args: add simple value accessor

pkg/args/args.go

@@ -4,6 +4,7 @@
 package args
 
 import (
+	"errors"
 	"fmt"
 	"iter"
 	"sort"
@@ -18,6 +19,8 @@ import (
 	"github.com/ucan-wg/go-ucan/pkg/policy/literal"
 )
 
+var ErrNotFound = errors.New("key not found in meta")
+
 // Args are the Command's arguments when an invocation Token is processed by the executor.
 // This also serves as a way to construct the underlying IPLD data with minimum allocations
 // and transformations, while hiding the IPLD complexity from the caller.
@@ -36,6 +39,16 @@ func New() *Args {
 	}
 }
 
+// GetNode retrieves a value as a raw IPLD node.
+// Returns ErrNotFound if the given key is missing.
+func (a *Args) GetNode(key string) (ipld.Node, error) {
+	v, ok := a.Values[key]
+	if !ok {
+		return nil, ErrNotFound
+	}
+	return v, nil
+}
+
 // Add inserts a key/value pair in the Args set.
 //
 // Accepted types for val are any CBOR compatible type, or directly IPLD values.

pkg/args/readonly.go

@@ -10,6 +10,10 @@ type ReadOnly struct {
 	args *Args
 }
 
+func (r ReadOnly) GetNode(key string) (ipld.Node, error) {
+	return r.args.GetNode(key)
+}
+
 func (r ReadOnly) Iter() iter.Seq2[string, ipld.Node] {
 	return r.args.Iter()
 }

pkg/meta/meta.go

@@ -121,7 +121,6 @@ func (m *Meta) GetEncryptedBytes(key string, encryptionKey []byte) ([]byte, erro
 
 // GetNode retrieves a value as a raw IPLD node.
 // Returns ErrNotFound if the given key is missing.
-// Returns datamodel.ErrWrongKind if the value has the wrong type.
 func (m *Meta) GetNode(key string) (ipld.Node, error) {
 	v, ok := m.Values[key]
 	if !ok {