feat: add Options::apply_merge()

Merges `<<` keys into the surrounding mapping.

Closes: #2

src/options.rs

@@ -8,18 +8,64 @@ use crate::{Compose, YamlValue};
 #[allow(missing_copy_implementations)] // Will include interpolation vars as a HashMap.
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
 pub struct Options {
-    /// TODO
-    merge_anchors: bool,
+    /// Whether to perform merging of `<<` keys.
+    apply_merge: bool,
 }
 
 impl Options {
+    /// Set whether to merge `<<` keys into the surrounding mapping.
+    ///
+    /// ```
+    /// use compose_spec::Compose;
+    ///
+    /// let yaml = "
+    /// services:
+    ///   one:
+    ///     environment: &env
+    ///       FOO: foo
+    ///       BAR: bar
+    ///   two:
+    ///     environment:
+    ///       <<: *env
+    ///       BAR: baz
+    /// ";
+    ///
+    /// let compose = Compose::options()
+    ///     .apply_merge(true)
+    ///     .from_yaml_str(yaml)
+    ///     .unwrap();
+    ///
+    /// let two_env = compose.services["two"]
+    ///     .environment
+    ///     .clone()
+    ///     .into_map()
+    ///     .unwrap();
+    ///
+    /// assert_eq!(two_env["FOO"].as_ref().unwrap().as_string().unwrap(), "foo");
+    /// assert_eq!(two_env["BAR"].as_ref().unwrap().as_string().unwrap(), "baz");
+    /// ```
+    pub fn apply_merge(&mut self, apply_merge: bool) -> &mut Self {
+        self.apply_merge = apply_merge;
+        self
+    }
+
+    /// Return `true` if any options are set.
+    const fn any(&self) -> bool {
+        let Self { apply_merge } = *self;
+        apply_merge
+    }
+
     /// Use the set options to deserialize a [`Compose`] file from a string slice of YAML.
     ///
     /// # Errors
     ///
     /// Returns an error if deserialization fails.
     pub fn from_yaml_str(&self, yaml: &str) -> serde_yaml::Result<Compose> {
-        serde_yaml::from_str(yaml)
+        if self.any() {
+            self.from_yaml_value(serde_yaml::from_str(yaml)?)
+        } else {
+            serde_yaml::from_str(yaml)
+        }
     }
 
     /// Use the set options to deserialize a [`Compose`] file from an IO stream of YAML.
@@ -28,7 +74,11 @@ impl Options {
     ///
     /// Returns an error if deserialization fails.
     pub fn from_yaml_reader<R: Read>(&self, reader: R) -> serde_yaml::Result<Compose> {
-        serde_yaml::from_reader(reader)
+        if self.any() {
+            self.from_yaml_value(serde_yaml::from_reader(reader)?)
+        } else {
+            serde_yaml::from_reader(reader)
+        }
     }
 
     /// Use the set options to deserialize a [`Compose`] file from bytes of YAML.
@@ -37,15 +87,22 @@ impl Options {
     ///
     /// Returns an error if deserialization fails.
     pub fn from_yaml_slice(&self, slice: &[u8]) -> serde_yaml::Result<Compose> {
-        serde_yaml::from_slice(slice)
+        if self.any() {
+            self.from_yaml_value(serde_yaml::from_slice(slice)?)
+        } else {
+            serde_yaml::from_slice(slice)
+        }
     }
 
     /// Use the set options to deserialize a [`Compose`] file from a YAML [`Value`](YamlValue).
     ///
     /// # Errors
     ///
     /// Returns an error if deserialization fails.
-    pub fn from_yaml_value(&self, value: YamlValue) -> serde_yaml::Result<Compose> {
+    pub fn from_yaml_value(&self, mut value: YamlValue) -> serde_yaml::Result<Compose> {
+        if self.apply_merge {
+            value.apply_merge()?;
+        }
         serde_yaml::from_value(value)
     }
 }