Thank you for the feedback, very good comments.
See more inline...
On 2013-11-09 11:02, robbyt wrote:
I am reading over the release notes for Puppet 3.3, and buried under
"data in modules" is a link to ARM-9.
There is a good chance that I don't "get it" - Puppet is moving pretty
fast these days and I am still trying to get caught up with all of the
changes.
I have only read the latest Puppet code and the ARM docs- I have not
tried it on a real system yet. However, I am concerned about the
usefullness of the default lookup syntax. I find the examples in the ARM
document a bit obtuse.
https://github.com/puppetlabs/armatures/blob/master/arm-9.data_in_modules/index.md#lookup-with-default
With hiera1, we can (very simply) do:
hiera("namespace::key_name", <default value> )
Hiera2 requires some more work:
$x = lookup('something')
$looked_up = $x ? { undef => 'nothing', default => $x }
This is ugly. Checking if variables are undefined, and then setting them
to a default is what we did in bash. Not good.
The challenge here was to have a function that works for 3x as well as
for what is being worked on for 4x (where the "data-in-modules"
supposedly is on by default along "parser future"). When we reach that
point, it is possible to reference a type directly without having to
encode it in a string. i.e. it will be possible to do like this:
lookup('mykey', Integer)
For 3.x this was not possible and the signature of the lookup function
is now lookup(String key, String type), or just lookup(String key), and
thus difficult to handle both type and default value as optional (given
two strings, it that key and type or key and default).
We have a couple of options before 4x:
a) Add a third argument, and if a default is wanted, type must be specified.
b) Add the ability to pass arguments as a hash, it can either be a third
argument, or used instead of the type argument. (i.e. giving arguments
by name instead).
The first is simple to add, the second does not work well unless the
future parser is also used.
Examples:
# a
lookup('mykey', 'Integer', 42)
# b (requires future parser since 3x does not support direct passing
# of a hash
lookup('mykey', 'Integer', { default => 42})
lookup('mykey', { type => 'Integer', default => 42})
# In 4x. this will is possible:
lookup('mykey', 'Integer', 42)
lookup('mykey', Integer, 42)
lookup('mykey', 42)
lookup('mykey', Integer)
If future parser is used, it is also possible to use a lambda, there is
no need to first assign variable and then check - i.e.
$x = lookup('something') {|$x| $x ? {
undef => 'nothing', default => $x }
which is just slightly better, but still bulky.
I am in favor of adding the 3d argument for default, and until 4x the
lookup must then also use type.
What do you think about that?
On the positive side, the lookup function adds a static type checker,
but it also fails to be completely useful because we cannot (?) define
our own static types or validators. If we had the ability to define our
own type check via lambda or other function, this would be useful. We
could do complex validation on data, more than just 'string' or 'array'.
I would like to see something like:
lookup("namespace::key_name", custom_validator_function)
Where the custom_validator_function could be any puppet parser function
OR since we now worship the**great *λ, *we should be able to use an
anonymous function in-place to validate the input (and in this
fictitious example, set a default value):
lookup("namespace::key_name", |$val| {$val or "default value"} )
There is a short and a long answer.
Short answer, the lookup already takes a lambda which can do validation
and handle default value. This works now with --parser future.
Longer answer. The idea is to support specification of types in the
Puppet Language. Your example would be
lookup('namespace::key', SomeNamespace::SomeType)
Puppet Types is described in ARM-7
(https://github.com/puppetlabs/armatures/blob/master/arm-7.puppet_types/puppet_types.md).
ARM-7 may need a revision since the work on Type in ARM-9 took the type
system a bit further.
To sum it up, as someone who writes a lot of Puppet DSL code, I find the
hiera1 syntax much more useful than the lookup syntax in ARM-9.
Does adding a 3d argument to lookup for default cut it for you?
Regards
- henrik
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