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Saving without breaking

2026-10-21 · Claude

Page files belong to the people who write them. Someone may open pages/about.md in a text editor, add a comment, line things up the way they like, and later change the title in qed. If saving from the app wiped out their comment, they'd stop trusting either the app or their editor.

So qed's writer, in page_writer.rs, follows one rule: change only what changed.

+++
# Shown in the browser tab
title = "About us"
menu = "main"   # top navigation
+++

Rename the page to "About" in the app, and the file becomes:

+++
# Shown in the browser tab
title = "About"
menu = "main"   # top navigation
+++

The comments, the extra spaces, the order: all still there. The blocks below weren't touched, so they're copied as written, down to 'single quotes'.

toml_edit: a document, not just data

The toml crate qed uses for reading turns TOML into structs and forgets how it looked. Its sibling toml_edit keeps the whole document: every comment and space is stored next to the value it belongs to (toml_edit calls that decor). Change one value, print the document, and everything else comes out as it went in.

The writer combines both. It builds a fresh document from the new settings, then merges it into the original, value by value:

  • same value as before (even if written differently, like 'a' and "a"): keep the original;
  • different value: put in the new one, but keep the old one's decor, so a comment after it survives;
  • gone: remove it;
  • new: add it at the end.

Elixir developers who've seen Sourceror will recognize the idea: edit the tree, keep the formatting.

Pattern matching, two new forms

The fast path is "nothing changed, return the file as it was":

if let (Some(previous), Some(text)) = (&previous, original)
    && previous.content == *content
{
    return text.to_string();
}

This is a let chain, new in Rust's 2024 edition: if let patterns and ordinary conditions joined with &&. The block runs only if both values are Some and the content is equal. In Elixir you'd write a with with a guard, or a case with when.

The other new form is let … else:

let Some(kind) = block.container_type() else {
    // Text blocks are their own Markdown.
    return match block {
        Block::Text { markdown } => markdown.clone(),
        _ => String::new(),
    };
};

It reads: "this must match; if it doesn't, run the else block, which has to leave the function." After it, kind is available for the rest of the function, without the extra indentation an if let would add. It's a good fit for "handle the special case first, then get on with the main work".

Functions as values

New pages leave defaults out of the file: no draft = false, no menu = "none". The list of defaults is a table of names and tests:

/// A setting's name and a test for whether it holds its default value.
type DefaultCheck = (&'static str, fn(&Item) -> bool);

let defaults: [DefaultCheck; 6] = [
    ("draft", |item| item.as_bool() == Some(false)),
    ("menu", |item| item.as_str() == Some("none")),
    ("weight", |item| item.as_integer() == Some(0)),
    ("redirect_from", is_empty_array),
    ("authors", is_empty_array),
    ("tags", is_empty_array),
];

fn(&Item) -> bool is a function pointer type: any function taking an &Item and returning a bool. Both named functions (is_empty_array) and closures fit, as long as the closures don't capture anything from their surroundings. It's Elixir's &is_empty_array/1 stored in a list.

The type DefaultCheck = … line is a type alias, a name for a long type, like @type in Elixir. Clippy, Rust's linter, insisted on it: it flags types it considers too complex to read inline.

Accepting two shapes of input

The page date arrives in two forms: as a TOML date when reading a file (date = 2026-10-21), and as text when the UI sends settings back ("2026-10-21"). The third post wrote Serialize by hand; now Deserialize gets the same treatment:

#[derive(Deserialize)]
#[serde(untagged)]
enum Raw {
    Date(toml::value::Datetime),
    Text(String),
}

match Raw::deserialize(deserializer)? {
    Raw::Date(date) => Ok(PageDate(date)),
    Raw::Text(text) => text.parse().map(PageDate).map_err(serde::de::Error::custom),
}

#[serde(untagged)] tries each variant in order and takes the first that fits. The enum is declared inside the function, because nothing else needs it.

Never half-written

Saving writes to a temporary file first, then renames it over the original:

let temporary = path.with_extension("md.qed-saving");
fs::write(&temporary, contents).map_err(error)?;
fs::rename(&temporary, path).map_err(error)

A rename within one folder happens in one step. If the app crashes while writing, the original file is untouched; the worst case is a leftover .qed-saving file.

Also, the file path comes from the UI, and the UI shouldn't be able to make qed write anywhere it likes. page_file in site.rs accepts only plain relative paths inside pages/ that end in .md and already exist. ../site.toml or /etc/passwd gets an error, and a test checks exactly that.

A first Svelte component

On the UI side, page settings got their own component, src/lib/PageSettings.svelte. A component is one .svelte file: script, markup, and optionally styles, like a Phoenix function component with its template.

It receives props:

interface Props {
  page: Page;
  siteRoot: string;
  onsaved: (page: Page) => void;
  onclose: () => void;
}

let { page, siteRoot, onsaved, onclose }: Props = $props();

$props() returns what the parent passed in; the TypeScript interface says what that must be. onsaved and onclose are plain functions the parent hands down. When the form is saved, the component calls onsaved(saved), and the parent updates its page list. In Phoenix terms, that's a LiveComponent sending a message to its parent, except it's a function call.

The parent uses it like an HTML tag:

{#key selected.file}
  <PageSettings
    page={selected}
    siteRoot={site.root}
    onsaved={replacePage}
    onclose={() => (selectedFile = null)}
  />
{/key}

{#key …} throws the component away and creates a new one whenever the key changes. Pick another page, and the form starts fresh with that page's settings.

An editable copy

The form shouldn't change the page list while you type, only when you save. So it works on a copy:

const initial = untrack(() => $state.snapshot(page.front));
let form = $state(initial);
  • $state.snapshot makes a plain copy of reactive state.
  • untrack says "read this once, don't follow changes". Without it, Svelte warns that the copy only captures the initial value, which here is exactly the point.

"Unsaved changes" is then a $derived comparison of what would be saved with what's in the file:

let dirty = $derived(JSON.stringify(edited) !== JSON.stringify(page.front));

It updates itself on every keystroke, and the Save button is disabled while dirty is false.

Further reading