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On Tue, Feb 17, 2015 at 7:06 AM, 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]):
>

A really long time ago I posted this patch for Heat:
https://review.openstack.org/#/c/41858/2/doc/source/template_guide/functions.rst
(adds a jinja2 function to Heat http://jinja.pocoo.org/docs/dev/)

I also used <% %>, it seems to be what people use when using jinja2 on yaml.

This was rejected because of security concerns of Jinja2.


>
> 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” }
>

You *could* have the expression as a list, like this (but might not work in
all cases):
{ yaql: [1, +, 1, $.my.var, <, 100] }

Generally in Heat we make the functions and args a natural part of the yaml
so it's not one big string that gets parsed separately.
Tho' it would be nice to have a common approach to this, so I am partial to
the one you have here.

-Angus


>
> 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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