Forum: Cfengine Help
Subject: Re: How to extract values (not keys) from an array to slist?
Author: yrouxel
Link to topic: https://cfengine.com/forum/read.php?3,21174,21211#msg-21211
Hi Mike,
I had a problem related to yours, but not quite the same. Sometimes, we want to
build a new list from an existing one by doing a transformation upon those
values. In functionnal programing this is the 'map' operator; in pseudo-code:
{"file1", "file2", "file3" } map (x -> '/etc/httpd/'+x) ==
{"/etc/httpd/file1", "/etc/httpd/file2", "/etc/httpd/file3" }
With CFEngine, this could be:
"myList1" slist => {"file1", "file2", "file3" }
"myList2" slist => {"/etc/httpd/$(myList1)"}
Unfortunately, this does not work because, the iteration over 'myList1' is done
over the whole '"myList2" slist =>...' declaration; as a consequence, each
iteration is flagged by CFEgine as a redefinition of the variable 'myList1'.
I did not manage to find a function or a way for CFEngine to provide the
equivalent of the 'map' operator. I think this is something that is missing in
the toolbox of CFEngine to manipulate data. There are the functions
{readstringlist, splitstring} which produce list, but 'readstringlist' reads
from a file and 'splitstring' is cumbersome to use.
Eventually, I found the function 'getindices' the best candidate to emulate the
'map' operator, though in a contrived way.
Rather than trying to explaining, I apply the technique to the above example:
"myList1" slist => {"file1", "file2", "file3" }
"myList2_array["/etc/httpd/$(myList1)"] => ""
"myList2" slist => getindices("myList2_array")
With this unconventionnal use of the 'getindices' function, we can mimic the
'map' operator. For our purpose, only the indices are of interest, the content
of the array 'myList2_array' is not important, so we use the empty string. The
real mapping is carried out in the index déclaration
''
It can be as sophisticated as the CFEngine syntax allows for the definition of
variables.
Regards
Yann Rouxel
_______________________________________________
Help-cfengine mailing list
Help-cfengine@cfengine.org
https://cfengine.org/mailman/listinfo/help-cfengine
