Add function vec returns a list mutable from the minijinja context (#143)

The `ValueVec` helps with the efforts to emit already formatted code.

It's has a simple `push` method, and it implements `enumerate` so I will be able to iterate through the vec from within a template.

Author: svix-mman

src/cli_v2/mod.rs

@@ -0,0 +1 @@
+pub(crate) mod value_vec;

src/cli_v2/value_vec.rs

@@ -0,0 +1,84 @@
+use std::sync::{Arc, RwLock};
+
+use minijinja::{ErrorKind, value::DynObject};
+
+pub(crate) fn new_value_vec() -> DynObject {
+    DynObject::new(Arc::new(ValueVec(RwLock::new(vec![]))))
+}
+
+impl minijinja::value::Object for ValueVec {
+    fn repr(self: &Arc<Self>) -> minijinja::value::ObjectRepr {
+        minijinja::value::ObjectRepr::Iterable
+    }
+
+    fn call_method(
+        self: &Arc<Self>,
+        _state: &minijinja::State<'_, '_>,
+        method: &str,
+        args: &[minijinja::Value],
+    ) -> Result<minijinja::Value, minijinja::Error> {
+        match method {
+            "push" => self.push(args),
+            _ => Err(minijinja::Error::new(
+                ErrorKind::UnknownMethod,
+                format!("Unexpected method {method}"),
+            )),
+        }
+    }
+
+    fn enumerate(self: &Arc<Self>) -> minijinja::value::Enumerator {
+        let vals = self
+            .0
+            .read()
+            .expect("Unable to read from ValueVec, RwLock was poisoned")
+            .iter()
+            .map(|s| s.to_owned())
+            .collect::<Vec<_>>();
+        minijinja::value::Enumerator::Iter(Box::new(vals.into_iter()))
+    }
+}
+
+/// List of `minijinja::Value`, a workaround for `minijinja`s mutability limitations.
+///
+/// Like `minijinja`s own `namespace`, but in array form instead of dict form.
+#[derive(Debug)]
+struct ValueVec(RwLock<Vec<minijinja::Value>>);
+
+impl ValueVec {
+    fn push(
+        self: &Arc<Self>,
+        args: &[minijinja::Value],
+    ) -> Result<minijinja::Value, minijinja::Error> {
+        ensure_n_args("push", 1, args)?;
+        {
+            let mut list = self
+                .0
+                .try_write()
+                .map_err(|e| minijinja::Error::new(ErrorKind::InvalidOperation, e.to_string()))?;
+            list.push(args[0].clone());
+        }
+        Ok(minijinja::Value::UNDEFINED)
+    }
+}
+
+fn ensure_n_args(
+    method: &str,
+    n: usize,
+    args: &[minijinja::Value],
+) -> Result<(), minijinja::Error> {
+    let err = |kind| -> Result<(), minijinja::Error> {
+        Err(minijinja::Error::new(
+            kind,
+            format!(
+                "{method} | Expected: {n} args, got {} arguments",
+                args.len()
+            ),
+        ))
+    };
+
+    match args.len().cmp(&n) {
+        std::cmp::Ordering::Less => err(ErrorKind::MissingArgument),
+        std::cmp::Ordering::Greater => err(ErrorKind::TooManyArguments),
+        std::cmp::Ordering::Equal => Ok(()),
+    }
+}

src/lib.rs

@@ -1,5 +1,6 @@
 mod api;
 mod cli_v1;
+pub(crate) mod cli_v2;
 mod codesamples;
 mod generator;
 mod postprocessing;

src/template.rs

@@ -21,6 +21,9 @@ pub fn env_with_dir(
 pub fn populate_env(
     mut env: minijinja::Environment<'static>,
 ) -> Result<minijinja::Environment<'static>, minijinja::Error> {
+    // === Custom functions ===
+    env.add_function("vec", crate::cli_v2::value_vec::new_value_vec);
+
     // === Custom filters ===
 
     // --- Case conversion ---