deftemplate and dynamic vars

[Anthony Grimes]

I created https://github.com/cgrand/enlive/issues/30 last night.

The issue seems to be that when you call a template function, it is not executed within the dynamic scope of the caller. Here is my specific code: https://refheap.com/paste/4f39738de4b07508daafd022

In my code, I always call 'layout' inside of a Noir defpage which sets session/*noir-session* to a session map. However, whenever I call this code, *noir-session* remains unbound. I've also tested by trying to bind something innocent like *out* and it doesn't get communicated to the template either. It's as if deftemplate creates its function with bound-fn (which doesn't seem to be the case).

I talked to Chas Emerick on IRC and he encouraged me to ping the ML about it. He seems to have confirmed the bug himself.

[Christophe Grand]

Hi,

Templates return a lazy-seq, so it's the opposite behaviour of being created by bound-fn that you observe.

The most expedient way to solve this problem is to have a middleware which calls doall (or map (bound-fn [x] x) if you want to preserve laziness) on the :body of the response map -- thus the lazy seq evaluation will happen inside the dynamic scope set by upstream middlewares.

Given that per Ring's spec, :body may be lazy seqs, I would tend to say that it's up to the authors of middlewares which introduce dynamic binding to ensure that this case is covered.

I don't want to fix this systematically at the heart of Enlive because bound-fn incurs a overhead.
However in a first time, I'm ok with shipping a body-forcing middleware before finding a better solution. Would it be ok with you?

hth,

Christophe

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[Christophe Grand]

While on the road I realized that the laziness at the origin of the problem is uncorelated with the desired laziness.
https://github.com/cgrand/enlive/commit/698343e456287c5f744bf6170426d33ccd6d6e3a

Thanks for the report.

(I still think in the general case, middleware writers should take care of lazy bodies.)

Christophe