Ever since 1.18 came out I've been struggling to find an idiomatic way to
implement a certain kind of adapter. I'm starting to suspect I'm Holding It
Wrong (TM) so here I am.
In short, I have an adapter that is effectively a collection of static
methods and it doesn't look right.
Imagine you have a generic key-value database (transaction and life-cycle
methods omitted for brevity):
type SimpleDatabase interface {
Get(t ViewTransaction, key string) ([]byte, bool, error)
Put(t UpdateTransaction, key string, value []byte) error
}
You then want to build something more specific on top using generics to
avoid repetition of tricky code:
type ApplicationDatabase struct {
impl *SimpleDatabase
}
func (db *ApplicationDatabase) PutProjectRecord(t UpdateTransaction, r
*ProjectRecord) error {
return put[ProjectRecord](db.impl, projectRecordFactory, t, r)
}
type recordFactory[V any] interface {
dbKey(value V) string
encodeValue(value V) []byte
decodeValue(data []byte) (V, error)
}
func put[V any](db *SimpleDatabase, f *recordFactory[V], t
UpdateTransaction, value V) error {
key := f.dbKey(value)
valueBytes := f.encodeValue(value)
if err := db.put(t, key, valueBytes); err != nil
return fmt.Errorf("db error: %w", err)
}
return nil
}
This is nice and clean -- adding a new record type is just a matter of
adding a new recordFactory implementation. Adding more complexity to the
put/get (recovery, etc.) only has to happen once. The part that bugs me,
though, is that the implementation has no state and really looks like it
shouldn't be instantiated:
type projectRecordFactory struct{}
func (f *projectRecordFactory) dbKey() string { return .... }
func (f *projectRecordFactory) encodeValue(value V) []byte {
json.Marshal(...) }
func (f *projectRecordFactory) decodeValue(data []byte) (V, error) {
json.Unmarshal(...) }
and, frankly, if I didn't instantiate it, it would feel like either Java or
C++ metaprogramming.
I'm open to a completely different structure but I am really hoping to add
new record types by adding a new package or type.
Thanks!
-- Salvatore
smile.
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