interface implementing subset not compatible

[Miha Vrhovnik]

I'm trying to put external library under the interface but it seems that this is not possible the way I thought it would be...

My interfaces are as following...

type Beginner interface {
    Begin(ctx context.Context) (Tx, error)
}

type Querier interface {
    Exec(ctx context.Context, sql string, arguments ...interface{}) (commandTag pgconn.CommandTag, err error)
    Query(ctx context.Context, sql string, args ...interface{}) (pgx.Rows, error)
    QueryRow(ctx context.Context, sql string, args ...interface{}) pgx.Row
}

type Tx interface {
    Commit(ctx context.Context) error
    Rollback(ctx context.Context) error

    Querier
}

Now I have a function....

func NewFoo (b Beginer) {
}

Into that function I want to pass a struct instance with the method that has a following signature. (This function is not under my control).

Begin(ctx context.Context) (pgx.Tx, error)

Now the pgx.Tx has a following signature

type Tx interface {
    Begin(ctx context.Context) (Tx, error)
    Commit(ctx context.Context) error

    Rollback(ctx context.Context) error

    CopyFrom(ctx context.Context, tableName Identifier, columnNames []string, rowSrc CopyFromSource) (int64, error)
    SendBatch(ctx context.Context, b *Batch) BatchResults
    LargeObjects() LargeObjects

    Prepare(ctx context.Context, name, sql string) (*pgconn.StatementDescription, error)

    Exec(ctx context.Context, sql string, arguments ...interface{}) (commandTag pgconn.CommandTag, err error)
    Query(ctx context.Context, sql string, args ...interface{}) (Rows, error)
    QueryRow(ctx context.Context, sql string, args ...interface{}) Row

    // Conn returns the underlying *Conn that on which this transaction is executing.
    Conn() *Conn
}

The go is giving me the following error.

cannot use me.DBpgx() (type *pgxpool.Pool) as type db.Beginner in argument to service.NewFoo:
    *pgxpool.Pool does not implement db.Beginner (wrong type for Begin method)
        have Begin(context.Context) (pgx.Tx, error)
        want Begin(context.Context) (db.Tx, error)
       
how come that this is forbidden as my Tx interface is clearly a subset of pgx.Tx

BR,

Miha

[Axel Wagner]

Hi,

this is answered (to a degree) in the FAQ: https://golang.org/doc/faq#covariant_types

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