Add command "reduce" (#2292)

* initial

* fold working

* tests and cleanup

* change command to reduce, with fold flag

* move complex example to tests

* add --numbered flag

crates/nu-cli/tests/commands/mod.rs

@@ -34,6 +34,7 @@ mod parse;
 mod prepend;
 mod random;
 mod range;
+mod reduce;
 mod rename;
 mod reverse;
 mod rm;

crates/nu-cli/tests/commands/reduce.rs

@@ -0,0 +1,89 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn reduce_table_column() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "[{month:2,total:30}, {month:3,total:10}, {month:4,total:3}, {month:5,total:60}]"
+        | from json
+        | get total
+        | reduce -f 20 { = $it + $( math eval `{{$acc}}^1.05` )}
+        | str from -d 1
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "180.6");
+
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "[{month:2,total:30}, {month:3,total:10}, {month:4,total:3}, {month:5,total:60}]"
+        | from json
+        | reduce -f 20 { = $it.total + $( math eval `{{$acc}}^1.05` )}
+        | str from -d 1
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "180.6");
+}
+
+#[test]
+fn reduce_rows_example() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo a,b 1,2 3,4
+        | split column ,
+        | headers
+        | reduce -f 1.6 { = $acc * $(echo $it.a | str to-int) + $(echo $it.b | str to-int) }
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "14.8");
+}
+
+#[test]
+fn reduce_numbered_example() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo one longest three bar
+        | reduce -n { if $(echo $it.item | str length) > $(echo $acc.item | str length) {echo $it} {echo $acc}}
+        | get index
+        | echo $it
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "1");
+}
+
+#[test]
+fn error_reduce_fold_type_mismatch() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo a b c | reduce -f 0 { = $acc + $it }
+        "#
+        )
+    );
+
+    assert!(actual.err.contains("Coercion"));
+}
+
+#[test]
+fn error_reduce_empty() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        reduce { = $acc + $it }
+        "#
+        )
+    );
+
+    assert!(actual.err.contains("empty input"));
+}