Expand template rule about bazel-skylib HOT 9 CLOSED

Ran across this looking for the same thing. The code's handy--certainly looks like there's popular demand to me.

@laurentlb, would you be down to get your code in?

P.S. I just watched your immersion demo. It's super neat!

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FWIW we now have this in https://github.com/aspect-build/bazel-lib/blob/main/lib/expand_make_vars.bzl
Since bazel-skylib wants to reduce scope, we're adding these helpers over there.

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We've used laurentlb's sketchup internally for a while now, so I look forward to vertexwahn's PR landing.

I'm currently tweaking our internal version, however, to additionally accept substitution information via a SubstitutionInfo provider. Would this be useful to Skylib?

Use cases:

• Values of a string_dict aren't configurable. Suppose I have a template where I'm expanding 6 variable expressions, and assume 5 of those 6 are static, while the 6th varies by build configuration. If I have N configuration cases, I have to define N unique dictionaries with the common key-value items repeated. (I can reduce repetition w/ dicts.add, but it's still clumsy.)
• I have a template file with two variables to expand. I can define one expansion via an expand_template rule in a BUILD file. However, the other expansion is performed by a rule class which examines a Java library's runtime classpath and generates its own ctx.expand_template action. To apply both substitutions today, I have to define a chain of expansions, where the output of one expansion rule is the input to another.

Sketchup of a BUILD file using a provider-aware expand_template:

expand_template_info(
    name = "foo_subinfo",
    substitutions = {"%FOO%": "foo"},
)

expand_template_info(
    name = "bar_subinfo",
    substitutions = select({
        "//condition_1": {"%BAR%": "bar1"},
        "//conditions:default": {"%BAR%": "bar"},
    }),
)

# Returns a `SubstitutionInfo` containing a dict of `"%RUNTIME_CLASSPATH%": ":".join([...])`.
java_runtime_classpath(
    name = "libfoo_classpath",
    runtime_deps = [":foo"],
    marker = "%RUNTIME_CLASSPATH%",
)

expand_template(
    name = "dummy_json",
    template = "dummy.tpl.json",
    deps = [
        ":foo_subinfo",
        ":bar_subinfo",  # Or I could `select` here between a static `bar1..N_subinfo`.
        ":libfoo_classpath",
    ],
)

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@pauldraper sorry about that 😛

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meanwhile, I updated my PR (#330). Would be nice if anyone could review it

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We also have something similar internally and would like to have it in a shared lib.

The code is easy, but naming is hard. Having both expand_template and write_file in skylib for writing files could be confusing. How about generate_file or gen_file?

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Would also be helpful for tensorflow, kythe, and OpenEXR

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fyi, I'll let the code owners (@brandjon @tetromino) decide what to do here. Feel free to reuse my code sample in any way you want.

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I was very sad to learn that https://github.com/bazel-contrib/rules-template was not this.

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