merge(glittershark/achilles): Subtree import at 'b93268085a'

Imported from https://github.com/glittershark/achilles/

git-subtree-dir: users/glittershark/achilles
git-subtree-mainline: 4d193f2395
git-subtree-split: b93268085a
Change-Id: I64a583b454bbe03e20358ad7808939a4cbc212ba

users/glittershark/achilles/src/interpreter/error.rs

@@ -0,0 +1,19 @@
+use std::result;
+
+use thiserror::Error;
+
+use crate::ast::{Ident, Type};
+
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum Error {
+    #[error("Undefined variable {0}")]
+    UndefinedVariable(Ident<'static>),
+
+    #[error("Unexpected type {actual}, expected type {expected}")]
+    InvalidType {
+        actual: Type<'static>,
+        expected: Type<'static>,
+    },
+}
+
+pub type Result<T> = result::Result<T, Error>;