refactor: improve eval error handling (start)

src/nodes/eval.rs

@@ -7,6 +7,8 @@ use crate::nodes::{Function, Node, Nodes, Operator};
 
 #[derive(Clone, Debug, thiserror::Error)]
 pub enum Error {
+    #[error("out of bounds: index {0}, {1}")]
+    OutOfBounds(Nodes, usize),
     #[error("invalid operand: {0}")]
     InvalidOperand(Node),
     #[error("missing argument, got: {0}")]
@@ -48,8 +50,15 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
         })
         .next()
     {
-        match &mut nodes[start..end][index + 1] {
+        let Some(node) = nodes.get_mut(start + index + 1) else {
-            Node::Number(n) => match variant {
+            return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + index + 1));
+        };
+
+        let Node::Number(n) = node else {
+            return Err(Error::MissingArgument(node.clone()));
+        };
+
+        match variant {
             Function::Sqrt => n.sqrt_mut(),
 
             Function::Ln => n.ln_mut(),
@@ -62,8 +71,6 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             Function::ArcTangent => n.atan_mut(),
             Function::Secant => n.sec_mut(),
             Function::CoSecant => n.csc_mut(),
-            },
-            n => return Err(Error::MissingArgument(n.clone())),
         };
 
         nodes.remove(start + index);