Golang Array of functions in function argument

Question

Playing around with golang parquet package and came across the function s3.NewS3FileWriter that accepts the following arguement []func(*s3manager.Uploader)

func NewS3FileWriter(
    ctx context.Context,
    bucket string,
    key string,
    uploaderOptions []func(*s3manager.Uploader),
    cfgs ...*aws.Config,
) (source.ParquetFile, error) {
    if activeS3Session == nil {
        sessLock.Lock()
        if activeS3Session == nil {
            activeS3Session = session.Must(session.NewSession())
        }
        sessLock.Unlock()
    }

    file := &S3File{
        ctx:             ctx,
        client:          s3.New(activeS3Session, cfgs...),
        writeDone:       make(chan error),
        uploaderOptions: uploaderOptions,
        BucketName:      bucket,
        Key:             key,
    }

    return file.Create(key)
}

What does []func(*s3manager.Uploader) mean? And how do I pass a modified version of s3Manager.Uploader? If I wanted to change the defaults for example.

Read it from left to right: Slice of function with argument pointer to s3manager.Uploader. — Volker
– Volker, Commented Jul 5, 2019 at 9:32

novalagung · Accepted Answer · 2019-07-05 09:45:10Z

1

The uploaderOptions argument of NewS3FileWriter need to be filled with slice of functions during call. And each of the functions must have an argument with type of *s3manager.Uploader.

Example:

uploaderOptions := make([]func(*s3manager.Uploader), 0)

uploader1 := func (param *s3manager.Uploader) {
    fmt.Println("uploader 1", param)
}
uploaderOptions = append(uploaderOptions, uploader1)

uploader2 := func (param *s3manager.Uploader) {
    fmt.Println("uploader 2", param)
}
uploaderOptions = append(uploaderOptions, uploader2)

// ...

NewS3FileWriter(ctx, bucket, key, uploaderOptions)

answered Jul 5, 2019 at 9:45

novalagung

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1 Comment

amwill04 Over a year ago

Thanks - appreciate that

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