lang: interfaces: Add args to struct helper

We should consider if it's possible to avoid all of this transforming
entirely, but at least for now, do it all in one place by having an
available helper.

lang/interfaces/func.go

@@ -39,6 +39,7 @@ import (
 	"github.com/purpleidea/mgmt/engine/local"
 	"github.com/purpleidea/mgmt/lang/types"
 	"github.com/purpleidea/mgmt/pgraph"
+	"github.com/purpleidea/mgmt/util"
 )
 
 // FuncSig is the simple signature that is used throughout our implementations.
@@ -421,7 +422,9 @@ type Txn interface {
 }
 
 // StructToCallableArgs transforms the single value, graph representation of the
-// callable values into a linear, standard args list.
+// callable values into a linear, standard args list. The reverse of this
+// function is CallableArgsToStruct, with the caveat that this call looses any
+// argument names that there might have been.
 func StructToCallableArgs(st types.Value) ([]types.Value, error) {
 	args := []types.Value{}
 	if st == nil { // for functions that take no args
@@ -449,3 +452,36 @@ func StructToCallableArgs(st types.Value) ([]types.Value, error) {
 	}
 	return args, nil
 }
+
+// CallableArgsToStruct transforms the list of args, call representation of the
+// graph represenation into a standard single struct value. Note, this is not
+// the exact reverse operation of StructToCallableArgs because the arg names are
+// lost when performing that operation.
+func CallableArgsToStruct(args []types.Value) (types.Value, error) {
+	m := make(map[string]*types.Type, len(args))
+	ord := []string{}
+	v := make(map[string]types.Value, len(args))
+	for i, arg := range args {
+		s := util.NumToAlpha(i) // invent an arg name
+		m[s] = arg.Type()
+		ord = append(ord, s)
+		v[s] = arg
+	}
+
+	si := &types.Type{
+		// input to functions are structs
+		Kind: types.KindStruct,
+		Map:  m,
+		Ord:  ord,
+	}
+	st := types.NewStruct(si)
+	st.V = v // pass it in directly to avoid the below second iteration...
+	//for i, value := range args {
+	//	arg := ord[i]
+	//	if err := st.Set(arg, value); err != nil {
+	//		return nil, err
+	//	}
+	//}
+
+	return st, nil
+}