Spark - calling a udf from spark sql

spark_udf_sql.scala

scala> import org.apache.spark.sql.functions.{input_file_name, udf}
import org.apache.spark.sql.functions.{input_file_name, udf}

scala>   def extract_file_name(path: String): String =
     |     path.split("/").last
extract_file_name: (path: String)String

scala> spark.sqlContext.udf.register("extract_file_name", extract_file_name _);
res4: org.apache.spark.sql.expressions.UserDefinedFunction = UserDefinedFunction(<function1>,StringType,Some(List(StringType)))

scala> spark.sql("select extract_file_name('tmp/test.txt') file_name").show()
+---------+
|file_name|
+---------+
| test.txt|
+---------+