diff --git a/apb/src/field.rs b/apb/src/field.rs
new file mode 100644
index 0000000..336148e
--- /dev/null
+++ b/apb/src/field.rs
@@ -0,0 +1,18 @@
+#[derive(Debug, thiserror::Error)]
+#[error("missing field '{0}'")]
+pub struct FieldErr(pub &'static str);
+
+pub type Field<T> = Result<T, FieldErr>;
+
+
+// TODO this trait is really ad-hoc and has awful naming...
+
+pub trait OptionalString {
+	fn str(self) -> Option<String>;
+}
+
+impl OptionalString for Field<&str> {
+	fn str(self) -> Option<String> {
+		self.ok().map(|x| x.to_string())
+	}
+}
diff --git a/apb/src/lib.rs b/apb/src/lib.rs
index bf0feb4..691b983 100644
--- a/apb/src/lib.rs
+++ b/apb/src/lib.rs
@@ -98,6 +98,9 @@ pub mod target;
 mod key;
 pub use key::{PublicKey, PublicKeyMut};
 
+pub mod field;
+pub use field::{Field, FieldErr};
+
 #[cfg(feature = "jsonld")]
 mod jsonld;
 
@@ -131,12 +134,6 @@ pub use types::{
 	},
 };
 
-#[derive(Debug, thiserror::Error)]
-#[error("missing field '{0}'")]
-pub struct FieldErr(pub &'static str);
-
-pub type Field<T> = Result<T, FieldErr>;
-
 #[cfg(feature = "unstructured")]
 pub fn new() -> serde_json::Value {
 	serde_json::Value::Object(serde_json::Map::default())