1 files changed, 50 insertions, 0 deletions
diff --git a/src/eval/machine.rs b/src/eval/machine.rs
new file mode 100644
index 00000000..168cedcb
--- /dev/null
+++ b/src/eval/machine.rs
@@ -0,0 +1,50 @@
+use super::{Scopes, Value};
+use crate::diag::TypError;
+use crate::syntax::Span;
+use crate::Context;
+
+/// A virtual machine.
+pub struct Machine<'a> {
+    /// The core context.
+    pub ctx: &'a mut Context,
+    /// The stack of scopes.
+    pub scopes: Scopes<'a>,
+    /// A control flow event that is currently happening.
+    pub flow: Option<Flow>,
+}
+
+impl<'a> Machine<'a> {
+    /// Create a new virtual machine.
+    pub fn new(ctx: &'a mut Context, scopes: Scopes<'a>) -> Self {
+        Self { ctx, scopes, flow: None }
+    }
+}
+
+/// A control flow event that occurred during evaluation.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Flow {
+    /// Stop iteration in a loop.
+    Break(Span),
+    /// Skip the remainder of the current iteration in a loop.
+    Continue(Span),
+    /// Stop execution of a function early, optionally returning an explicit
+    /// value.
+    Return(Span, Option<Value>),
+}
+
+impl Flow {
+    /// Return an error stating that this control flow is forbidden.
+    pub fn forbidden(&self) -> TypError {
+        match *self {
+            Self::Break(span) => {
+                error!(span, "cannot break outside of loop")
+            }
+            Self::Continue(span) => {
+                error!(span, "cannot continue outside of loop")
+            }
+            Self::Return(span, _) => {
+                error!(span, "cannot return outside of function")
+            }
+        }
+    }
+}