I consider this get correct.

Possible alternative API

const result = buildFill(($) => {
  $.append("a");
  $.append("b");
  $.appendLine(line);
  $.append("c");
})

instead of

const builder = fillBuilder();
builder.append("a");
builder.append("b");
builder.appendLine(line);
builder.append("c");
const result = builder.build();

It might be better to encourage using fillBuilder() instead of fill() since this should be safer.
If you agree, let's discuss on another discussion or issue.

some functions went to util.

I can't find input that cover this path. I left it anyway.
https://app.codecov.io/gh/prettier/prettier/blob/main/src%2Flanguage-markdown%2Fprinter-markdown.js#L663

Will something like this work?

  const parts = []
  
  // Check last part / parts.index is ODD-EVEN
  switch (getDocType(doc)) {
      case DOC_TYPE_FILL: {
        // concat or turn the last element to array and push the first
        // push the rest
        break;
      }
      default:
        builder.append(doc);
        break;
    }

return fill(parts)

@fisker
I'm sorry but I don't understand what does it mean / aim at.
Do you mean

• it can be faster?
• it can be more readable?
• prefer cleaner doc? (e.g. prefer fill("a", line, "b") to fill(["a"], line, ["b"]))
• recursion should be avoided?
• fillBuilder looks over-engineering?

I'm not sure whether I understand the pseudo code or not but possibly following case can be a counter-example.
input: ["a", fill(["b", line, "c", line]), "b"]
expected output: fill(join(line, [["a", "b"], ["c"], ["b"]]))

It's simply, so we don't need fillBuilder. Can you find other places that we want a "flat" fill?

@fisker
Does 866b257 meet your request?

I'm not sure about this part, do we need go deeper? head and docs in rest can also be DOC_TYPE_ARRAY or DOC_TYPE_FILL.

I'm going to run some test first, prettier/prettier-regression-testing#435 (comment)

I'm not very sure it. But probably fill() isn't nested in paragraph. If we want to go deeper, we should take care that we should go deeper only even (odd for doc.parts) index element.

Searching "fill" in src/language-markdown, I confirmed fill() can't be nested in markdown. There are three use of fill():

• printSentence() in print-sentence.js: sentence only contains word and whitespace that turns into simple docs like string or line-like. It won't have fill() as children.
• if (shouldRemainTheSameContent(path)) { block in printer-markdown.js: same as printSentence().
• flattenFill() in print-paragraph.js: paragraph can't be nested and the other children neither yield nested as described above.

So I think we don't need go deeper for elements of fill() as of now. But feel free to ask me to add recursive flattening for fill if you still worry about it.