What obvious way of using Vavr's fold() am I missing here?

StringBuilderWithFolding.java

private String linesOf(List<String> lines) {
    StringBuilder stringBuilder = new StringBuilder();
    lines.map(line -> stringBuilder.append(line).append(System.lineSeparator()));
    return stringBuilder.toString();
}

I could fold using Strings, but I prefer to fold using StringBuilder. Which obvious idea am I missing here?

I guess if I map each String to a function of StringBuilder to StringBuilder, then I can fold with new StringBuilder().

What is fold when the accumulator is of a different type than the items in the sequence?

Hah, that's interesting!

In most other FP contexts the type of the accumulator of fold does not have to be the same as the elements of a foldable data structure at all.
e.g. F#: List.fold : ('State -> 'T -> 'State) -> 'State -> 'T list -> 'State

https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/list.fold%5B't%2C'state%5D-function-%5Bfsharp%5D

I have no JRE/vavr on my pc, but looking at the docs a foldLeft accepts an accumulator of a different type, give that a try!
https://static.javadoc.io/io.vavr/vavr/0.9.0/io/vavr/collection/Foldable.html#foldLeft-U-java.util.function.BiFunction-

Aha! So that differentiates "fold left" from "fold". I think I understand. Thank you.

@jovaneyck Much better, thanks!

package ca.jbrains.pos.test;

import io.vavr.collection.LinearSeq;

public class LineBuilder {

    private final StringBuilder stringBuilder = new StringBuilder();

    public static String linesOf(LinearSeq<String> lines) {
        return lines.foldLeft(new LineBuilder(), LineBuilder::appendAsALine).toString();
    }

    public LineBuilder appendAsALine(String line) {
        this.stringBuilder.append(line).append(System.lineSeparator());
        return this;
    }

    @Override
    public String toString() {
        return stringBuilder.toString();
    }
}