Added pipe and length docs, fix pipe precedence

pkg/yqlib/operator_length_test.go

@@ -0,0 +1,42 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var lengthOperatorScenarios = []expressionScenario{
+	{
+		description:    "String length",
+		subdescription: "returns length of string",
+		document:       `{a: cat}`,
+		expression:     `.a | length`,
+		expected: []string{
+			"D0, P[a], (!!int)::3\n",
+		},
+	},
+	{
+		description:    "Map length",
+		subdescription: "returns number of entries",
+		document:       `{a: cat, c: dog}`,
+		expression:     `length`,
+		expected: []string{
+			"D0, P[], (!!int)::2\n",
+		},
+	},
+	{
+		description:    "Array length",
+		subdescription: "returns number of elements",
+		document:       `[2,4,6,8]`,
+		expression:     `length`,
+		expected: []string{
+			"D0, P[], (!!int)::4\n",
+		},
+	},
+}
+
+func TestLengthOperatorScenarios(t *testing.T) {
+	for _, tt := range lengthOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentScenarios(t, "Length", lengthOperatorScenarios)
+}