add specOrNull delegate

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 ### Added
+- Add `specOrNull` delegate to meta and Scheme
 
 ### Changed
 - `Factory` is now `fun interface` and uses `build` instead of `invoke`. `invoke moved to an extension.

dataforge-meta/src/commonMain/kotlin/space/kscience/dataforge/meta/Specification.kt

@@ -43,7 +43,7 @@ public interface Specification<out T : Any> : ReadOnlySpecification<T> {
  */
 public fun <T : Any> MutableMeta.updateWith(
     spec: Specification<T>,
-    action: T.() -> Unit
+    action: T.() -> Unit,
 ): T = spec.write(this).apply(action)
 
 
@@ -82,6 +82,31 @@ public fun <T : Scheme> Scheme.spec(
     key: Name? = null,
 ): ReadWriteProperty<Any?, T> = meta.spec(spec, key)
 
+/**
+ * A delegate that uses a [Specification] to wrap a child of this provider.
+ * Returns null if meta with given name does not exist.
+ */
+public fun <T : Scheme> MutableMeta.specOrNull(
+    spec: Specification<T>,
+    key: Name? = null,
+): ReadWriteProperty<Any?, T?> = object : ReadWriteProperty<Any?, T?> {
+    override fun getValue(thisRef: Any?, property: KProperty<*>): T? {
+        val name = key ?: property.name.asName()
+        return if (get(name) == null) null else spec.write(getOrCreate(name))
+    }
+
+    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
+        val name = key ?: property.name.asName()
+        if (value == null) remove(name)
+        else set(name, value.toMeta())
+    }
+}
+
+public fun <T : Scheme> Scheme.specOrNull(
+    spec: Specification<T>,
+    key: Name? = null,
+): ReadWriteProperty<Any?, T?> = meta.specOrNull(spec, key)
+
 /**
  * A delegate that uses a [Specification] to wrap a list of child providers.
  * If children are mutable, the changes in list elements are reflected on them.