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Along with <% %> syntax here are some other alternatives that I checked for YAML friendliness with my short comments:

p1: ${1 + $.var}    	# Here it’s bad that $ sign is used for two different things
p2: ~{1 + $.var}    	# ~ is easy to miss in a text
p3: ^{1 + $.var}    	# For someone may be associated with regular expressions
p4: ?{1 + $.var}	
p5: <{1 + $.var}>	# This is kinda crazy
p6: e{1 + $.var}	# That looks a pretty interesting option to me, “e” could mean “expression” here.
p7: yaql{1 + $.var}	# This is interesting because it would give a clear and easy mechanism to plug in other expression languages, “yaql” here is a used dialect for the following expression
p8: y{1 + $.var}	# “y” here is just shortened “yaql"


Any ideas and thoughts would be really appreciated!

Renat Akhmerov
@ Mirantis Inc.



> On 17 Feb 2015, at 12:53, Renat Akhmerov <rakhmerov at mirantis.com> wrote:
> 
> Dmitri,
> 
> I agree with all your reasonings and fully support the idea of changing the syntax now as well as changing system’s API a little bit due to recently found issues in the current engine design that don’t allow us, for example, to fully implement ‘with-items’ (although that’s a little bit different story).
> 
> Just a general note about all changes happening now: Once we release kilo stable release our API, DSL of version 2 must be 100% stable. I was hoping to stabilize it much earlier but the start of production use revealed a number of things (I think this is normal) which we need to address, but not later than the end of Kilo.
> 
> As far as <% %> syntax. I see that it would solve a number of problems (YAML friendliness, type ambiguity) but my only not strong argument is that it doesn’t look that elegant in YAML as it looks, for example, in ERB templates. It really reminds me XML/HTML and looks like a bear in a grocery store (tried to make it close to one old russian saying :) ). So just for this only reason I’d suggest we think about other alternatives, maybe not so familiar to Ruby/Chef/Puppet users but looking better with YAML and at the same time being YAML friendly.
> 
> I would be good if we could here more feedback on this, especially from people who started using Mistral.
> 
> Thanks
> 
> Renat Akhmerov
> @ Mirantis Inc.
> 
> 
> 
>> On 17 Feb 2015, at 03:06, Dmitri Zimine <dzimine at stackstorm.com <mailto:dzimine at stackstorm.com>> wrote:
>> 
>> SUMMARY: 
>> ----------------
>> 
>> We are changing the syntax for inlining YAQL expressions in Mistral YAML from {1+$.my.var} (or “{1+$.my.var}”) to <% 1+$.my.var %>
>> 
>> Below I explain the rationale and the criteria for the choice. Comments and suggestions welcome.
>> 
>> DETAILS: 
>> -------------
>> 
>> We faced a number of problems with using YAQL expressions in Mistral DSL: [1] must handle any YAQL, not only the ones started with $; [2] must preserve types and [3] must comply with YAML. We fixed these problems by applying Ansible style syntax, requiring quotes around delimiters (e.g. “{1+$.my.yaql.var}”). However, it lead to unbearable confusion in DSL readability, in regards to types:
>> 
>>     publish:
>>        intvalue1: "{1+1}” # Confusing: you expect quotes to be string.
>>        intvalue2: "{int(1+1)}” # Even this doestn’ clean the confusion
>>        whatisthis:"{$.x + $.y}” # What type would this return? 
>> 
>> We got a very strong push back from users in the filed on this syntax. 
>> 
>> The crux of the problem is using { } as delimiters YAML. It is plain wrong to use the reserved character. The clean solution is to find a delimiter that won’t conflict with YAML.
>> 
>> Criteria for selecting best alternative are: 
>> 1) Consistently applies to to all cases of using YAML in DSL
>> 2) Complies with YAML 
>> 3) Familiar to target user audience - openstack and devops
>> 
>> We prefer using two-char delimiters to avoid requiring extra escaping within the expressions.
>> 
>> The current winner is <% %>. It fits YAML well. It is familiar to openstack/devops as this is used for embedding Ruby expressions in Puppet and Chef (for instance, [4]). It plays relatively well across all cases of using expressions in Mistral (see examples in [5]):
>> 
>> ALTERNATIVES considered:
>> --------------------------------------------------
>> 
>> 1) Use Ansible-like syntax: http://docs.ansible.com/YAMLSyntax.html#gotchas <http://docs.ansible.com/YAMLSyntax.html#gotchas>
>> Rejected for confusion around types. See above.
>> 
>> 2) Use functions, like Heat HOT or TOSCA:
>> 
>> HOT templates and TOSCA doesn’t seem to have a concept of typed variables to borrow from (please correct me if I missed it). But they have functions: function: { function_name: {foo: [parameter1, parameter 2], bar:"xxx”}}. Applied to Mistral, it would look like:
>> 
>>     publish:
>>      - bool_var: { yaql: “1+1+$.my.var < 100” } 
>> 
>> Not bad, but currently rejected as it reads worse than delimiter-based syntax, especially in simplified one-line action invocation.
>> 
>> 3) < > paired with other symbols: php-styoe  <? ..?>
>> 
>> 
>> REFERENCES: 
>> ----------------------
>> 
>> [1] Allow arbitrary YAQL expressions, not just ones started with $ : https://github.com/stackforge/mistral/commit/5c10fb4b773cd60d81ed93aec33345c0bf8f58fd <https://github.com/stackforge/mistral/commit/5c10fb4b773cd60d81ed93aec33345c0bf8f58fd>
>> [2] Use Ansible-like syntax to make YAQL expressions YAML complient https://github.com/stackforge/mistral/commit/d9517333b1fc9697d4847df33d3b774f881a111b <https://github.com/stackforge/mistral/commit/d9517333b1fc9697d4847df33d3b774f881a111b>
>> [3] Preserving types in YAQL https://github.com/stackforge/mistral/blob/d9517333b1fc9697d4847df33d3b774f881a111b/mistral/tests/unit/test_expressions.py#L152-L184 <https://github.com/stackforge/mistral/blob/d9517333b1fc9697d4847df33d3b774f881a111b/mistral/tests/unit/test_expressions.py#L152-L184>
>> [4]Using <% %> in Puppet https://docs.puppetlabs.com/guides/templating.html#erb-is-plain-text-with-embedded-ruby <https://docs.puppetlabs.com/guides/templating.html#erb-is-plain-text-with-embedded-ruby> 
>> [5] Etherpad with discussion https://etherpad.openstack.org/p/mistral-YAQL-delimiters <https://etherpad.openstack.org/p/mistral-YAQL-delimiters>
>> [6] Blueprint https://blueprints.launchpad.net/mistral/+spec/yaql-delimiters <https://blueprints.launchpad.net/mistral/+spec/yaql-delimiters>
>> 
> 

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