Add recommended hlint rules for proper isNothing usage (#214)

* Add recommended hlint rules for proper isNothing usage

* Update changelog.md

changelog.md

@@ -1,3 +1,9 @@
+3.3.4.1
+
+- @maxgabriel
+  - [#214](https://github.com/bitemyapp/esqueleto/pull/214)
+    - Add suggested hlint rules for proper `isNothing` usage
+
 3.3.4.0
 =======
 - @parsonsmatt

esqueleto.cabal

@@ -1,7 +1,7 @@
 cabal-version: 1.12
 
 name:           esqueleto
-version:        3.3.4.0
+version:        3.3.4.1
 synopsis:       Type-safe EDSL for SQL queries on persistent backends.
 description:    @esqueleto@ is a bare bones, type-safe EDSL for SQL queries that works with unmodified @persistent@ SQL backends.  Its language closely resembles SQL, so you don't have to learn new concepts, just new syntax, and it's fairly easy to predict the generated SQL and optimize it for your backend. Most kinds of errors committed when writing SQL are caught as compile-time errors---although it is possible to write type-checked @esqueleto@ queries that fail at runtime.
                 .

src/Database/Esqueleto/Internal/Internal.hs

@@ -599,6 +599,13 @@ val v = ERaw Never $ const ("?", [toPersistValue v])
 -- +----------------+----------------------------------+---------------+
 --
 -- In SQL, @= NULL@ and @!= NULL@ return NULL instead of true or false. For this reason, you very likely do not want to use @'!=.' Nothing@ in Esqueleto.
+-- You may find these @hlint@ rules helpful to enforce this:
+--
+-- > - error: {lhs: v ==. nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
+-- > - error: {lhs: v ==. val Nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
+-- > - error: {lhs: v !=. nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}
+-- > - error: {lhs: v !=. val Nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}
+
 
 
 isNothing :: PersistField typ => SqlExpr (Value (Maybe typ)) -> SqlExpr (Value Bool)