Update src/Rel8/Expr/Default.hs

Co-authored-by: Shane <shane.obrien@circuithub.com>

rel8.cabal

@@ -66,6 +66,7 @@ library
     Rel8.Expr.Aggregate
     Rel8.Expr.Array
     Rel8.Expr.Bool
+    Rel8.Expr.Default
     Rel8.Expr.Eq
     Rel8.Expr.Function
     Rel8.Expr.Null

src/Rel8.hs

@@ -250,14 +250,15 @@ module Rel8
   , Insert(..)
   , OnConflict(..)
   , insert
+  , unsafeDefault
 
     -- ** @DELETE@
   , Delete(..)
   , delete
 
     -- ** @UPDATE@
-  , update
   , Update(..)
+  , update
 
     -- ** @.. RETURNING@
   , Returning(..)
@@ -289,6 +290,7 @@ import Rel8.Column.These
 import Rel8.Expr
 import Rel8.Expr.Aggregate
 import Rel8.Expr.Bool
+import Rel8.Expr.Default
 import Rel8.Expr.Eq
 import Rel8.Expr.Function
 import Rel8.Expr.Null

src/Rel8/Expr/Default.hs

@@ -0,0 +1,35 @@
+module Rel8.Expr.Default
+  ( unsafeDefault
+  )
+where
+
+-- base
+import Prelude ()
+
+-- opaleye
+import qualified Opaleye.Internal.HaskellDB.PrimQuery as Opaleye
+
+-- rel8
+import Rel8.Expr ( Expr )
+import Rel8.Expr.Opaleye ( fromPrimExpr )
+
+
+-- | Corresponds to the SQL @DEFAULT@ expression.
+--
+-- This 'Expr' is unsafe for numerous reasons, and should be used with care:
+--
+-- 1. This 'Expr' only makes sense in an @INSERT@ or @UPDATE@ statement.
+--
+-- 2. Rel8 is not able to verify that a particular column actually has a
+-- @DEFAULT@ value. Trying to use @unsafeDefault@ where there is no default
+-- will cause a runtime crash
+--
+-- 3. @DEFAULT@ values can not be transformed. For example, the innocuous Rel8
+-- code @unsafeDefault + 1@ will crash, despite type checking.
+--
+-- Given all these caveats, we suggest avoiding the use of default values where
+-- possible, instead being explicit. A common scenario where default values are
+-- used is with auto-incrementing identifier columns. In this case, we suggest
+-- using 'Rel8.nextval' instead.
+unsafeDefault :: Expr a
+unsafeDefault = fromPrimExpr Opaleye.DefaultInsertExpr