pkgs.lux/packages/frontmatter/lib.lux

// frontmatter - YAML-like frontmatter parser for Lux
//
// Parses documents with --- delimited frontmatter:
//   ---
//   title: My Post
//   date: 2025-01-29
//   tags: web blog
//   ---
//   Body content here...

// A single key-value pair from the frontmatter
type Entry =
    | Entry(String, String)

// Parse result: list of key-value entries and the body text
type Document =
    | Document(List<Entry>, String)

// Internal parser state
type PState =
    | PState(Bool, Bool, List<Entry>, String)

fn psInFront(s: PState): Bool =
    match s {
    PState(f, _, _, _) => f,
}

fn psPastFront(s: PState): Bool =
    match s {
    PState(_, p, _, _) => p,
}

fn psEntries(s: PState): List<Entry> =
    match s {
    PState(_, _, e, _) => e,
}

fn psBody(s: PState): String =
    match s {
    PState(_, _, _, b) => b,
}

// Strip surrounding quotes from a value if present
fn stripQuotes(s: String): String =
    if String.length(s) >= 2 then
        if String.startsWith(s, "\"") then
            if String.endsWith(s, "\"") then
                String.substring(s, 1, String.length(s) - 1)
            else s
        else if String.startsWith(s, "'") then
            if String.endsWith(s, "'") then
                String.substring(s, 1, String.length(s) - 1)
            else s
        else s
    else s

// Parse a single line within the frontmatter block
fn parseFrontLine(entries: List<Entry>, line: String): List<Entry> =
    match String.indexOf(line, ": ") {
    Some(idx) => {
        let key = String.trim(String.substring(line, 0, idx))
        let rawVal = String.trim(String.substring(line, idx + 2, String.length(line)))
        let val = stripQuotes(rawVal)
        List.concat(entries, [Entry(key, val)])
    },
    None => {
        // Handle "key:" with no value (treat as empty string)
        let trimmed = String.trim(line)
        if String.endsWith(trimmed, ":") then {
            let key = String.substring(trimmed, 0, String.length(trimmed) - 1)
            List.concat(entries, [Entry(key, "")])
        } else
            entries
    },
}

// Fold function that processes one line at a time
fn foldLine(acc: PState, line: String): PState = {
    let inFront = psInFront(acc)
    let pastFront = psPastFront(acc)
    let entries = psEntries(acc)
    let body = psBody(acc)
    if pastFront then
        PState(inFront, pastFront, entries, body + line + "\n")
    else if String.trim(line) == "---" then
        if inFront then
            PState(false, true, entries, body)
        else
            PState(true, false, entries, body)
    else if inFront then
        PState(inFront, pastFront, parseFrontLine(entries, line), body)
    else
        acc
}

// Parse a document string into frontmatter entries and body content.
//
// Returns a Document with an empty entry list if no frontmatter is found.
pub fn parse(content: String): Document = {
    let lines = String.lines(content)
    let init = PState(false, false, [], "")
    let result = List.fold(lines, init, foldLine)
    Document(psEntries(result), psBody(result))
}

// Get the list of key-value entries from a Document
pub fn entries(doc: Document): List<Entry> =
    match doc {
    Document(e, _) => e,
}

// Get the body text from a Document
pub fn body(doc: Document): String =
    match doc {
    Document(_, b) => b,
}

// Look up a value by key. Returns the first matching entry.
pub fn get(doc: Document, key: String): Option<String> = {
    let es = entries(doc)
    getFromEntries(es, key)
}

fn getFromEntries(es: List<Entry>, key: String): Option<String> =
    match List.head(es) {
    Some(entry) => match entry {
        Entry(k, v) => if k == key then Some(v)
            else match List.tail(es) {
                Some(rest) => getFromEntries(rest, key),
                None => None,
            },
    },
    None => None,
}

// Get a value by key, returning a default if not found
pub fn getOrDefault(doc: Document, key: String, default: String): String =
    match get(doc, key) {
    Some(v) => v,
    None => default,
}

// Get the entry key
pub fn entryKey(e: Entry): String =
    match e {
    Entry(k, _) => k,
}

// Get the entry value
pub fn entryValue(e: Entry): String =
    match e {
    Entry(_, v) => v,
}

// Check if a document has frontmatter (at least one entry)
pub fn hasFrontmatter(doc: Document): Bool =
    List.length(entries(doc)) > 0

// Split a tags string into a list of tags (space-separated)
pub fn parseTags(tagsStr: String): List<String> =
    if tagsStr == "" then []
    else String.split(tagsStr, " ")

// Convenience: parse and get common blog fields
pub fn title(doc: Document): String = getOrDefault(doc, "title", "")
pub fn date(doc: Document): String = getOrDefault(doc, "date", "")
pub fn description(doc: Document): String = getOrDefault(doc, "description", "")
pub fn tags(doc: Document): List<String> = parseTags(getOrDefault(doc, "tags", ""))