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The import statement

Part of the point of introducing functions was to make it possible to modularize transforms so that they are more maintainable. For example, the many related amplitude-*.jstl2 transforms could all be based on a library of shared functions (amplitude-shared.jstl2) for the parts of the logic that are shared.

However, for this to work it has to be possible for one transform file to get logic from another file.

We have that same issue when it comes to the LoginStatePreserved transform, which now basically does a simple test, then runs one of two already existing transforms. It has to be implemented in Scala at the moment, because there is no import function.

The actual statement (simple)

The import statement could look like this:

import "reference/to/other/file.jstl"

where the import then makes all variables and functions defined in the other file available in the current scope. (This includes variables and functions imported from other files into file.jstl.)

The value of the actual transform would not be evaluated and would not be available. We would need to modify the grammar so that not having a final expression at all (only variables and functions) would be allowed.

This would solve the amplitude use case, but for the state preserved case to work we would need to rewrite those templates to be functions. That can be done, of course.

The actual statement (with prefix)

Import could also look like this

import "reference/to/other/file.jstl" as foo

Now all functions in that file would be available as foo:function and all variables as $foo:variable. The transform itself would be available as the function foo(input). Anything imported into file.jstl would be invisible.

With this we could still solve the amplitude use case, and now the state preserving case would be solved by doing:

import "transforms/pulse-identified.jstl2" as identified
import "transforms/pulse-anonymized.jstl2" as anonymized

if (.actor."spt:userId" and not(ends-with(":null", .actor."spt:userId")))
  identified(.)
else
  anonymized(.)

Refinements

We could perhaps allow both mechanisms. If you leave off the prefix the import goes straight into your local scope with no prefix, and the transform body (the top-level expression at the end) is not accessible. If you include the prefix you get the behaviour described above.

Resolving the file

By default, this should first attempt to resolve on the classpath. If that fails, resolve in the file system.

However, the Parser should support plugging in a resolver that can take the "reference/to/other/file.jstl" string and return a Reader. Those that need it can then override the resolution mechanism in their own projects.