Export materialize (#260)

The main reason I wasn't happy with this before is that there was nothing stopping you from writing `materialize query pure`. This returned query would produce invalid SQL if you actually tried to use it, because it would attempt to reference a common table expression outside the scope of the `WITH` statement.

The "solution" here is just to throw a `rebind` around the result such that `materialize` incurs an extra `Table Expr b` constraint, which means that you can't return `Query (Query a)` because `Query a` can't satisfy a `Table Expr` constraint.

src/Rel8.hs

@@ -251,6 +251,9 @@ module Rel8
   , without
   , withoutBy
 
+    -- ** @WITH@
+  , materialize
+
     -- ** @WITH RECURSIVE@
   , loop
 

src/Rel8/Query/Materialize.hs

@@ -33,11 +33,12 @@ import Rel8.Table.Opaleye ( unpackspec )
 -- 'materialize' to use the newer @WITH foo AS MATERIALIZED bar@ syntax
 -- introduced in PostgreSQL 12 in the future. Currently Rel8 does not use
 -- @AS MATERIALIZED@ to support earlier PostgreSQL versions.
-materialize :: Table Expr a => Query a -> (Query a -> Query b) -> Query b
+materialize :: (Table Expr a, Table Expr b)
+  => Query a -> (Query a -> Query b) -> Query b
 materialize query f =
-  fromOpaleye $
+  (>>= rebind "with") . fromOpaleye $
     withExplicit unpackspec
       (toOpaleye query')
       (toOpaleye . f . fromOpaleye)
   where
-    query' = query >>= rebind "with"
+    query' = query >>= rebind "materialize"

src/Rel8/Tabulate.hs

@@ -628,7 +628,7 @@ difference a b = a <* absent b
 
 
 -- | 'Q.materialize' for 'Tabulation's.
-materialize :: (Table Expr k, Table Expr a)
+materialize :: (Table Expr k, Table Expr a, Table Expr b)
   => Tabulation k a -> (Tabulation k a -> Query b) -> Query b
 materialize tabulation f = case peek tabulation of
   Tabulation query -> do