.PrimQuery as Opaleye
+import qualified Opaleye.Internal.QueryArr as Opaleye
+import qualified Opaleye.Internal.Tag as Opaleye
+
+-- rel8
+import Rel8.Expr ( Expr )
+import Rel8.Expr.Opaleye ( fromPrimExpr )
+import Rel8.Query ( Query )
+import Rel8.Query.Opaleye ( mapOpaleye )
+
+
+-- | Pair each row of a query with its index within the query.
+indexed :: Query a -> Query (Expr Int64, a)
+indexed = mapOpaleye $ \(Opaleye.QueryArr f) -> Opaleye.QueryArr $ \(_, tag) ->
+  let
+    (a, query, tag') = f ((), tag)
+    tag'' = Opaleye.next tag'
+    window = Opaleye.ConstExpr $ Opaleye.OtherLit "ROW_NUMBER() OVER () - 1"
+    (index, bindings) = Opaleye.run $ Opaleye.extractAttr "index" tag' window
+    query' lateral = Opaleye.Rebind True bindings . query lateral
+  in
+    ((fromPrimExpr index, a), query', tag'')