Change append operator to concatenation operator (#14344)

# Description

The "append" operator currently serves as both the append operator and
the concatenation operator. This dual role creates ambiguity when
operating on nested lists.

```nu
[1 2] ++ 3     # appends a value to a list [1 2 3]
[1 2] ++ [3 4] # concatenates two lists    [1 2 3 4]

[[1 2] [3 4]] ++ [5 6]
# does this give [[1 2] [3 4] [5 6]]
# or             [[1 2] [3 4] 5 6]  
```

Another problem is that `++=` can change the type of a variable:
```nu
mut str = 'hello '
$str ++= ['world']
($str | describe) == list<string>
```

Note that appending is only relevant for lists, but concatenation is
relevant for lists, strings, and binary values. Additionally, appending
can be expressed in terms of concatenation (see example below). So, this
PR changes the `++` operator to only perform concatenation.

# User-Facing Changes

Using the `++` operator with a list and a non-list value will now be a
compile time or runtime error.
```nu
mut list = []
$list ++= 1 # error
```
Instead, concatenate a list with one element:
```nu
$list ++= [1]
```
Or use `append`:
```nu
$list = $list | append 1
```

# After Submitting

Update book and docs.

---------

Co-authored-by: Douglas <32344964+NotTheDr01ds@users.noreply.github.com>

crates/nu-cli/src/completions/operator_completions.rs

@@ -60,10 +60,6 @@ impl Completer for OperatorCompletion {
                 ("bit-shr", "Bitwise shift right"),
                 ("in", "Is a member of (doesn't use regex)"),
                 ("not-in", "Is not a member of (doesn't use regex)"),
-                (
-                    "++",
-                    "Appends two lists, a list and a value, two strings, or two binary values",
-                ),
             ],
             Expr::String(_) => vec![
                 ("=~", "Contains regex match"),
@@ -72,7 +68,7 @@ impl Completer for OperatorCompletion {
                 ("not-like", "Does not contain regex match"),
                 (
                     "++",
-                    "Appends two lists, a list and a value, two strings, or two binary values",
+                    "Concatenates two lists, two strings, or two binary values",
                 ),
                 ("in", "Is a member of (doesn't use regex)"),
                 ("not-in", "Is not a member of (doesn't use regex)"),
@@ -95,10 +91,6 @@ impl Completer for OperatorCompletion {
                 ("**", "Power of"),
                 ("in", "Is a member of (doesn't use regex)"),
                 ("not-in", "Is not a member of (doesn't use regex)"),
-                (
-                    "++",
-                    "Appends two lists, a list and a value, two strings, or two binary values",
-                ),
             ],
             Expr::Bool(_) => vec![
                 (
@@ -113,15 +105,11 @@ impl Completer for OperatorCompletion {
                 ("not", "Negates a value or expression"),
                 ("in", "Is a member of (doesn't use regex)"),
                 ("not-in", "Is not a member of (doesn't use regex)"),
-                (
-                    "++",
-                    "Appends two lists, a list and a value, two strings, or two binary values",
-                ),
             ],
             Expr::FullCellPath(path) => match path.head.expr {
                 Expr::List(_) => vec![(
                     "++",
-                    "Appends two lists, a list and a value, two strings, or two binary values",
+                    "Concatenates two lists, two strings, or two binary values",
                 )],
                 Expr::Var(id) => get_variable_completions(id, working_set),
                 _ => vec![],
@@ -161,7 +149,7 @@ pub fn get_variable_completions<'a>(
         Type::List(_) | Type::String | Type::Binary => vec![
             (
                 "++=",
-                "Appends a list, a value, a string, or a binary value to a variable.",
+                "Concatenates two lists, two strings, or two binary values",
             ),
             ("=", "Assigns a value to a variable."),
         ],

crates/nu-command/src/help/help_operators.rs

@@ -31,7 +31,7 @@ impl Command for HelpOperators {
         let mut operators = [
             Operator::Assignment(Assignment::Assign),
             Operator::Assignment(Assignment::PlusAssign),
-            Operator::Assignment(Assignment::AppendAssign),
+            Operator::Assignment(Assignment::ConcatAssign),
             Operator::Assignment(Assignment::MinusAssign),
             Operator::Assignment(Assignment::MultiplyAssign),
             Operator::Assignment(Assignment::DivideAssign),
@@ -48,7 +48,7 @@ impl Command for HelpOperators {
             Operator::Comparison(Comparison::StartsWith),
             Operator::Comparison(Comparison::EndsWith),
             Operator::Math(Math::Plus),
-            Operator::Math(Math::Append),
+            Operator::Math(Math::Concat),
             Operator::Math(Math::Minus),
             Operator::Math(Math::Multiply),
             Operator::Math(Math::Divide),
@@ -144,8 +144,8 @@ fn description(operator: &Operator) -> &'static str {
         Operator::Comparison(Comparison::StartsWith) => "Checks if a string starts with another.",
         Operator::Comparison(Comparison::EndsWith) => "Checks if a string ends with another.",
         Operator::Math(Math::Plus) => "Adds two values.",
-        Operator::Math(Math::Append) => {
-            "Appends two lists, a list and a value, two strings, or two binary values."
+        Operator::Math(Math::Concat) => {
+            "Concatenates two lists, two strings, or two binary values."
         }
         Operator::Math(Math::Minus) => "Subtracts two values.",
         Operator::Math(Math::Multiply) => "Multiplies two values.",
@@ -163,8 +163,8 @@ fn description(operator: &Operator) -> &'static str {
         Operator::Bits(Bits::ShiftRight) => "Bitwise shifts a value right by another.",
         Operator::Assignment(Assignment::Assign) => "Assigns a value to a variable.",
         Operator::Assignment(Assignment::PlusAssign) => "Adds a value to a variable.",
-        Operator::Assignment(Assignment::AppendAssign) => {
-            "Appends a list, a value, a string, or a binary value to a variable."
+        Operator::Assignment(Assignment::ConcatAssign) => {
+            "Concatenates two lists, two strings, or two binary values."
         }
         Operator::Assignment(Assignment::MinusAssign) => "Subtracts a value from a variable.",
         Operator::Assignment(Assignment::MultiplyAssign) => "Multiplies a variable by a value.",

crates/nu-command/tests/commands/assignment/append_assign.rs

@@ -1,88 +0,0 @@
-use nu_test_support::nu;
-
-#[test]
-fn append_assign_int() {
-    let actual = nu!(r#"
-            mut a = [1 2];
-            $a ++= [3 4];
-            $a == [1 2 3 4]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_string() {
-    let actual = nu!(r#"
-            mut a = [a b];
-            $a ++= [c d];
-            $a == [a b c d]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_any() {
-    let actual = nu!(r#"
-            mut a = [1 2 a];
-            $a ++= [b 3];
-            $a == [1 2 a b 3]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_both_empty() {
-    let actual = nu!(r#"
-            mut a = [];
-            $a ++= [];
-            $a == []
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_type_mismatch() {
-    let actual = nu!(r#"
-            mut a = [1 2];
-            $a ++= [a];
-            $a == [1 2 "a"]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_single_element() {
-    let actual = nu!(r#"
-            mut a = ["list" "and"];
-            $a ++= "a single element";
-	    $a == ["list" "and" "a single element"]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_to_single_element() {
-    let actual = nu!(r#"
-            mut a = "string";
-            $a ++= ["and" "the" "list"];
-	    $a == ["string" "and" "the" "list"]
-        "#);
-
-    assert_eq!(actual.out, "true")
-}
-
-#[test]
-fn append_assign_single_to_single() {
-    let actual = nu!(r#"
-            mut a = 1;
-            $a ++= "and a single element";
-        "#);
-
-    assert!(actual.err.contains("nu::parser::unsupported_operation"));
-}

crates/nu-command/tests/commands/assignment/concat.rs

@@ -0,0 +1,76 @@
+use nu_test_support::nu;
+
+#[test]
+fn concat_assign_list_int() {
+    let actual = nu!(r#"
+        mut a = [1 2];
+        $a ++= [3 4];
+        $a == [1 2 3 4]
+    "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn concat_assign_list_string() {
+    let actual = nu!(r#"
+        mut a = [a b];
+        $a ++= [c d];
+        $a == [a b c d]
+    "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn concat_assign_any() {
+    let actual = nu!(r#"
+        mut a = [1 2 a];
+        $a ++= [b 3];
+        $a == [1 2 a b 3]
+    "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn concat_assign_both_empty() {
+    let actual = nu!(r#"
+        mut a = [];
+        $a ++= [];
+        $a == []
+    "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn concat_assign_string() {
+    let actual = nu!(r#"
+        mut a = 'hello';
+        $a ++= ' world';
+        $a == 'hello world'
+    "#);
+
+    assert_eq!(actual.out, "true")
+}
+
+#[test]
+fn concat_assign_type_mismatch() {
+    let actual = nu!(r#"
+        mut a = [];
+        $a ++= 'str'
+    "#);
+
+    assert!(actual.err.contains("nu::parser::unsupported_operation"));
+}
+
+#[test]
+fn concat_assign_runtime_type_mismatch() {
+    let actual = nu!(r#"
+        mut a = [];
+        $a ++= if true { 'str' }
+    "#);
+
+    assert!(actual.err.contains("nu::shell::type_mismatch"));
+}

crates/nu-command/tests/commands/assignment/mod.rs

@@ -1 +1 @@
-mod append_assign;
+mod concat;

crates/nu-command/tests/commands/math/mod.rs

@@ -483,7 +483,7 @@ fn compound_where_paren() {
 // TODO: these ++ tests are not really testing *math* functionality, maybe find another place for them
 
 #[test]
-fn adding_lists() {
+fn concat_lists() {
     let actual = nu!(pipeline(
         r#"
             [1 3] ++ [5 6] | to nuon
@@ -494,29 +494,7 @@ fn adding_lists() {
 }
 
 #[test]
-fn adding_list_and_value() {
-    let actual = nu!(pipeline(
-        r#"
-            [1 3] ++ 5 | to nuon
-        "#
-    ));
-
-    assert_eq!(actual.out, "[1, 3, 5]");
-}
-
-#[test]
-fn adding_value_and_list() {
-    let actual = nu!(pipeline(
-        r#"
-            1 ++ [3 5] | to nuon
-        "#
-    ));
-
-    assert_eq!(actual.out, "[1, 3, 5]");
-}
-
-#[test]
-fn adding_tables() {
+fn concat_tables() {
     let actual = nu!(pipeline(
         r#"
             [[a b]; [1 2]] ++ [[c d]; [10 11]] | to nuon
@@ -526,7 +504,7 @@ fn adding_tables() {
 }
 
 #[test]
-fn append_strings() {
+fn concat_strings() {
     let actual = nu!(pipeline(
         r#"
             "foo" ++ "bar"
@@ -536,7 +514,7 @@ fn append_strings() {
 }
 
 #[test]
-fn append_binary_values() {
+fn concat_binary_values() {
     let actual = nu!(pipeline(
         r#"
             0x[01 02] ++ 0x[03 04] | to nuon

crates/nu-command/tests/commands/url/join.rs

@@ -27,7 +27,7 @@ fn url_join_with_only_user() {
                     "password": "",
                     "host": "localhost",
                     "port": "",
-                } | url join 
+                } | url join
             "#
     ));
 
@@ -44,7 +44,7 @@ fn url_join_with_only_pwd() {
                     "password": "pwd",
                     "host": "localhost",
                     "port": "",
-                } | url join 
+                } | url join
             "#
     ));
 
@@ -61,7 +61,7 @@ fn url_join_with_user_and_pwd() {
                     "password": "pwd",
                     "host": "localhost",
                     "port": "",
-                } | url join 
+                } | url join
             "#
     ));
 
@@ -79,7 +79,7 @@ fn url_join_with_query() {
                     "host": "localhost",
                     "query": "par_1=aaa&par_2=bbb"
                     "port": "",
-                } | url join 
+                } | url join
             "#
     ));
 
@@ -411,12 +411,9 @@ fn url_join_with_params_invalid_table() {
                 "host": "localhost",
                 "params": (
                     [
-                        ["key", "value"];
-                        ["par_1", "aaa"],
-                        ["par_2", "bbb"],
-                        ["par_1", "ccc"],
-                        ["par_2", "ddd"],
-                    ] ++ ["not a record"]
+                        { key: foo, value: bar }
+                        "not a record"
+                    ]
                 ),
                 "port": "1234",
             } | url join

crates/nu-engine/src/compile/operator.rs

@@ -155,7 +155,7 @@ pub(crate) fn decompose_assignment(assignment: Assignment) -> Option<Operator> {
     match assignment {
         Assignment::Assign => None,
         Assignment::PlusAssign => Some(Operator::Math(Math::Plus)),
-        Assignment::AppendAssign => Some(Operator::Math(Math::Append)),
+        Assignment::ConcatAssign => Some(Operator::Math(Math::Concat)),
         Assignment::MinusAssign => Some(Operator::Math(Math::Minus)),
         Assignment::MultiplyAssign => Some(Operator::Math(Math::Multiply)),
         Assignment::DivideAssign => Some(Operator::Math(Math::Divide)),

crates/nu-engine/src/eval.rs

@@ -547,9 +547,9 @@ impl Eval for EvalRuntime {
                 let lhs = eval_expression::<D>(engine_state, stack, lhs)?;
                 lhs.div(op_span, &rhs, op_span)?
             }
-            Assignment::AppendAssign => {
+            Assignment::ConcatAssign => {
                 let lhs = eval_expression::<D>(engine_state, stack, lhs)?;
-                lhs.append(op_span, &rhs, op_span)?
+                lhs.concat(op_span, &rhs, op_span)?
             }
         };
 

crates/nu-engine/src/eval_ir.rs

@@ -956,7 +956,7 @@ fn binary_op(
         },
         Operator::Math(mat) => match mat {
             Math::Plus => lhs_val.add(op_span, &rhs_val, span)?,
-            Math::Append => lhs_val.append(op_span, &rhs_val, span)?,
+            Math::Concat => lhs_val.concat(op_span, &rhs_val, span)?,
             Math::Minus => lhs_val.sub(op_span, &rhs_val, span)?,
             Math::Multiply => lhs_val.mul(op_span, &rhs_val, span)?,
             Math::Divide => lhs_val.div(op_span, &rhs_val, span)?,

crates/nu-parser/src/parser.rs

@@ -4887,7 +4887,7 @@ pub fn parse_assignment_operator(working_set: &mut StateWorkingSet, span: Span)
     let operator = match contents {
         b"=" => Operator::Assignment(Assignment::Assign),
         b"+=" => Operator::Assignment(Assignment::PlusAssign),
-        b"++=" => Operator::Assignment(Assignment::AppendAssign),
+        b"++=" => Operator::Assignment(Assignment::ConcatAssign),
         b"-=" => Operator::Assignment(Assignment::MinusAssign),
         b"*=" => Operator::Assignment(Assignment::MultiplyAssign),
         b"/=" => Operator::Assignment(Assignment::DivideAssign),
@@ -5013,7 +5013,7 @@ pub fn parse_operator(working_set: &mut StateWorkingSet, span: Span) -> Expressi
         b"=~" | b"like" => Operator::Comparison(Comparison::RegexMatch),
         b"!~" | b"not-like" => Operator::Comparison(Comparison::NotRegexMatch),
         b"+" => Operator::Math(Math::Plus),
-        b"++" => Operator::Math(Math::Append),
+        b"++" => Operator::Math(Math::Concat),
         b"-" => Operator::Math(Math::Minus),
         b"*" => Operator::Math(Math::Multiply),
         b"/" => Operator::Math(Math::Divide),

crates/nu-parser/src/type_check.rs

@@ -132,7 +132,7 @@ pub fn math_result_type(
                     )
                 }
             },
-            Operator::Math(Math::Append) => check_append(working_set, lhs, rhs, op),
+            Operator::Math(Math::Concat) => check_concat(working_set, lhs, rhs, op),
             Operator::Math(Math::Minus) => match (&lhs.ty, &rhs.ty) {
                 (Type::Int, Type::Int) => (Type::Int, None),
                 (Type::Float, Type::Int) => (Type::Float, None),
@@ -935,8 +935,8 @@ pub fn math_result_type(
                     )
                 }
             },
-            Operator::Assignment(Assignment::AppendAssign) => {
-                check_append(working_set, lhs, rhs, op)
+            Operator::Assignment(Assignment::ConcatAssign) => {
+                check_concat(working_set, lhs, rhs, op)
             }
             Operator::Assignment(_) => match (&lhs.ty, &rhs.ty) {
                 (x, y) if x == y => (Type::Nothing, None),
@@ -1085,7 +1085,7 @@ pub fn check_block_input_output(working_set: &StateWorkingSet, block: &Block) ->
     output_errors
 }
 
-fn check_append(
+fn check_concat(
     working_set: &mut StateWorkingSet,
     lhs: &Expression,
     rhs: &Expression,
@@ -1099,23 +1099,17 @@ fn check_append(
                 (Type::List(Box::new(Type::Any)), None)
             }
         }
-        (Type::List(a), b) | (b, Type::List(a)) => {
-            if a == &Box::new(b.clone()) {
-                (Type::List(a.clone()), None)
-            } else {
-                (Type::List(Box::new(Type::Any)), None)
-            }
-        }
         (Type::Table(a), Type::Table(_)) => (Type::Table(a.clone()), None),
         (Type::String, Type::String) => (Type::String, None),
         (Type::Binary, Type::Binary) => (Type::Binary, None),
         (Type::Any, _) | (_, Type::Any) => (Type::Any, None),
-        (Type::Table(_) | Type::String | Type::Binary, _) => {
+        (Type::Table(_) | Type::List(_) | Type::String | Type::Binary, _)
+        | (_, Type::Table(_) | Type::List(_) | Type::String | Type::Binary) => {
             *op = Expression::garbage(working_set, op.span);
             (
                 Type::Any,
                 Some(ParseError::UnsupportedOperationRHS(
-                    "append".into(),
+                    "concatenation".into(),
                     op.span,
                     lhs.span,
                     lhs.ty.clone(),
@@ -1129,7 +1123,7 @@ fn check_append(
             (
                 Type::Any,
                 Some(ParseError::UnsupportedOperationLHS(
-                    "append".into(),
+                    "concatenation".into(),
                     op.span,
                     lhs.span,
                     lhs.ty.clone(),

crates/nu-plugin-engine/src/interface/tests.rs

@@ -1485,7 +1485,7 @@ fn prepare_plugin_call_custom_value_op() {
                     span,
                 },
                 CustomValueOp::Operation(
-                    Operator::Math(Math::Append).into_spanned(span),
+                    Operator::Math(Math::Concat).into_spanned(span),
                     cv_ok_val.clone(),
                 ),
             ),
@@ -1498,7 +1498,7 @@ fn prepare_plugin_call_custom_value_op() {
                     span,
                 },
                 CustomValueOp::Operation(
-                    Operator::Math(Math::Append).into_spanned(span),
+                    Operator::Math(Math::Concat).into_spanned(span),
                     cv_bad_val.clone(),
                 ),
             ),

crates/nu-protocol/src/ast/operator.rs

@@ -24,7 +24,7 @@ pub enum Comparison {
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum Math {
     Plus,
-    Append,
+    Concat,
     Minus,
     Multiply,
     Divide,
@@ -53,7 +53,7 @@ pub enum Bits {
 pub enum Assignment {
     Assign,
     PlusAssign,
-    AppendAssign,
+    ConcatAssign,
     MinusAssign,
     MultiplyAssign,
     DivideAssign,
@@ -90,7 +90,7 @@ impl Operator {
             | Self::Comparison(Comparison::NotEqual)
             | Self::Comparison(Comparison::In)
             | Self::Comparison(Comparison::NotIn)
-            | Self::Math(Math::Append) => 80,
+            | Self::Math(Math::Concat) => 80,
             Self::Bits(Bits::BitAnd) => 75,
             Self::Bits(Bits::BitXor) => 70,
             Self::Bits(Bits::BitOr) => 60,
@@ -107,7 +107,7 @@ impl Display for Operator {
         match self {
             Operator::Assignment(Assignment::Assign) => write!(f, "="),
             Operator::Assignment(Assignment::PlusAssign) => write!(f, "+="),
-            Operator::Assignment(Assignment::AppendAssign) => write!(f, "++="),
+            Operator::Assignment(Assignment::ConcatAssign) => write!(f, "++="),
             Operator::Assignment(Assignment::MinusAssign) => write!(f, "-="),
             Operator::Assignment(Assignment::MultiplyAssign) => write!(f, "*="),
             Operator::Assignment(Assignment::DivideAssign) => write!(f, "/="),
@@ -124,7 +124,7 @@ impl Display for Operator {
             Operator::Comparison(Comparison::In) => write!(f, "in"),
             Operator::Comparison(Comparison::NotIn) => write!(f, "not-in"),
             Operator::Math(Math::Plus) => write!(f, "+"),
-            Operator::Math(Math::Append) => write!(f, "++"),
+            Operator::Math(Math::Concat) => write!(f, "++"),
             Operator::Math(Math::Minus) => write!(f, "-"),
             Operator::Math(Math::Multiply) => write!(f, "*"),
             Operator::Math(Math::Divide) => write!(f, "/"),

crates/nu-protocol/src/eval_base.rs

@@ -238,7 +238,7 @@ pub trait Eval {
                             Math::Minus => lhs.sub(op_span, &rhs, expr_span),
                             Math::Multiply => lhs.mul(op_span, &rhs, expr_span),
                             Math::Divide => lhs.div(op_span, &rhs, expr_span),
-                            Math::Append => lhs.append(op_span, &rhs, expr_span),
+                            Math::Concat => lhs.concat(op_span, &rhs, expr_span),
                             Math::Modulo => lhs.modulo(op_span, &rhs, expr_span),
                             Math::FloorDivision => lhs.floor_div(op_span, &rhs, expr_span),
                             Math::Pow => lhs.pow(op_span, &rhs, expr_span),

crates/nu-protocol/src/value/mod.rs

@@ -2503,34 +2503,20 @@ impl Value {
         }
     }
 
-    pub fn append(&self, op: Span, rhs: &Value, span: Span) -> Result<Value, ShellError> {
+    pub fn concat(&self, op: Span, rhs: &Value, span: Span) -> Result<Value, ShellError> {
         match (self, rhs) {
             (Value::List { vals: lhs, .. }, Value::List { vals: rhs, .. }) => {
-                let mut lhs = lhs.clone();
-                let mut rhs = rhs.clone();
-                lhs.append(&mut rhs);
-                Ok(Value::list(lhs, span))
-            }
-            (Value::List { vals: lhs, .. }, val) => {
-                let mut lhs = lhs.clone();
-                lhs.push(val.clone());
-                Ok(Value::list(lhs, span))
-            }
-            (val, Value::List { vals: rhs, .. }) => {
-                let mut rhs = rhs.clone();
-                rhs.insert(0, val.clone());
-                Ok(Value::list(rhs, span))
+                Ok(Value::list([lhs.as_slice(), rhs.as_slice()].concat(), span))
             }
             (Value::String { val: lhs, .. }, Value::String { val: rhs, .. }) => {
-                Ok(Value::string(lhs.to_string() + rhs, span))
-            }
-            (Value::Binary { val: lhs, .. }, Value::Binary { val: rhs, .. }) => {
-                let mut val = lhs.clone();
-                val.extend(rhs);
-                Ok(Value::binary(val, span))
+                Ok(Value::string([lhs.as_str(), rhs.as_str()].join(""), span))
             }
+            (Value::Binary { val: lhs, .. }, Value::Binary { val: rhs, .. }) => Ok(Value::binary(
+                [lhs.as_slice(), rhs.as_slice()].concat(),
+                span,
+            )),
             (Value::Custom { val: lhs, .. }, rhs) => {
-                lhs.operation(self.span(), Operator::Math(Math::Append), op, rhs)
+                lhs.operation(self.span(), Operator::Math(Math::Concat), op, rhs)
             }
             _ => Err(ShellError::OperatorMismatch {
                 op_span: op,

crates/nu-std/std/help/mod.nu

@@ -37,7 +37,7 @@ def get-all-operators [] { return [
 
     [Assignment, =, Assign, "Assigns a value to a variable.", 10]
     [Assignment, +=, PlusAssign, "Adds a value to a variable.", 10]
-    [Assignment, ++=, AppendAssign, "Appends a list or a value to a variable.", 10]
+    [Assignment, ++=, ConcatAssign, "Concatenate two lists, two strings, or two binary values.", 10]
     [Assignment, -=, MinusAssign, "Subtracts a value from a variable.", 10]
     [Assignment, *=, MultiplyAssign, "Multiplies a variable by a value.", 10]
     [Assignment, /=, DivideAssign, "Divides a variable by a value.", 10]
@@ -55,7 +55,7 @@ def get-all-operators [] { return [
     [Comparison, ends-with, EndsWith, "Checks if a string ends with another.", 80]
     [Comparison, not, UnaryNot, "Negates a value or expression.", 0]
     [Math, +, Plus, "Adds two values.", 90]
-    [Math, ++, Append, "Appends two lists or a list and a value.", 80]
+    [Math, ++, Concat, "Concatenate two lists, two strings, or two binary values.", 80]
     [Math, -, Minus, "Subtracts two values.", 90]
     [Math, *, Multiply, "Multiplies two values.", 95]
     [Math, /, Divide, "Divides two values.", 95]

crates/nu_plugin_custom_values/src/cool_custom_value.rs

@@ -112,7 +112,7 @@ impl CustomValue for CoolCustomValue {
     ) -> Result<Value, ShellError> {
         match operator {
             // Append the string inside `cool`
-            ast::Operator::Math(ast::Math::Append) => {
+            ast::Operator::Math(ast::Math::Concat) => {
                 if let Some(right) = right
                     .as_custom_value()
                     .ok()

tests/const_/mod.rs

@@ -154,7 +154,6 @@ fn const_unary_operator(#[case] inp: &[&str], #[case] expect: &str) {
 #[case(&[r#"const x = "a" ++ "b" "#, "$x"], "ab")]
 #[case(&[r#"const x = [1,2] ++ [3]"#, "$x | describe"], "list<int>")]
 #[case(&[r#"const x = 0x[1,2] ++ 0x[3]"#, "$x | describe"], "binary")]
-#[case(&[r#"const x = 0x[1,2] ++ [3]"#, "$x | describe"], "list<any>")]
 #[case(&["const x = 1 < 2", "$x"], "true")]
 #[case(&["const x = (3 * 200) > (2 * 100)", "$x"], "true")]
 #[case(&["const x = (3 * 200) < (2 * 100)", "$x"], "false")]

tests/eval/mod.rs

@@ -159,7 +159,7 @@ fn record_spread() {
 #[test]
 fn binary_op_example() {
     test_eval(
-        "(([1 2] ++ [3 4]) == [1 2 3 4]) and (([1 2 3] ++ 4) == ([1] ++ [2 3 4]))",
+        "(([1 2] ++ [3 4]) == [1 2 3 4]) and (([1] ++ [2 3 4]) == [1 2 3 4])",
         Eq("true"),
     )
 }