Make Expr's role nominal

changelog.d/20260921_122034_teofilcamarasu_expr_role.md

@@ -0,0 +1,3 @@
+### Fixed
+
+- Make `Expr` have a representational role, which disallows freely coercing the type within `Expr`. Users can still use `unsafeCastExpr` if they want this behaviour.

rel8-internal/src/Rel8/Internal/Expr.hs

@@ -9,6 +9,7 @@
 {-# language TypeApplications #-}
 {-# language TypeFamilies #-}
 {-# language UndecidableInstances #-}
+{-# language RoleAnnotations #-}
 
 module Rel8.Internal.Expr
   ( Expr(..)
@@ -53,6 +54,7 @@ import Data.Scientific (fromRationalRepetendLimited)
 
 -- | Typed SQL expressions.
 type Expr :: K.Context
+type role Expr nominal
 newtype Expr a = Expr Opaleye.PrimExpr
   deriving stock Show
 

rel8-internal/src/Rel8/Internal/Expr.hs-boot

@@ -1,5 +1,6 @@
 {-# language DataKinds #-}
 {-# language StandaloneKindSignatures #-}
+{-# language RoleAnnotations #-}
 
 module Rel8.Internal.Expr
   ( Expr(..)
@@ -17,4 +18,5 @@ import Rel8.Internal.Schema.Kind ( Context )
 
 
 type Expr :: Context
+type role Expr nominal
 newtype Expr a = Expr Opaleye.PrimExpr