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 My main concern with the {} delimiters in YAQL is that the curly brace
already has a defined use within YAML.  We most definitely will eventually
run in to parsing errors with whatever delimiter we choose but I don't feel
that it should conflict with the markup language it is directly embedded
in.  It gets quite difficult to, at a glance, identify YAQL expressions.
 <% %> may appear ugly to some but I feel that it works as a clear
delimiter of both the beginning AND the end of the YAQL query. The options
that only escape the beginning look fine in small examples like this but
the workflows that we have written or seen in the wild tend to have some
fairly large expressions.  If the opening and closing delimiters don't
match, it gets quite difficult to read.

>
> *From: *Anastasia Kuznetsova <akuznetsova at mirantis.com>
> *Subject: **Re: [openstack-dev] [Mistral] Changing "expression"
> delimiters in Mistral DSL*
> *Date: *February 17, 2015 at 8:28:27 AM PST
> *To: *"OpenStack Development Mailing List (not for usage questions)" <
> openstack-dev at lists.openstack.org>
> *Reply-To: *"OpenStack Development Mailing List \(not for usage
> questions\)" <openstack-dev at lists.openstack.org>
>
> As for me, I think that <% ... %> is not an elegant solution and looks
> massive because of '%' sign. Also I agree with Renat, that <% ... %>
> reminds HTML/Jinja2 syntax.
>
> I am not sure that similarity with something should be one of the main
> criteria, because we don't know who will use Mistral.
>
> I like:
> - <{1 + $.var}> Renat's example
> - variant with using some functions (item 2 in Dmitry's list):  { yaql:
> “1+1+$.my.var < 100” } or <yaql: 'Hello' + $.name >
> - my two cents, maybe we can use something like: result: -< "Hello" +
> $.name >-
>
>
> Regards,
> Anastasia Kuznetsova
>
> On Tue, Feb 17, 2015 at 1:17 PM, Nikolay Makhotkin <
> nmakhotkin at mirantis.com> wrote:
>
>> Some suggestions from me:
>>
>> 1. <y 1 + $.var > # (short from yaql).
>> 2. <{ 1 + $.var }>  # as for me, looks more elegant than <% %>. And
>> visually it is more strong
>>
>> A also like p7 and p8 suggested by Renat.
>>
>> On Tue, Feb 17, 2015 at 11:43 AM, Renat Akhmerov <rakhmerov at mirantis.com>
>> wrote:
>>
>>> One more:
>>>
>>> p9: \{1 + $.var} # That’s pretty much what
>>> https://review.openstack.org/#/c/155348/ addresses but it’s not exactly
>>> that. Note that we don’t have to put it in quotes in this case to deal with
>>> YAML {} semantics, it’s just a string
>>>
>>>
>>>
>>> Renat Akhmerov
>>> @ Mirantis Inc.
>>>
>>>
>>>
>>> On 17 Feb 2015, at 13:37, Renat Akhmerov <rakhmerov at mirantis.com> wrote:
>>>
>>> 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 + $.var} # “y” here is just shortened “yaql"
>>>
>>>
>>> Any ideas and thoughts would be really appreciated!
>>>
>>> Renat Akhmerov
>>> @ Mirantis Inc.
>>>
>>>
>>>
>>> On 17 Feb 2015, at 12:53, Renat Akhmerov <rakhmerov at mirantis.com> wrote:
>>>
>>> Dmitri,
>>>
>>> I agree with all your reasonings and fully support the idea of changing
>>> the syntax now as well as changing system’s API a little bit due to
>>> recently found issues in the current engine design that don’t allow us, for
>>> example, to fully implement ‘with-items’ (although that’s a little bit
>>> different story).
>>>
>>> Just a general note about all changes happening now: *Once we release
>>> kilo stable release our API, DSL of version 2 must be 100% stable*. I
>>> was hoping to stabilize it much earlier but the start of production use
>>> revealed a number of things (I think this is normal) which we need to
>>> address, but not later than the end of Kilo.
>>>
>>> As far as <% %> syntax. I see that it would solve a number of problems
>>> (YAML friendliness, type ambiguity) but my only not strong argument is that
>>> it doesn’t look that elegant in YAML as it looks, for example, in ERB
>>> templates. It really reminds me XML/HTML and looks like a bear in a grocery
>>> store (tried to make it close to one old russian saying :) ). So just for
>>> this only reason I’d suggest we think about other alternatives, maybe not
>>> so familiar to Ruby/Chef/Puppet users but looking better with YAML and at
>>> the same time being YAML friendly.
>>>
>>> I would be good if we could here more feedback on this, especially from
>>> people who started using Mistral.
>>>
>>> Thanks
>>>
>>> Renat Akhmerov
>>> @ Mirantis Inc.
>>>
>>>
>>>
>>> On 17 Feb 2015, at 03:06, Dmitri Zimine <dzimine at stackstorm.com> wrote:
>>>
>>> SUMMARY:
>>> ----------------
>>>
>>> We are changing the syntax for inlining YAQL expressions in Mistral YAML
>>> from {1+$.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 + $.y}” # What type would this return?
>>>
>>> We got a very strong push back from users in the filed on this syntax.
>>>
>>> The crux of the problem is using { } as delimiters YAML. It is plain
>>> wrong to use the reserved character. The clean solution is to find a
>>> delimiter that won’t conflict with YAML.
>>>
>>> Criteria for selecting best alternative are:
>>> 1) Consistently applies to to all cases of using YAML in DSL
>>> 2) Complies with YAML
>>> 3) Familiar to target user audience - openstack and devops
>>>
>>> We prefer using two-char delimiters to avoid requiring extra escaping
>>> within the expressions.
>>>
>>> The current winner is <% %>. It fits YAML well. It is familiar to
>>> openstack/devops as this is used for embedding Ruby expressions in Puppet
>>> and Chef (for instance, [4]). It plays relatively well across all cases of
>>> using expressions in Mistral (see examples in [5]):
>>>
>>> ALTERNATIVES considered:
>>> --------------------------------------------------
>>>
>>> 1) Use Ansible-like syntax:
>>> http://docs.ansible.com/YAMLSyntax.html#gotchas
>>> Rejected for confusion around types. See above.
>>>
>>> 2) Use functions, like Heat HOT or TOSCA:
>>>
>>> HOT templates and TOSCA doesn’t seem to have a concept of typed
>>> variables to borrow from (please correct me if I missed it). But they have
>>> functions: function: { function_name: {foo: [parameter1, parameter 2],
>>> bar:"xxx”}}. Applied to Mistral, it would look like:
>>>
>>>     publish:
>>>      - bool_var: { yaql: “1+1+$.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
>>> [2] Use Ansible-like syntax to make YAQL expressions YAML complient
>>> 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
>>> [4]Using <% %> in Puppet
>>> 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
>>> [6] Blueprint
>>> https://blueprints.launchpad.net/mistral/+spec/yaql-delimiters
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>
>>
>> --
>> Best Regards,
>> Nikolay
>>
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