feat: Custom const for ERROR_TYPE (#756)

src/extension/prelude.rs

@@ -137,12 +137,11 @@ pub fn new_array_op(element_ty: Type, size: u64) -> LeafOp {
         .into()
 }
 
+/// The custom type for Errors.
+pub const ERROR_CUSTOM_TYPE: CustomType =
+    CustomType::new_simple(ERROR_TYPE_NAME, PRELUDE_ID, TypeBound::Eq);
 /// Unspecified opaque error type.
-pub const ERROR_TYPE: Type = Type::new_extension(CustomType::new_simple(
-    ERROR_TYPE_NAME,
-    PRELUDE_ID,
-    TypeBound::Eq,
-));
+pub const ERROR_TYPE: Type = Type::new_extension(ERROR_CUSTOM_TYPE);
 
 /// The string name of the error type.
 pub const ERROR_TYPE_NAME: SmolStr = SmolStr::new_inline("error");
@@ -191,6 +190,48 @@ impl KnownTypeConst for ConstUsize {
     const TYPE: CustomType = USIZE_CUSTOM_T;
 }
 
+#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
+/// Structure for holding constant usize values.
+pub struct ConstError {
+    /// Integer tag/signal for the error.
+    pub signal: u32,
+    /// Error message.
+    pub message: String,
+}
+
+impl ConstError {
+    /// Define a new error value.
+    pub fn new(signal: u32, message: impl ToString) -> Self {
+        Self {
+            signal,
+            message: message.to_string(),
+        }
+    }
+}
+
+#[typetag::serde]
+impl CustomConst for ConstError {
+    fn name(&self) -> SmolStr {
+        format!("ConstError({:?}, {:?})", self.signal, self.message).into()
+    }
+
+    fn check_custom_type(&self, typ: &CustomType) -> Result<(), CustomCheckFailure> {
+        self.check_known_type(typ)
+    }
+
+    fn equal_consts(&self, other: &dyn CustomConst) -> bool {
+        crate::values::downcast_equal_consts(self, other)
+    }
+
+    fn extension_reqs(&self) -> ExtensionSet {
+        ExtensionSet::singleton(&PRELUDE_ID)
+    }
+}
+
+impl KnownTypeConst for ConstError {
+    const TYPE: CustomType = ERROR_CUSTOM_TYPE;
+}
+
 #[cfg(test)]
 mod test {
     use crate::{
@@ -219,7 +260,7 @@ mod test {
     }
 
     #[test]
-    /// Test building a HUGR involving a new_array operation.
+    /// test the prelude error type.
     fn test_error_type() {
         let ext_def = PRELUDE
             .get_type(&ERROR_TYPE_NAME)
@@ -229,5 +270,18 @@ mod test {
 
         let ext_type = Type::new_extension(ext_def);
         assert_eq!(ext_type, ERROR_TYPE);
+
+        let error_val = ConstError::new(2, "my message");
+
+        assert_eq!(error_val.name(), "ConstError(2, \"my message\")");
+
+        assert!(error_val.check_custom_type(&ERROR_CUSTOM_TYPE).is_ok());
+
+        assert_eq!(
+            error_val.extension_reqs(),
+            ExtensionSet::singleton(&PRELUDE_ID)
+        );
+        assert!(error_val.equal_consts(&ConstError::new(2, "my message")));
+        assert!(!error_val.equal_consts(&ConstError::new(3, "my message")));
     }
 }