impl Display for StructValue

- Further on #670.
- Fixes a bug in `sturct_.rs` wherein the failure condition was reversed.
- Permit `as_list` to convert a `Null` into a `None`.

Examples:

    {}

    {foo:3}

    {foo:3,bar:true}

vortex-scalar/src/display.rs

@@ -6,6 +6,7 @@ use vortex_dtype::{match_each_native_ptype, DType};
 use crate::binary::BinaryScalar;
 use crate::bool::BoolScalar;
 use crate::primitive::PrimitiveScalar;
+use crate::struct_::StructScalar;
 use crate::utf8::Utf8Scalar;
 use crate::Scalar;
 
@@ -50,7 +51,26 @@ impl Display for Scalar {
                     }
                 }
             }
-            DType::Struct(..) => todo!(),
+            DType::Struct(dtype, _) => {
+                let v = StructScalar::try_from(self).map_err(|_| std::fmt::Error)?;
+
+                if v.is_null() {
+                    write!(f, "null")
+                } else {
+                    write!(f, "{{")?;
+                    let formatted_fields = dtype
+                        .names()
+                        .iter()
+                        .enumerate()
+                        .map(|(idx, name)| match v.field_by_idx(idx) {
+                            None => format!("{name}:null"),
+                            Some(val) => format!("{name}:{val}"),
+                        })
+                        .format(",");
+                    write!(f, "{}", formatted_fields)?;
+                    write!(f, "}}")
+                }
+            }
             DType::List(..) => todo!(),
             DType::Extension(..) => todo!(),
         }
@@ -59,11 +79,13 @@ impl Display for Scalar {
 
 #[cfg(test)]
 mod tests {
+    use std::sync::Arc;
+
     use vortex_buffer::Buffer;
     use vortex_dtype::Nullability::{NonNullable, Nullable};
-    use vortex_dtype::{DType, PType};
+    use vortex_dtype::{DType, PType, StructDType};
 
-    use crate::Scalar;
+    use crate::{PValue, Scalar, ScalarValue};
 
     #[test]
     fn display_bool() {
@@ -109,4 +131,88 @@ mod tests {
         );
         assert_eq!(format!("{}", Scalar::null(DType::Binary(Nullable))), "null");
     }
+
+    #[test]
+    fn display_empty_struct() {
+        fn dtype() -> DType {
+            DType::Struct(StructDType::new(Arc::new([]), vec![]), Nullable)
+        }
+
+        assert_eq!(format!("{}", Scalar::null(dtype())), "null");
+
+        assert_eq!(format!("{}", Scalar::r#struct(dtype(), vec![])), "{}");
+    }
+
+    #[test]
+    fn display_one_field_struct() {
+        fn dtype() -> DType {
+            DType::Struct(
+                StructDType::new(
+                    Arc::new([Arc::from("foo")]),
+                    vec![DType::Primitive(PType::U32, Nullable)],
+                ),
+                Nullable,
+            )
+        }
+
+        assert_eq!(format!("{}", Scalar::null(dtype())), "null");
+
+        assert_eq!(
+            format!("{}", Scalar::r#struct(dtype(), vec![ScalarValue::Null])),
+            "{foo:null}"
+        );
+
+        assert_eq!(
+            format!(
+                "{}",
+                Scalar::r#struct(dtype(), vec![ScalarValue::Primitive(PValue::U32(32))])
+            ),
+            "{foo:32}"
+        );
+    }
+
+    #[test]
+    fn display_two_field_struct() {
+        fn dtype() -> DType {
+            DType::Struct(
+                StructDType::new(
+                    Arc::new([Arc::from("foo"), Arc::from("bar")]),
+                    vec![
+                        DType::Bool(Nullable),
+                        DType::Primitive(PType::U32, Nullable),
+                    ],
+                ),
+                Nullable,
+            )
+        }
+
+        assert_eq!(format!("{}", Scalar::null(dtype())), "null");
+
+        assert_eq!(
+            format!("{}", Scalar::r#struct(dtype(), vec![])),
+            "{foo:null,bar:null}"
+        );
+
+        assert_eq!(
+            format!(
+                "{}",
+                Scalar::r#struct(dtype(), vec![ScalarValue::Bool(true)])
+            ),
+            "{foo:true,bar:null}"
+        );
+
+        assert_eq!(
+            format!(
+                "{}",
+                Scalar::r#struct(
+                    dtype(),
+                    vec![
+                        ScalarValue::Bool(true),
+                        ScalarValue::Primitive(PValue::U32(32))
+                    ]
+                )
+            ),
+            "{foo:true,bar:32}"
+        );
+    }
 }

vortex-scalar/src/struct_.rs

@@ -17,6 +17,10 @@ impl<'a> StructScalar<'a> {
         self.dtype
     }
 
+    pub fn is_null(&self) -> bool {
+        self.fields.is_none()
+    }
+
     pub fn field_by_idx(&self, idx: usize) -> Option<Scalar> {
         let DType::Struct(st, _) = self.dtype() else {
             unreachable!()
@@ -56,7 +60,7 @@ impl<'a> TryFrom<&'a Scalar> for StructScalar<'a> {
     type Error = VortexError;
 
     fn try_from(value: &'a Scalar) -> Result<Self, Self::Error> {
-        if matches!(value.dtype(), DType::Struct(..)) {
+        if !matches!(value.dtype(), DType::Struct(..)) {
             vortex_bail!("Expected struct scalar, found {}", value.dtype())
         }
         Ok(Self {

vortex-scalar/src/value.rs

@@ -61,6 +61,7 @@ impl ScalarValue {
 
     pub fn as_list(&self) -> VortexResult<Option<&Arc<[Self]>>> {
         match self {
+            Self::Null => Ok(None),
             Self::List(l) => Ok(Some(l)),
             _ => Err(vortex_err!("Expected a list scalar, found {:?}", self)),
         }