A little more documentation on PrimOpError.

src/eval/primop.rs

@@ -5,10 +5,19 @@ use crate::eval::value::Value;
 pub enum PrimOpError {
     #[error("Math error (underflow or overflow) computing {0} operator")]
     MathFailure(&'static str),
+    /// This particular variant covers the case in which a primitive
+    /// operator takes two arguments that are supposed to be the same,
+    /// but they differ. (So, like, all the math operators.)
     #[error("Type mismatch ({1} vs {2}) computing {0} operator")]
     TypeMismatch(String, Value, Value),
+    /// This variant covers when an operator must take a particular
+    /// type, but the user has provided a different one.
     #[error("Bad type for operator {0}: {1}")]
     BadTypeFor(&'static str, Value),
+    /// Probably obvious from the name, but just to be very clear: this
+    /// happens when you pass three arguments to a two argument operator,
+    /// etc. Technically that's a type error of some sort, but we split
+    /// it out.
     #[error("Illegal number of arguments for {0}: {1} arguments found")]
     BadArgCount(String, usize),
     #[error("Unknown primitive operation {0}")]