leanRcoding: Leaner R Coding

Recursive/nested if-statements can pop up when the latter statements need the former statements to be true in order to be well-defined, in which case it might be rather messy to formulate an "all-must-be-TRUE" statement. Such nested if-statements can e.g. occur when inspecting objects stored in recursive lists where different branches can have quite different structures, in which case any statement regarding a given level of a list must be preceeded by a test of the kind !identical(list$level, NULL).

1 2	nested_if(if_list = list(), expr_all_TRUE = TRUE, expr_not_all_TRUE = invisible(NULL), env = parent.frame())

`if_list`	A list of the statements we want to test, default value `list()`.
`expr_all_TRUE`	The result to return if the nested structure turns out to contain well defined stuff that passes the tests. The default value for this is `TRUE`.
`expr_not_all_TRUE`	The result to return if it turns out that something goes wrong (not defined, or false statements). The default value for this is `NULL`.
`env`	The environment at which the statements should be evaluated, the default value is `parent.frame()`

The result will be TRUE if all the statements given in if_list is TRUE, otherwise the result will be FALSE. The result will be FALSE if if_list has length zero.