Java config evaluation at compile time

[dev92]

Hi,
From the java code-gen example I see we generate java classes from pkl template and then during runtime we evaluate with a pkl file that has the actual values.
Is it possible to check pkl file during compilation phase in Java?, instead of getting runtime errors

[edward3h]

I did this by adding an evaluator in my Gradle configuration.
Example:

pkl {
    javaCodeGenerators {
        register("genJava") {
            generateGetters = true
            sourceModules.add(file("src/main/resources/ConfigurationTemplate.pkl"))
        }
    }
    evaluators {
        register("evalDev") {
            sourceModules.add(file("src/main/resources/dev.pkl"))
            transitiveModules.from(file("src/main/resources/ConfigurationTemplate.pkl"))
            outputFile = layout.buildDirectory.file("evaldev.pcf")
        }
    }
}

When my dev.pkl file does not set a value for a property defined in ConfigurationTemplate.pkl, this fails the Gradle build.

[holzensp]

This is the solution I would have proposed also; although... if your scenario is entirely build-time, you could even remove your (run-time) Pkl dependency and just read the rendered JSON. (edit: since you’re using codegen, there are risks going via a less expressive format, because of e.g. inheritance and/or non-JSON types)

[bioball]

I would recommend setting up a Java unit test to evaluate Pkl. This would be the most representative of how you will be using Pkl in production.

Adding evaluation via Gradle is pretty good, but those are textual evaluators, and they are slightly different than how Java evaluates Pkl.

Our textual evaluators evaluate the output.text (or output.files) property of a Pkl program, while our Java evaluator's evaluate method evaluates module (the top-level object).

For example, this Pkl code would pass if run through our evaluators extension, but would fail through our Java-side evaluate().
The evaluators extension will pass because it will evaluate to the string "I am totally okay". OTOH, the Java-side evaluate() will fail because it evaluates module. And module is not valid because property foo is not set.

foo: String

output {
  text = "I am totally okay"
}