lamdas with :stencil/pass-render or :stencil/pass-context lose higher-level contexts

[timclemons]

Given the following params:

{:history [{:text "foo"} {:text "bar}]
 :prefix "item:"
 :format-text (with-meta (fn [text context render]
                                         (clojure.string/uppercase (render text context)))
                                       {:stencil/pass-render true})}

And this template:

{{#history}}{{#format-text}}{{prefix}} {{text}}{{/format-text}}{{/history}}

Each invocation of :format-text will be passed only the context of the history item, so top-level items like :prefix will be inaccessible.

[timclemons]

Having had a look at the code, what I would propose is to have callers of call-lambda pass in the entire context-stack, then have the lamda use context-get to obtain the values it needs. This way the context is still fully able to work with calls to 'render'.

[davidsantiago]

Hm. I looked at the Mustache spec, and it doesn't really say anything about this. The Mustache spec only goes as far as specifying that lambdas should get the text of their sections, but never says anything about receiving the context. That is something Stencil users have added as a usability extension, and I think it was reasonable.

I remember choosing to only give the current context to a lambda, instead of the entire context stack. My reasoning was that the API was going to be much harder to explain if it was passing a stack of maps, and the writer of the lambda function would have to know the proper way to traverse upwards in this data structure on missing keys. Furthermore, I reasoned, the user of the template usually controls the data map, and could make map entries with the data they needed, if they really needed to. It's a cost of doing business with a very simple template language.

The other issue is that would be a breaking change to the API, and would require any current users of the lambda functionality to rewrite their lambdas to keep them working. I'm open to working on the usability here, but I'm not sure I'm willing to go that far.

[timclemons]

Ok, so instead of the full context stack, how about collapsing it into a single map using (reduce merge ...) so that higher-level context values override lower-level. This would preserve the API while still allowing access to the lower-level value.

[timclemons]

Another idea: add another supported metadata :stencil/pass-full-context where call-lambda invokes the lambda with the full stack. If the metadata :stencil/pass-render or :stencil/pass-context are defined, strip off the first item off the context stack and pass that to the lambda.

[timclemons]

Went with my second idea. Let me know your thoughts.