UPDATING.md

Updating to MiniJinja 2

MiniJinja 2.0 is a major update to MiniJinja that changes a lot of core internals and cleans up some APIs. In particular it resolves some limitations in the engine in relation to working with dynamic objects, unlocks potentials for future performance improvements and enhancements. This document helps with upgrading to that version.

Syntax Config

If you want to use custom delimiters, the way to configure this was slightly changed to enable future improvements to this feature.

Old:

use minijinja::{Environment, Syntax};

let mut env = Environment::new();
env.set_syntax(minijinja::Syntax {
    block_start: "{".into(),
    block_end: "}".into(),
    variable_start: "${".into(),
    variable_end: "}".into(),
    comment_start: "{*".into(),
    comment_end: "*}".into(),
})
.unwrap();

New:

use minijinja::{Environment, syntax::SyntaxConfig};

let mut env = Environment::new();
env.set_syntax(
    SyntaxConfig::builder()
        .block_delimiters("{", "}")
        .variable_delimiters("${", "}")
        .comment_delimiters("{*", "*}")
        .build()
        .unwrap(),
);

Iterators

In MiniJinja 1.x you could create iterators with the Value::from_iterator function. That same function is now called Value::make_one_shot_iterator and the use is discouraged. Instead most uses should instead use Value::make_iterable which takes a function returning an iterator. This has the advantage that the value can be iterated over multiple times.

Old:

let value = Value::from_iterator(1..10);

New Preferred:

let value = Value::make_iterable(|| 1..10);

Additionally you can now also make iterables that borrow from other values by using Value::make_object_iterable:

let value = Value::make_iterable(vec![1, 2, 3], |obj| {
    Box::new(obj.iter().map(|x| Value::from(*x * 2)))
});

Objects

The largest change is the new object systems. In MiniJinja 2, Objects are now using an entire new trait. (Object, SeqObject and StructObject) were replaced by a single trait called Object. It is however a completely new trait unrelated to the old one, though it retains some common ideas.

In a nutshell:

• All method use &Arc<Self> instead of &self as receiver. This allows one to clone out of the object when needed.
• Object::kind is gone and was replaced with Object::repr in spirit
• All trait methods are directly on the Object trait and the StructObject and SeqObject functionality is moved onto the object trait.
• Formatting is done via Object::render rather than fmt::Display.
• fmt::Debug is required in all cases now.
• Iteration now is implemented via Object::enumerate.

When working with objects of an unknown type, you can use the new DynObject struct which is a type erased box over Arc<Object>. Value::as_object now returns an Option<&DynObject> compared to previously an Option<&dyn Object> as an example. The DynObject can be cheaply cloned which bumps the reference count.

On the value type, the object related APIs were changed a bit to better accommodate for the new trait:

• Value::as_struct was removed, use Value::as_object instead.
• Value::as_seq was removed, use Value::as_object instead.
• Value::as_object now returns a Option<&DynObject>.
• Value::downcast_object was added which returns an Option<Arc<T>>
• ValueKind is now non exhaustive and has more variants.

Structs and Maps

Objects can now directly implement structs and maps. That gives them greater flexibility. Because the receiver is an Arc<Self> we can also efficiently borrow from them.

#[derive(Debug)]
struct User {
    username: String,
    roles: Vec<String>,
}

Old:

use minijinja::value::{Value, StructObject};

impl StructObject for User {
    fn get_field(&self, field: &str) -> Option<Value> {
        Some(match field {
            "username" => Value::from(&self.username),
            "roles" => Value::from(self.roles.clone()),
            _ => return None,
        })
    }

    fn static_fields(&self) -> Option<&'static [&'static str]> {
        Some(&["username", "roles"])
    }
}

let user = Value::from_struct_object(User { ... });

New:

The big changes are that get_value is now used instead of get_field and the field that is looked up is a &Value. To match on a string we need to call as_str() on it. For the roles here we can keep using the old pattern, or use the more efficient Value::make_object_iterable which can borrow from the object and make a lazy iterable. For iteration an Enumerator::Str over all keys is returned from Object::enumerate.

use std::sync::Arc;
use minijinja::value::{Value, Object, Enumerator};

impl Object for User {
    fn get_value(self: &Arc<Self>, field: &Value) -> Option<Value> {
        Some(match field.as_str()? {
            "username" => Value::from(&self.username),
            "roles" => Value::make_object_iterable(self.clone(), |o| {
                Box::new(o.roles.iter().map(Value::from))
            }),
            _ => return None,
        })
    }

    fn enumerate(self: &Arc<Self>) -> Enumerator {
        Enumerator::Str(&["foo", "bar"])
    }
}

let value = Value::from_object(User { ... });

Sequences

Sequences are now also just an Object.

#[derive(Debug)]
struct SimpleDynamicSeq([char; 4]);

Old:

use minijinja::value::SeqObject;

impl SeqObject for SimpleDynamicSeq {
    fn get_item(&self, idx: usize) -> Option<Value> {
        self.0.get(idx).copied().map(Value::from)
    }

    fn item_count(&self) -> usize {
        4
    }
}

let value = Value::from_seq_object(SimpleDynamicSeq(...));

New:

Because the default object representation is a Map, we need to change it to ObjectRepr::seq in the repr method. As sequences iterate over their values, we can use the convenient Enumerator::Seq enumerator which instructs the engine to sequentially iterate over the object from 0 to the given length. Otherwise the interface is the same as with the map above, which means that rather than implementing get_item you now also implement get_value which replaces it. To match over the index, use as_usize() on the value.

use minijinja::value::{Object, ObjectRepr, Enumerator, Value};

#[derive(Debug)]
struct SimpleDynamicSeq([char; 4]);

impl Object for SimpleDynamicSeq {
    fn repr(self: &Arc<Self>) -> ObjectRepr {
        ObjectRepr::Seq
    }

    fn get_value(self: &Arc<Self>, idx: &Value) -> Option<Value> {
        self.0.get(idx.as_usize()?).copied().map(Value::from)
    }

    fn enumerate(self: &Arc<Self>) -> Enumerator {
        Enumerator::Seq(self.0.len())
    }
}

let value = Value::from_object(SimpleDynamicSeq(...));

Methods, Callables and Rendering

The interface for callables is largely unchanged other than the new receiver.

Old:

use minijinja::{Error, ErrorKind};
use minijinja::value::Object;

#[derive(Debug)]
struct Markdown(String);

impl fmt::Display for Markdown {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", &self.0)
    }
}

impl Object for Markdown {
    fn call_method(
        &self,
        _state: &State,
        name: &str,
        args: &[Value],
    ) -> Result<Value, Error> {
        if name == "render" {
            // assert no arguments
            from_args(args)?;
            Ok(Value::from(render_markdown(&self.0)))
        } else {
            Err(Error::new(
                ErrorKind::UnknownMethod,
                format!("object has no method named {name}"),
            ))
        }
    }
}

New:

The replacement for fmt::Display is the new Object::render method. If you implement it, it overrides the implied default. Additionally if you leave out the error message in the UnknownMethod error the engine provides a useful one by default.

use minijinja::{Error, ErrorKind};
use minijinja::value::Object;

#[derive(Debug)]
struct Markdown(String);

impl Object for Markdown {
    fn call_method(
        self: &Arc<Self>,
        _state: &State,
        name: &str,
        args: &[Value],
    ) -> Result<Value, Error> {
        if name == "render" {
            from_args(args)?;
            Ok(Value::from(render_markdown(&self.0)))
        } else {
            Err(Error::from(ErrorKind::UnknownMethod))
        }
    }

    fn render(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", &self.0)
    }
}

Stack Ref

The old minijinja-stack-ref module was removed as it can no longer accommodate the new object model. However that module largely is no longer useful as the new object system is powerful enough to support it for the most part. While it's not possible any more to return references to objects on the stack, you can now trivially work with reference counted externally held objects which should resolve a lot of the needs for the stack-ref module.

For examples of how to do that, look at the new object-ref example that is modelled after the old stack-ref example where you can see the differences between the two.

Lazy Iterables

With MiniJinja 2 various things that were previously sequences, are now just iterables. For instance using |reverse will only return an iterable, not a sequence. This means that you cannot index into this for instance. On the other hand it performs better and more efficiently. The same is now true for slicing into things that are not strings with the [:] operator.

If you do still want a list, you can force it into a list with the |list operator.