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content/Rustでの評価文脈（環境）のRAIIっぽい実装.md

@@ -20,14 +20,28 @@ fn eval<'a>(expr:Expr,env:&'a mut Vec<(String,Val)>)->Result<Val,Error>{
 
 Environmentは評価全体で見ればLetやLambdaごとに分岐していく構造だけれど、局所的には1列のベクタで表現できる
 
+こんな感じすかねえ
+
 ```rust
-struct<'a,T> Environment(&'a mut Vec<T>)
+struct EnvironmentT<'a, T>(&'a mut Vec<(String, T)>, usize);
 
-impl<'a,T> Environment{
-	pub fn new()->Self{
-		Self(&mut Vec::new())
+impl<'a, T> EnvironmentT<'a, T> {
+    pub fn new(vec: &'a mut Vec<(String, T)>, mut names: Vec<(String, T)>) -> Self {
+        let len = vec.len();
+        vec.append(&mut names);
+        Self(vec, len)
+    }
+    pub fn drop(&mut self) {
+        self.0.truncate(self.1);
+    }
+    pub fn lookup(&self, name: &String) -> Option<&'a T> {
+        let res = self
+            .0
+            .iter()
+            .rev()
+            .filter(|(n, _v)| name == n)
+            .collect::<Vec<_>>();
+        res.get(0).map(|(_, v)| v)
     }
-
 }
-
 ```