Templating with classes does not fail if expected #100

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2025-12-30T01:20:48+01:00

adam commented

2025-12-30 01:20:48 +01:00

Originally created by @StefMa on GitHub (Mar 2, 2024).

Given a similar template as you have in the documentation "How to write a template".

title: String
interactive: Boolean
seats: Int
occupancy: Float
duration: Duration
`abstract`: String
class Event {
-  name: String
+  name: String?
-  year: Int
+  year: Int?
}
event: Event
instructors: Listing<String>
class Session {
  time: Duration
  date: String
}
sessions: Listing<Session>
assistants: Mapping<String, String>

The implementation leaves out the event property:

title = "Pkl: Configure your Systems in New Ways"
interactive = true
seats = 100
occupancy = 0.85
duration = 1.5.h
`abstract` = """
  With more systems to configure, the software industry is drowning in repetitive and brittle configuration files.
  YAML and other configuration formats have been turned into programming languages against their will.
  Unsurprisingly, they don’t live up to the task.
  Pkl puts you back in control.
  """
-event {
-  name = "Migrating Birds between hemispheres"
-  year = 2024
-}
instructors {
  "Kate Sparrow"
  "Jerome Owl"
}
sessions {
  new {
    date = "2/1/2024"
    time = 30.min
  }
  new {
    date = "2/1/2024"
    time = 30.min
  }
}
assistants {
  ["kevin"] = "Kevin Parrot"
  ["betty"] = "Betty Harrier"
}

Expected:
Error with message

Tried to read property event but its value is undefined.

But I get:
Not error, outpout contains

event {
  name = null
  year = null
}

For me looks wrong.
event: Event from the template is not overriden so it should fail.

Fun fact:
If I leave a property within Event non-nullable, it will fail with the correct error message:

Tried to read property name but its value is undefined.
...
The above error occurred when rendering path event.name of module "..."

Another (simpler) example:
Given the following template:

class On {
  first: Int?
  second: Int?
  third: Int?
}

on: On

My template expect to override On.
However, non of the properties of On are required to set.

Implementation:

☝️ No joke, just the amend statement of course 😁

What will be rendered?

on {
  first = null
  second = null
  third = null
}

Technically this behaviour seems to be correct.
It might be the case that pkl will "simply" add the "null class" for you to the implementation.
Because not defining on at all or using on {} will lead to the same output.
The latter (on {}) is perfectly fine and shouldn't print an error,
Nevertheless, I think we should fail here because it behaves differently to other errors that says "value is undefined".

Originally created by @StefMa on GitHub (Mar 2, 2024). Given a similar template as you have in the documentation "How to write a template". ```diff title: String interactive: Boolean seats: Int occupancy: Float duration: Duration `abstract`: String class Event { - name: String + name: String? - year: Int + year: Int? } event: Event instructors: Listing<String> class Session { time: Duration date: String } sessions: Listing<Session> assistants: Mapping<String, String> ``` The implementation leaves out the `event` property: ```diff title = "Pkl: Configure your Systems in New Ways" interactive = true seats = 100 occupancy = 0.85 duration = 1.5.h `abstract` = """ With more systems to configure, the software industry is drowning in repetitive and brittle configuration files. YAML and other configuration formats have been turned into programming languages against their will. Unsurprisingly, they don’t live up to the task. Pkl puts you back in control. """ -event { - name = "Migrating Birds between hemispheres" - year = 2024 -} instructors { "Kate Sparrow" "Jerome Owl" } sessions { new { date = "2/1/2024" time = 30.min } new { date = "2/1/2024" time = 30.min } } assistants { ["kevin"] = "Kevin Parrot" ["betty"] = "Betty Harrier" } ``` **Expected**: Error with message > Tried to read property `event` but its value is undefined. **But I get**: Not error, outpout contains ``` event { name = null year = null } ``` For me looks *wrong*. `event: Event` from the template **is not overriden** so it should fail. Fun fact: If I leave a property *within* `Event` non-nullable, it will fail with the correct error message: > Tried to read property `name` but its value is undefined. > ... > The above error occurred when rendering path `event.name` of module "..." **Another (simpler) example**: Given the following template: ``` class On { first: Int? second: Int? third: Int? } on: On ``` My template expect to override `On`. However, non of the properties of `On` are required to set. Implementation: ``` ``` ☝️ No joke, just the `amend` statement of course 😁 What will be rendered? ``` on { first = null second = null third = null } ``` --- Technically **this behaviour seems to be correct**. It might be the case that `pkl` will "simply" add the "null class" for you to the implementation. Because *not* defining `on` at all **or** using `on {}` will lead to the same output. The latter (`on {}`) is perfectly fine and shouldn't print an error, Nevertheless, I think we should fail here because it behaves differently to other errors that says "value is undefined".

adam closed this issue

2025-12-30 01:20:48 +01:00

adam commented

2025-12-30 01:20:48 +01:00

@stackoverflow commented on GitHub (Mar 4, 2024):

Most non-primitive types in Pkl have a default value. The default value of nullable types is null.
If you want to force people to give a value, you have two options:

Make the property non-null. This indicates that it has to be provided and can't be left empty.
Make the default an error: first: Int? = throw("error: no value provided for property 'first'")

@stackoverflow commented on GitHub (Mar 4, 2024): Most non-primitive types in Pkl have a default value. The default value of nullable types is `null`. If you want to force people to give a value, you have two options: - Make the property non-null. This indicates that it has to be provided and can't be left empty. - Make the default an error: `first: Int? = throw("error: no value provided for property 'first'")`

adam commented

2025-12-30 01:20:48 +01:00

@StefMa commented on GitHub (Mar 4, 2024):

Hey @stackoverflow ,

thanks for the answer.

Make the property non-null. This indicates that it has to be provided and can't be left empty.

How do I mark a property as non-null?
Isn't declaring it as on: On enough?
In fact, I have this!
See https://github.com/StefMa/pkl-gha/blob/d266b8b2f9a541774b2f648a1c583d057c931462/GitHubAction.pkl#L139-L149

It also works with build-in classes like String or Int.
In these cases it works:

However, on and jobs will be rendered empty...

Package used:

amends "package://pkg.pkl-lang.org/github.com/stefma/pkl-gha/com.github.action@0.0.1#/GitHubAction.pkl"

@StefMa commented on GitHub (Mar 4, 2024): Hey @stackoverflow , thanks for the answer. > Make the property non-null. This indicates that it has to be provided and can't be left empty. How do I mark a property as `non-null`? Isn't declaring it as `on: On` enough? In fact, I have this! See https://github.com/StefMa/pkl-gha/blob/d266b8b2f9a541774b2f648a1c583d057c931462/GitHubAction.pkl#L139-L149 It also works with build-in classes like `String` or `Int`. In these cases it works: <img width="1397" alt="Screenshot 2024-03-04 at 3 57 43 PM" src="https://github.com/apple/pkl/assets/10229883/dd88d18b-8194-47b9-ac9f-286ed0d28789"> However, `on` and `jobs` will be rendered empty... <img width="1413" alt="Screenshot 2024-03-04 at 3 58 16 PM" src="https://github.com/apple/pkl/assets/10229883/54088bff-5556-48cd-838c-f1580ead2d50"> Package used: ``` amends "package://pkg.pkl-lang.org/github.com/stefma/pkl-gha/com.github.action@0.0.1#/GitHubAction.pkl" ```

adam commented

2025-12-30 01:20:49 +01:00

@stackoverflow commented on GitHub (Mar 4, 2024):

How do I mark a property as non-null?
Just remove the ? from the end of the type: first: Int instead of first: Int?

Here's a list of default values for different types, in case you are confused: https://pkl-lang.org/main/current/language-reference/index.html#default-values

Primitive types like String, Int, Boolean, etc have no default values. That's why you get an error if you don't provide them.

@stackoverflow commented on GitHub (Mar 4, 2024): > How do I mark a property as non-null? Just remove the `?` from the end of the type: `first: Int` instead of `first: Int?` Here's a list of default values for different types, in case you are confused: https://pkl-lang.org/main/current/language-reference/index.html#default-values Primitive types like `String`, `Int`, `Boolean`, etc have no default values. That's why you get an error if you don't provide them.

adam commented

2025-12-30 01:20:49 +01:00

@bioball commented on GitHub (Mar 4, 2024):

Is this invalid output?

on: {}

If this is valid, then there is nothing to do here.

If it this invalid, then you will want to think about what the validation rules are, and define them in your template.

For example, maybe you want to say that at least one sub-property is defined. If so, then you can do:

local const hasAtLeastOneProperty = (it: On) -> it.first != null || it.second != null || it.third != null

on: On(hasAtLeastOneProperty)

Then, Pkl will throw if some property is not defined.

Alternatively, if on: {} is valid, but you would rather omit this from your output altogether if it's empty. In that case, you can just make the property nullable, and have your renderer omit null properties.

For the YAML use-case, it already does omit null properties so nothing else needs to be set.

on: On?

output {
  renderer = new YamlRenderer {}
}

@bioball commented on GitHub (Mar 4, 2024): Is this invalid output? ``` on: {} ``` If this is _valid_, then there is nothing to do here. If it this _invalid_, then you will want to think about what the validation rules are, and define them in your template. For example, maybe you want to say that at least one sub-property is defined. If so, then you can do: ```groovy local const hasAtLeastOneProperty = (it: On) -> it.first != null || it.second != null || it.third != null on: On(hasAtLeastOneProperty) ``` Then, Pkl will throw if some property is not defined. Alternatively, if `on: {}` is _valid_, but you would rather omit this from your output altogether if it's empty. In that case, you can just make the property nullable, and have your renderer omit null properties. For the YAML use-case, it already does omit null properties so nothing else needs to be set. ```groovy on: On? output { renderer = new YamlRenderer {} } ```

adam commented

2025-12-30 01:20:49 +01:00

@StefMa commented on GitHub (Mar 4, 2024):

@stackoverflow I was not aware of the concept of default properties 🤯 thanks for sharing! Know it make sense!

@bioball thanks for the explanation!
Indeed, on {}without a single property set is invalid. Seems I wanna introduce a logic you provided.

Thanks for your support here guys.
Much appreciated.

Going to close it as it everything works as expected.

@StefMa commented on GitHub (Mar 4, 2024): @stackoverflow I was not aware of the concept of default properties 🤯 thanks for sharing! Know it make sense! @bioball thanks for the explanation! Indeed, `on {}`without a single property set is *invalid*. Seems I wanna introduce a logic you provided. Thanks for your support here guys. Much appreciated. Going to close it as it everything works as expected.

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Reference: starred/pkl#100