diff --git a/src/types/simple.rs b/src/types/simple.rs
index 43ed169d9..63d885c92 100644
--- a/src/types/simple.rs
+++ b/src/types/simple.rs
@@ -97,6 +97,10 @@ pub trait PrimType: TypeRowElem + std::fmt::Debug + sealed::Sealed {
     fn tag(&self) -> TypeTag;
 }
 
+impl TypeRowElem for SimpleType {}
+impl TypeRowElem for ClassicType {}
+impl TypeRowElem for HashableType {}
+
 // sealed trait pattern to prevent users extending PrimType
 mod sealed {
     use super::{ClassicType, HashableType, SimpleType};
diff --git a/src/types/simple/serialize.rs b/src/types/simple/serialize.rs
index bb09fcd93..470388b43 100644
--- a/src/types/simple/serialize.rs
+++ b/src/types/simple/serialize.rs
@@ -6,6 +6,7 @@ use super::HashableType;
 use super::PrimType;
 use super::TypeTag;
 
+use itertools::Itertools;
 use smol_str::SmolStr;
 
 use super::super::custom::CustomType;
@@ -17,6 +18,7 @@ use super::SimpleType;
 use super::super::AbstractSignature;
 
 use crate::ops::constant::HugrIntWidthStore;
+use crate::types::type_row::TypeRowElem;
 
 #[derive(serde::Serialize, serde::Deserialize, Clone, Debug)]
 #[serde(tag = "t")]
@@ -30,11 +32,11 @@ pub(crate) enum SerSimpleType {
         signature: Box<AbstractSignature>,
     },
     Tuple {
-        row: Box<TypeRow<SerSimpleType>>,
+        row: Vec<SerSimpleType>,
         c: TypeTag,
     },
     Sum {
-        row: Box<TypeRow<SerSimpleType>>,
+        row: Vec<SerSimpleType>,
         c: TypeTag,
     },
     Array {
@@ -79,15 +81,15 @@ where
     fn from(value: Container<T>) -> Self {
         match value {
             Container::Sum(inner) => SerSimpleType::Sum {
-                row: Box::new(inner.map_into()),
+                row: inner.into_owned().into_iter().map_into().collect(),
                 c: T::TAG, // We could inspect inner.containing_tag(), but this should have been done already
             },
             Container::Tuple(inner) => SerSimpleType::Tuple {
-                row: Box::new(inner.map_into()),
+                row: inner.into_owned().into_iter().map_into().collect(),
                 c: T::TAG,
             },
             Container::Array(inner, len) => SerSimpleType::Array {
-                inner: box_convert(*inner),
+                inner: Box::new((*inner).into()),
                 len,
                 c: T::TAG,
             },
@@ -130,19 +132,14 @@ impl From<SimpleType> for SerSimpleType {
     }
 }
 
-pub(crate) fn box_convert_try<T, F>(value: T) -> Box<F>
-where
-    T: TryInto<F>,
-    <T as TryInto<F>>::Error: std::fmt::Debug,
-{
-    Box::new((value).try_into().unwrap())
-}
-
-pub(crate) fn box_convert<T, F>(value: T) -> Box<F>
-where
-    T: Into<F>,
-{
-    Box::new((value).into())
+fn try_convert_list<T: TryInto<T2>, T2: TypeRowElem>(
+    values: Vec<T>,
+) -> Result<TypeRow<T2>, T::Error> {
+    let vals = values
+        .into_iter()
+        .map(T::try_into)
+        .collect::<Result<Vec<T2>, T::Error>>()?;
+    Ok(TypeRow::from(vals))
 }
 
 macro_rules! handle_container {
@@ -163,13 +160,13 @@ impl From<SerSimpleType> for SimpleType {
             SerSimpleType::S => HashableType::String.into(),
             SerSimpleType::G { signature } => ClassicType::Graph(Box::new(*signature)).into(),
             SerSimpleType::Tuple { row: inner, c } => {
-                handle_container!(c, Tuple(Box::new(inner.try_convert_elems().unwrap())))
+                handle_container!(c, Tuple(Box::new(try_convert_list(inner).unwrap())))
             }
             SerSimpleType::Sum { row: inner, c } => {
-                handle_container!(c, Sum(Box::new(inner.try_convert_elems().unwrap())))
+                handle_container!(c, Sum(Box::new(try_convert_list(inner).unwrap())))
             }
             SerSimpleType::Array { inner, len, c } => {
-                handle_container!(c, Array(box_convert_try(*inner), len))
+                handle_container!(c, Array(Box::new((*inner).try_into().unwrap()), len))
             }
             SerSimpleType::Alias { name: s, c } => handle_container!(c, Alias(s)),
             SerSimpleType::Opaque { custom, c } => {
diff --git a/src/types/type_row.rs b/src/types/type_row.rs
index eee7928bc..37af0c1dd 100644
--- a/src/types/type_row.rs
+++ b/src/types/type_row.rs
@@ -12,8 +12,6 @@ use crate::utils::display_list;
 /// Base trait for anything that can be put in a [TypeRow]
 pub trait TypeRowElem: Clone + 'static {}
 
-impl<T: Clone + 'static> TypeRowElem for T {}
-
 /// List of types, used for function signatures.
 #[derive(Clone, PartialEq, Eq, Debug, serde::Serialize, serde::Deserialize)]
 //#[cfg_attr(feature = "pyo3", pyclass)] // TODO: expose unparameterized versions