add length function that can work with more types

hcl2template/function/length.go

@@ -0,0 +1,53 @@
+package function
+
+import (
+	"errors"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+	"github.com/zclconf/go-cty/cty/function/stdlib"
+)
+
+var LengthFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name:             "value",
+			Type:             cty.DynamicPseudoType,
+			AllowDynamicType: true,
+			AllowUnknown:     true,
+		},
+	},
+	Type: func(args []cty.Value) (cty.Type, error) {
+		collTy := args[0].Type()
+		switch {
+		case collTy == cty.String || collTy.IsTupleType() || collTy.IsObjectType() || collTy.IsListType() || collTy.IsMapType() || collTy.IsSetType() || collTy == cty.DynamicPseudoType:
+			return cty.Number, nil
+		default:
+			return cty.Number, errors.New("argument must be a string, a collection type, or a structural type")
+		}
+	},
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		coll := args[0]
+		collTy := args[0].Type()
+		switch {
+		case collTy == cty.DynamicPseudoType:
+			return cty.UnknownVal(cty.Number), nil
+		case collTy.IsTupleType():
+			l := len(collTy.TupleElementTypes())
+			return cty.NumberIntVal(int64(l)), nil
+		case collTy.IsObjectType():
+			l := len(collTy.AttributeTypes())
+			return cty.NumberIntVal(int64(l)), nil
+		case collTy == cty.String:
+			// We'll delegate to the cty stdlib strlen function here, because
+			// it deals with all of the complexities of tokenizing unicode
+			// grapheme clusters.
+			return stdlib.Strlen(coll)
+		case collTy.IsListType() || collTy.IsSetType() || collTy.IsMapType():
+			return coll.Length(), nil
+		default:
+			// Should never happen, because of the checks in our Type func above
+			return cty.UnknownVal(cty.Number), errors.New("impossible value type for length(...)")
+		}
+	},
+})

hcl2template/functions.go

@@ -68,7 +68,7 @@ func Functions(basedir string) map[string]function.Function {
 		"jsondecode":         stdlib.JSONDecodeFunc,
 		"jsonencode":         stdlib.JSONEncodeFunc,
 		"keys":               stdlib.KeysFunc,
-		"length":             stdlib.LengthFunc,
+		"length":             pkrfunction.LengthFunc,
 		"log":                stdlib.LogFunc,
 		"lookup":             stdlib.LookupFunc,
 		"lower":              stdlib.LowerFunc,