private boolean isTypeCompatible(ObjectInspector argument) {
    PrimitiveObjectInspector poi = ((PrimitiveObjectInspector) argument);
    return
      poi.getPrimitiveCategory() == PrimitiveObjectInspector.PrimitiveCategory.STRING ||
      poi.getPrimitiveCategory() == PrimitiveObjectInspector.PrimitiveCategory.CHAR ||
      poi.getPrimitiveCategory() == PrimitiveObjectInspector.PrimitiveCategory.VARCHAR ||
      poi.getPrimitiveCategory() == PrimitiveObjectInspector.PrimitiveCategory.VOID;
  }
IN_FILE(str, filename) returns true if 'str' appears in the file specified
by 'filename'.  A string is considered to be in the file if it that string
appears as a line in the file.

If either argument is NULL then NULL is returned.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFInFile.java