Add null for implementing serdes

CHANGELOG.md

@@ -2,6 +2,9 @@
 All notable changes to this project will be documented in this file. This change log follows the conventions of [keepachangelog.com](http://keepachangelog.com/).
 
 ## [UNRELEASED]
+### Added
+- New `coffi.mem/null` var for implementing custom types
+
 ### Fixed
 - Usage of deprecated `(Class/STATIC_FIELD)` access pattern
 

src/clj/coffi/ffi.clj

@@ -292,7 +292,7 @@
 
                  ;; cast null pointers to something understood by panama
                  (#{::mem/pointer} type)
-                 `(or ~sym MemorySegment/NULL)
+                 `(or ~sym mem/null)
 
                  (mem/primitive-type type)
                  `(mem/serialize* ~sym ~type-sym ~arena)

src/clj/coffi/mem.clj

@@ -112,10 +112,18 @@
   ^long [addressable]
   (.address ^MemorySegment addressable))
 
+(def ^MemorySegment null
+  "The NULL pointer object.
+
+  While this object is safe to pass to functions which serialize to a pointer,
+  it's generally encouraged to simply pass `nil`. This value primarily exists to
+  make it easier to write custom types with a primitive pointer representation."
+  MemorySegment/NULL)
+
 (defn null?
   "Checks if a memory address is null."
   [addr]
-  (or (.equals MemorySegment/NULL addr) (not addr)))
+  (or (.equals null addr) (not addr)))
 
 (defn address?
   "Checks if an object is a memory address.
@@ -872,7 +880,7 @@
         (serialize-into obj (second type) segment arena)
         (address-of segment))
       obj)
-    MemorySegment/NULL))
+    null))
 
 (defmethod serialize* ::void
   [_obj _type _arena]
@@ -1132,7 +1140,7 @@
   [obj _type ^Arena arena]
   (if obj
     (.allocateFrom arena ^String obj)
-    MemorySegment/NULL))
+    null))
 
 (defmethod deserialize* ::c-string
   [addr _type]