Add length function

examples/input.json

@@ -2,5 +2,6 @@
     "i": ${i},
     "j": "${join(", ", h)}",
     "other": "${upper("foobar")}",
-    "foo": "${join(" | ", list("foo", "bar"))}"
+    "foo": "${join(" | ", list("foo", "bar"))}",
+    "len": ${length(h)}
 }

interpolation/funcs.go

@@ -2,6 +2,7 @@ package interpolation
 
 import (
 	"errors"
+	"fmt"
 	"os"
 
 	"github.com/hashicorp/hil/ast"
@@ -23,6 +24,7 @@ var CoreFunctions = map[string]ast.Function{
 	"min":      interpolationFuncMin(),
 	"contains": interpolationFuncContains(),
 	"split":    interpolationFuncSplit(),
+	"length":   interpolationFuncLength(),
 }
 
 // interpolationFuncEnv will extract a variable out of the env
@@ -40,3 +42,26 @@ func interpolationFuncEnv() ast.Function {
 		},
 	}
 }
+
+// interpolationFuncLength will determine the length of the input
+// if the input is a list, it will count the length of the items
+// if the input is a map, it will count the keys
+// if the input is a string, it will count the characters
+func interpolationFuncLength() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeAny},
+		ReturnType: ast.TypeInt,
+		Callback: func(inputs []interface{}) (interface{}, error) {
+			switch input := inputs[0].(type) {
+			case string:
+				return len(input), nil
+			case []ast.Variable:
+				return len(input), nil
+			case map[string]ast.Variable:
+				return len(input), nil
+			default:
+				return nil, fmt.Errorf("Must provide either a list, map or string, got %T", input)
+			}
+		},
+	}
+}

interpolation/funcs_test.go

@@ -132,3 +132,40 @@ func TestInterpolationFuncEnv(t *testing.T) {
 		})
 	}
 }
+
+func TestInterpolationFuncLength(t *testing.T) {
+	testCases := []functionTestCase{
+		{
+			description: "Length of string",
+			text:        `${length("FOO")}`,
+			expectation: "3",
+		},
+		{
+			description: "Length of list",
+			text:        `${length("${list("foo", "bar")}")}`,
+			expectation: "2",
+		},
+		{
+			description: "Length of map",
+			text:        `${length("${map("foo", "bar")}")}`,
+			expectation: "1",
+		},
+		{
+			description: "Invalid type errors",
+			text:        `${length(4)}`,
+			evalError:   true,
+		},
+	}
+
+	lengthTestFunc := testInterpolationFunc(keyFuncs{
+		"length": interpolationFuncLength,
+		"list":   interpolationFuncList,
+		"map":    interpolationFuncMap,
+	})
+
+	for _, tc := range testCases {
+		t.Run(tc.description, func(t *testing.T) {
+			lengthTestFunc(t, tc)
+		})
+	}
+}