Reuse template computation

[miquecg]

Let's say I have a small "computation" using some data stored in chezmoi.toml.tmpl and this is repeated over several templates.

{{ index (. | sortAlpha | reverse) 0 }}

I'm curious to know if there would be ways to reuse template code like this. I thought of two possible options:

• Compute the result on chezmoi.toml.tmpl with the caveat that it uses data defined there, so I imagine it will not be available during evaluation
• Compute the result in another template and then include it where needed. All data would be available.

For something that small I'm fine with copying/pasting, but I'm wondering how one would solve this.

[halostatue]

What I’m going to suggest here is useful only if the | sortAlpha | reverse is expensive to do, because it’s not shorter.

As of the moment, storing the result is your best bet. I’d suggest a before script (templated) that does something like:

#!/bin/bash

cat > /tmp/sortedData <<EOF
{{ .dataToSort | sortAlpha | reverse | toJson }}
EOF

Then you should be able do index ((output "cat" "/tmp/sortedData") | fromJson) 0 in your remaining templates.

At the end, you can have an after script that just does rm -f /tmp/sortedData.

[miquecg]

Sorry, just edited my question. I was thinking that simply using a template as storage mechanism would work.

[halostatue]

As @twpayne you could put {{ . | sortAlpha | reverse }} in .chezmoitemplates/reverseAlpha (I don’t remember if .tmpl is required in .chezmoitemplates) and do {{ index (includeTemplate "reverseAlpha" .) 0 }} so that you have a "function" like template call.

[halostatue]

@twpayne @bradenhilton this might be a good idea for the #2673 templating. Something like {{- .dataToSort | sortAlpha | reverse | storeDataAs "key" -}} and {{ index (readDataAs "key") 0 }} to have temporary files (or in-memory storage) associated with a specific execution context of Chezmoi (so the files might be done as /tmp/chezmoi-cache-$PID.json or something like that, and cleaned up on removal).

[twpayne]

You can reuse template code by using .chezmoitemplates.

There is no way in chezmoi to store the result of a template execution and re-use it elsewhere. Such a thing is both a very bad idea and impossible to implement.

It is impossible to implement because chezmoi's templates are neither hermetic nor idempotent so chezmoi cannot determine when the result of a template execution can be reused. Consider the small "computation" {{ output "head" "-c1" "/dev/random" }}.

It is a very bad idea because, even it were possible to implement correctly (which would first require solving the halting problem) it would result in a large, shared, global state, with all the nightmares that that brings. Think how bad global variables are. This would be even worse.

[miquecg]

Thanks both for engaging :) So basically that's option 2, reuse template code by including them. The word "compute" could be a bit misleading, but in reality I was talking about code reuse.