Added lengthOrNull variant

tribune-core/src/main/kotlin/com/sksamuel/tribune/core/nulls.kt

@@ -42,7 +42,7 @@ fun <I, A, E> Parser<I, A?, E>.notNull(ifNull: () -> E): Parser<I, A, E> =
    Parser { input: I -> this@notNull.parse(input).flatMap { it?.right() ?: ifNull().leftNel() } }
 
 /**
- * Maps a [Parser] to never fail, by replacing any failing values with null.
+ * Transforms a [Parser] to never fail, by replacing any failing values with null.
  */
 fun <I, A, E> Parser<I, A, E>.failAsNull(): Parser<I, A?, Nothing> =
    Parser { input: I -> this@failAsNull.parse(input).fold({ Either.Right(null) }, { it.right() }) }

tribune-core/src/main/kotlin/com/sksamuel/tribune/core/strings/lengths.kt

@@ -9,7 +9,7 @@ import com.sksamuel.tribune.core.flatMap
 /**
  * Narrows an existing String -> String [Parser] by enforcing an exact length on the input string.
  *
- * For strings which have length that differs than the given [len] arguments, an invalid is
+ * For strings which have length that differs than the given [len] arguments, a failure is
  * returned with the error message generated by [ifError].
  *
  * @param len the max length, inclusive
@@ -25,6 +25,27 @@ fun <I, E> Parser<I, String, E>.length(len: Int, ifError: (String) -> E): Parser
       }
    }
 
+/**
+ * Narrows an existing String -> String [Parser] by enforcing an exact length on the input string.
+ *
+ * For strings which have length that differs than the given [len] arguments, a failure is
+ * returned with the error message generated by [ifError].
+ *
+ * @param len the max length, inclusive
+ * @param ifError the error generating function
+ *
+ * @return valid if the input string is less than or equal to [len] or an invalid otherwise.
+ */
+@JvmName("lengthOrNull")
+fun <I, E> Parser<I, String?, E>.length(len: Int, ifError: (String) -> E): Parser<I, String?, E> =
+   flatMap {
+      when {
+         it == null -> Either.Right(null)
+         it.length == len -> it.right()
+         else -> ifError(it).leftNel()
+      }
+   }
+
 /**
  * Maps a string producing [Parser] by enforcing a length on the string.
  *