Functions are not inherited by amending modules

[philippemerle]

The language reference says that a module amending another module inherits from all the members of the amended module. But functions of the amended module seem to be not inherited from as shown in the following example.

File base_template.pkl

persons: Listing<Person>
class Person {
  name: String
}
function buildPerson(val: String): Person = new {
  name = val
}

File my_template.pkl

amends "base_template.pkl"

persons {
  new Person { name = "Me"}
  buildPerson("you")
}

Evaluating my_template.pkl produces the following error

–– Pkl Error ––
Cannot find method `buildPerson`.

5 | buildPerson("you")
    ^^^^^^^^^^^^^^^^^^
at my_template#persons[#2] (file:///.../my_template.pkl, line 5)

3 | persons {
    ^^^^^^^^^
at my_template#persons (file:///.../my_template.pkl, line 3)

106 | text = renderer.renderDocument(value)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
at pkl.base#Module.output.text (https://github.com/apple/pkl/blob/0.25.2/stdlib/base.pkl#L106)

Do I miss something?

[stackoverflow]

Scoping rules in Pkl can be a bit tricky. You can use module to reference top level variables and functions:

amends "base_template.pkl"

persons {
  new Person { name = "Me"}
  module.buildPerson("you")
}

Also: if you use our IntelliJ plugin you should see an error telling you this function is not found.

[philippemerle]

Thank you the provided solution. But this will be simpler if inherited functions could be called directly without prefixing them by module.. Perhaps in a future pkl release?

[bioball]

The scoping rules in place here provide an important safety net; that the resolved member can't change by adding something else in the (grand)parent module.

Without this rule, it would be possible to change what a variable/method resolved to by changing something in the base module. It would make it so that adding names to a module might produce very unexpected results in the generated configuration.

For example, if this worked:

parent.pkl

function foo() = "bar"

class MyObject {
  prop: String
}

obj: MyObject

child.pkl

obj {
  prop = foo()
}

Then, a new function foo() was introduced:

 function foo() = "bar"

 class MyObject {
   prop: String
+
+  function foo() = "baz"
 }
 
 obj: MyObject

This change seems innocent when just looking at parent.pkl, but it would change foo() within child.pkl to resolve to a different method. This behavior would make it too easy for pkl code to break in hidden ways.

The same resolution rules apply to properties, too.

[bioball]

Closing as not planned, as this works as-intended.