Allow callback_args to be NULL

DESCRIPTION

@@ -2,7 +2,7 @@ Encoding: UTF-8
 Package: optparse
 Type: Package
 Title: Command Line Option Parser
-Version: 1.6.0-0
+Version: 1.6.0-1
 Authors@R: c(person("Trevor L", "Davis", role = c("aut", "cre"), email="[email protected]"),
     person("Allen", "Day", role="ctb", comment="Some documentation and examples ported from the getopt package."),
     person("Python Software Foundation", role="ctb", comment="Some documentation from the optparse Python module."),

R/optparse.R

@@ -274,6 +274,8 @@ make_option <- function(opt_str, action="store", type=NULL, dest=NULL, default=N
         if(!is.null(callback_args))      
             warning(sprintf("callback_args argument is supplied for non-callback action")) 
     }
+    if (is.null(callback_args))
+        callback_args <- list()
 
     return(new("OptionParserOption", short_flag=short_flag, long_flag=long_flag,
                         action=action, type=type, dest=dest, default=default, 

tests/testthat/test-optparse.R

@@ -143,6 +143,11 @@ test_that("callback works as expected", {
     opts <- parse_args(parser2, args = c("--value=2"))
     expect_equal(opts$value, 8)
 
+    parser3 <- add_option(parser0, c("-v", "--value"), type="integer",
+                         action="callback", callback=callback_fn)
+    opts <- parse_args(parser3, args = c("--value=2"))
+    expect_equal(opts$value, 4)
+
     expect_warning(add_option(parser0, "--warning", callback=as.list), "callback argument is supplied for non-callback action")
     expect_warning(add_option(parser0, "--warning", callback_args=list(3,b=4)), "callback_args argument is supplied for non-callback action")
     expect_error(add_option(parser0, "--warning", action="callback", callback="hello"), "Option type cannot be inferred")