使用Go包中的私有“小写”功能

I'm trying so use the following function in my main package:

html.go (from blackfriday):

func doubleSpace(out *bytes.Buffer) {
    if out.Len() > 0 {
        out.WriteByte('
')
    }
}

main.go:

func (options *renderer) Paragraph(out *bytes.Buffer, text func() bool) {
    marker := out.Len()
    doubleSpace(out)

    out.WriteString("<p class='custom'>")
    if !text() {
        out.Truncate(marker)
        return
    }
    out.WriteString("</p>
")
}

I tried doing blackfriday.doubleSpace and blackfriday.DoubleSpace but in the three cases I get undefined.

What's the correct way to do it?

You can't. That is a deliberate decision by the authors of blackfriday. Your only option is to define your own function which does the same thing. It's just like accessing a private member in a language like Java.

See the spec here: https://golang.org/ref/spec#Exported_identifiers