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 + $.var} # “y” here is just shortened “yaql" Any ideas and thoughts would be really appreciated! Renat Akhmerov @ Mirantis Inc. > On 17 Feb 2015, at 12:53, Renat Akhmerov <rakhme...@mirantis.com> wrote: > > Dmitri, > > I agree with all your reasonings and fully support the idea of changing the > syntax now as well as changing system’s API a little bit due to recently > found issues in the current engine design that don’t allow us, for example, > to fully implement ‘with-items’ (although that’s a little bit different > story). > > Just a general note about all changes happening now: Once we release kilo > stable release our API, DSL of version 2 must be 100% stable. I was hoping to > stabilize it much earlier but the start of production use revealed a number > of things (I think this is normal) which we need to address, but not later > than the end of Kilo. > > As far as <% %> syntax. I see that it would solve a number of problems (YAML > friendliness, type ambiguity) but my only not strong argument is that it > doesn’t look that elegant in YAML as it looks, for example, in ERB templates. > It really reminds me XML/HTML and looks like a bear in a grocery store (tried > to make it close to one old russian saying :) ). So just for this only reason > I’d suggest we think about other alternatives, maybe not so familiar to > Ruby/Chef/Puppet users but looking better with YAML and at the same time > being YAML friendly. > > I would be good if we could here more feedback on this, especially from > people who started using Mistral. > > Thanks > > Renat Akhmerov > @ Mirantis Inc. > > > >> On 17 Feb 2015, at 03:06, Dmitri Zimine <dzim...@stackstorm.com >> <mailto:dzim...@stackstorm.com>> wrote: >> >> SUMMARY: >> ---------------- >> >> We are changing the syntax for inlining YAQL expressions in Mistral YAML >> from {1+$.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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