Add pretty errors example; Add validation in math

dust-lang/examples/pretty_errors.rs

@@ -0,0 +1,29 @@
+// It's very easy to get nice-looking error messages from the Dust's top-level error type.
+
+use std::{io::stderr, sync::Arc};
+
+use ariadne::sources;
+use dust_lang::Interpreter;
+
+fn main() {
+    let interpreter = Interpreter::new();
+
+    // First, we'll run some bad code.
+    let error = interpreter
+        .run(
+            Arc::from("bad code"),
+            Arc::from(
+                "
+            x = 1 + 'a'
+            y: float = 'hello'
+        ",
+            ),
+        )
+        .unwrap_err();
+
+    for report in error.build_reports() {
+        report
+            .write_for_stdout(sources(interpreter.sources()), stderr())
+            .unwrap();
+    }
+}

dust-lang/src/abstract_tree/math.rs

@@ -40,20 +40,24 @@ impl AbstractNode for Math {
                     return Err(ValidationError::ExpectedValueStatement(right.position()));
                 };
 
-                if let Type::Integer | Type::Float | Type::String = left_type {
-                    if let Type::Integer | Type::Float | Type::String = right_type {
-                        Ok(())
-                    } else {
-                        Err(ValidationError::ExpectedIntegerFloatOrString {
-                            actual: right_type,
-                            position: right.position(),
-                        })
+                match (&left_type, &right_type) {
+                    (Type::Integer, Type::Integer) => Ok(()),
+                    (Type::Float, Type::Float) => Ok(()),
+                    (Type::String, Type::String) => Ok(()),
+                    (Type::Integer, _) => {
+                        Err(ValidationError::ExpectedIntegerOrFloat(right.position()))
                     }
-                } else {
-                    Err(ValidationError::ExpectedIntegerFloatOrString {
-                        actual: left_type,
-                        position: left.position(),
-                    })
+                    (Type::Float, _) => {
+                        Err(ValidationError::ExpectedIntegerOrFloat(right.position()))
+                    }
+                    (Type::String, _) => Err(ValidationError::ExpectedString {
+                        actual: right_type,
+                        position: right.position(),
+                    }),
+                    (_, _) => Err(ValidationError::ExpectedIntegerFloatOrString {
+                        actual: right_type,
+                        position: right.position(),
+                    }),
                 }
             }
             Math::Subtract(left, right)