Add ability to use custom aggregation functions with `aggregateFuncti…

…on` (#283)

changelog.d/20231009_170238_shane.obrien_aggregateFunction.md

@@ -0,0 +1,3 @@
+### Added
+
+- `aggregationFunction`, which allows custom aggregation functions to be used.

rel8.cabal

@@ -70,6 +70,7 @@ library
   other-modules:
     Rel8.Aggregate
     Rel8.Aggregate.Fold
+    Rel8.Aggregate.Function
 
     Rel8.Column
     Rel8.Column.ADT

src/Rel8.hs

@@ -284,6 +284,7 @@ module Rel8
   , countWhere, countWhereOn
   , and, andOn
   , or, orOn
+  , aggregateFunction
 
   , mode, modeOn
   , percentile, percentileOn
@@ -383,6 +384,7 @@ import Prelude ()
 -- rel8
 import Rel8.Aggregate
 import Rel8.Aggregate.Fold
+import Rel8.Aggregate.Function
 import Rel8.Column
 import Rel8.Column.ADT
 import Rel8.Column.Either

src/Rel8/Aggregate/Function.hs

@@ -0,0 +1,40 @@
+{-# language FlexibleContexts #-}
+{-# language MonoLocalBinds #-}
+
+module Rel8.Aggregate.Function (
+  aggregateFunction,
+) where
+
+-- base
+import Prelude
+
+-- opaleye
+import qualified Opaleye.Internal.Aggregate as Opaleye
+import qualified Opaleye.Internal.HaskellDB.PrimQuery as Opaleye
+
+-- rel8
+import Rel8.Aggregate (Aggregator1, unsafeMakeAggregator)
+import Rel8.Aggregate.Fold (Fallback (Empty))
+import Rel8.Expr (Expr)
+import Rel8.Expr.Opaleye (castExpr, fromColumn, fromPrimExpr)
+import Rel8.Schema.Null (Sql)
+import Rel8.Schema.QualifiedName (QualifiedName, showQualifiedName)
+import Rel8.Table (Table)
+import Rel8.Table.Opaleye (unpackspec)
+import Rel8.Type (DBType)
+
+
+-- | 'aggregateFunction' allows the use use of custom aggregation functions
+-- or PostgreSQL aggregation functions which are not otherwise supported by
+-- Rel8.
+aggregateFunction ::
+  (Table Expr i, Sql DBType a) =>
+  QualifiedName ->
+  Aggregator1 i (Expr a)
+aggregateFunction name =
+  unsafeMakeAggregator
+    id
+    (castExpr . fromPrimExpr . fromColumn)
+    Empty
+    (Opaleye.makeAggrExplicit unpackspec
+      (Opaleye.AggrOther (showQualifiedName name)))