commit 7b3f782aae46f3e70dc2b5713d61ef77500b5fe7
Author: John <j@soft.fish>
Date:   Fri Apr 19 07:30:17 2024 -0500

    repline: Promote to its own crate!
    
    cl-repl: Major refactor based on the design of typeck.rs

diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..0437167
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "repline"
+repository.workspace = true
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+publish.workspace = true
+
+[dependencies]
+crossterm = "0.27.0"
diff --git a/src/editor.rs b/src/editor.rs
new file mode 100644
index 0000000..9fdb6fb
--- /dev/null
+++ b/src/editor.rs
@@ -0,0 +1,283 @@
+//! The [Editor]
+
+use crossterm::{cursor::*, execute, queue, style::*, terminal::*};
+use std::{collections::VecDeque, fmt::Display, io::Write};
+
+use super::error::{Error, ReplResult};
+
+fn is_newline(c: &char) -> bool {
+    *c == '\n'
+}
+
+fn write_chars<'a, W: Write>(
+    c: impl IntoIterator<Item = &'a char>,
+    w: &mut W,
+) -> std::io::Result<()> {
+    for c in c {
+        write!(w, "{c}")?;
+    }
+    Ok(())
+}
+
+#[derive(Clone, Debug)]
+pub struct Editor<'a> {
+    head: VecDeque<char>,
+    tail: VecDeque<char>,
+
+    pub color: &'a str,
+    begin: &'a str,
+    again: &'a str,
+}
+
+impl<'a> Editor<'a> {
+    pub fn new(color: &'a str, begin: &'a str, again: &'a str) -> Self {
+        Self { head: Default::default(), tail: Default::default(), color, begin, again }
+    }
+    pub fn iter(&self) -> impl Iterator<Item = &char> {
+        let Self { head, tail, .. } = self;
+        head.iter().chain(tail.iter())
+    }
+    pub fn undraw<W: Write>(&self, w: &mut W) -> ReplResult<()> {
+        let Self { head, .. } = self;
+        match head.iter().copied().filter(is_newline).count() {
+            0 => write!(w, "\x1b[0G"),
+            lines => write!(w, "\x1b[{}F", lines),
+        }?;
+        queue!(w, Clear(ClearType::FromCursorDown))?;
+        // write!(w, "\x1b[0J")?;
+        Ok(())
+    }
+    pub fn redraw<W: Write>(&self, w: &mut W) -> ReplResult<()> {
+        let Self { head, tail, color, begin, again } = self;
+        write!(w, "{color}{begin}\x1b[0m ")?;
+        // draw head
+        for c in head {
+            match c {
+                '\n' => write!(w, "\r\n{color}{again}\x1b[0m "),
+                _ => w.write_all({ *c as u32 }.to_le_bytes().as_slice()),
+            }?
+        }
+        // save cursor
+        execute!(w, SavePosition)?;
+        // draw tail
+        for c in tail {
+            match c {
+                '\n' => write!(w, "\r\n{color}{again}\x1b[0m "),
+                _ => write!(w, "{c}"),
+            }?
+        }
+        // restore cursor
+        execute!(w, RestorePosition)?;
+        Ok(())
+    }
+    pub fn prompt<W: Write>(&self, w: &mut W) -> ReplResult<()> {
+        let Self { head, color, begin, again, .. } = self;
+        queue!(
+            w,
+            MoveToColumn(0),
+            Print(color),
+            Print(if head.is_empty() { begin } else { again }),
+            ResetColor,
+            Print(' '),
+        )?;
+        Ok(())
+    }
+    pub fn print_head<W: Write>(&self, w: &mut W) -> ReplResult<()> {
+        self.prompt(w)?;
+        write_chars(
+            self.head.iter().skip(
+                self.head
+                    .iter()
+                    .rposition(is_newline)
+                    .unwrap_or(self.head.len())
+                    + 1,
+            ),
+            w,
+        )?;
+        Ok(())
+    }
+    pub fn print_tail<W: Write>(&self, w: &mut W) -> ReplResult<()> {
+        let Self { tail, .. } = self;
+        queue!(w, SavePosition, Clear(ClearType::UntilNewLine))?;
+        write_chars(tail.iter().take_while(|&c| !is_newline(c)), w)?;
+        queue!(w, RestorePosition)?;
+        Ok(())
+    }
+    pub fn push<W: Write>(&mut self, c: char, w: &mut W) -> ReplResult<()> {
+        // Tail optimization: if the tail is empty,
+        //we don't have to undraw and redraw on newline
+        if self.tail.is_empty() {
+            self.head.push_back(c);
+            match c {
+                '\n' => {
+                    write!(w, "\r\n")?;
+                    self.print_head(w)?;
+                }
+                c => {
+                    queue!(w, Print(c))?;
+                }
+            };
+            return Ok(());
+        }
+
+        if '\n' == c {
+            self.undraw(w)?;
+        }
+        self.head.push_back(c);
+        match c {
+            '\n' => self.redraw(w)?,
+            _ => {
+                write!(w, "{c}")?;
+                self.print_tail(w)?;
+            }
+        }
+        Ok(())
+    }
+    pub fn pop<W: Write>(&mut self, w: &mut W) -> ReplResult<Option<char>> {
+        if let Some('\n') = self.head.back() {
+            self.undraw(w)?;
+        }
+        let c = self.head.pop_back();
+        // if the character was a newline, we need to go back a line
+        match c {
+            Some('\n') => self.redraw(w)?,
+            Some(_) => {
+                // go back a char
+                queue!(w, MoveLeft(1), Print(' '), MoveLeft(1))?;
+                self.print_tail(w)?;
+            }
+            None => {}
+        }
+        Ok(c)
+    }
+
+    pub fn extend<T: IntoIterator<Item = char>, W: Write>(
+        &mut self,
+        iter: T,
+        w: &mut W,
+    ) -> ReplResult<()> {
+        for c in iter {
+            self.push(c, w)?;
+        }
+        Ok(())
+    }
+
+    pub fn restore(&mut self, s: &str) {
+        self.clear();
+        self.head.extend(s.chars())
+    }
+    pub fn clear(&mut self) {
+        self.head.clear();
+        self.tail.clear();
+    }
+    pub fn delete<W: Write>(&mut self, w: &mut W) -> ReplResult<char> {
+        match self.tail.front() {
+            Some('\n') => {
+                self.undraw(w)?;
+                let out = self.tail.pop_front();
+                self.redraw(w)?;
+                out
+            }
+            _ => {
+                let out = self.tail.pop_front();
+                self.print_tail(w)?;
+                out
+            }
+        }
+        .ok_or(Error::EndOfInput)
+    }
+    pub fn erase_word<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        while self.pop(w)?.filter(|c| !c.is_whitespace()).is_some() {}
+        Ok(())
+    }
+    pub fn len(&self) -> usize {
+        self.head.len() + self.tail.len()
+    }
+    pub fn is_empty(&self) -> bool {
+        self.head.is_empty() && self.tail.is_empty()
+    }
+    pub fn ends_with(&self, iter: impl DoubleEndedIterator<Item = char>) -> bool {
+        let mut iter = iter.rev();
+        let mut head = self.head.iter().rev();
+        loop {
+            match (iter.next(), head.next()) {
+                (None, _) => break true,
+                (Some(_), None) => break false,
+                (Some(a), Some(b)) if a != *b => break false,
+                (Some(_), Some(_)) => continue,
+            }
+        }
+    }
+    /// Moves the cursor back `steps` steps
+    pub fn cursor_back<W: Write>(&mut self, steps: usize, w: &mut W) -> ReplResult<()> {
+        for _ in 0..steps {
+            if let Some('\n') = self.head.back() {
+                self.undraw(w)?;
+            }
+            let Some(c) = self.head.pop_back() else {
+                return Ok(());
+            };
+            self.tail.push_front(c);
+            match c {
+                '\n' => self.redraw(w)?,
+                _ => queue!(w, MoveLeft(1))?,
+            }
+        }
+        Ok(())
+    }
+    /// Moves the cursor forward `steps` steps
+    pub fn cursor_forward<W: Write>(&mut self, steps: usize, w: &mut W) -> ReplResult<()> {
+        for _ in 0..steps {
+            if let Some('\n') = self.tail.front() {
+                self.undraw(w)?
+            }
+            let Some(c) = self.tail.pop_front() else {
+                return Ok(());
+            };
+            self.head.push_back(c);
+            match c {
+                '\n' => self.redraw(w)?,
+                _ => queue!(w, MoveRight(1))?,
+            }
+        }
+        Ok(())
+    }
+    /// Goes to the beginning of the current line
+    pub fn home<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        loop {
+            match self.head.back() {
+                Some('\n') | None => break Ok(()),
+                Some(_) => self.cursor_back(1, w)?,
+            }
+        }
+    }
+    /// Goes to the end of the current line
+    pub fn end<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        loop {
+            match self.tail.front() {
+                Some('\n') | None => break Ok(()),
+                Some(_) => self.cursor_forward(1, w)?,
+            }
+        }
+    }
+}
+
+impl<'a, 'e> IntoIterator for &'e Editor<'a> {
+    type Item = &'e char;
+    type IntoIter = std::iter::Chain<
+        std::collections::vec_deque::Iter<'e, char>,
+        std::collections::vec_deque::Iter<'e, char>,
+    >;
+    fn into_iter(self) -> Self::IntoIter {
+        self.head.iter().chain(self.tail.iter())
+    }
+}
+impl<'a> Display for Editor<'a> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use std::fmt::Write;
+        for c in self.iter() {
+            f.write_char(*c)?;
+        }
+        Ok(())
+    }
+}
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..241254e
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,42 @@
+use crate::iter::chars::BadUnicode;
+
+/// Result type for Repline
+pub type ReplResult<T> = std::result::Result<T, Error>;
+/// Borrowed error (does not implement [Error](std::error::Error)!)
+#[derive(Debug)]
+pub enum Error {
+    /// User broke with Ctrl+C
+    CtrlC(String),
+    /// User broke with Ctrl+D
+    CtrlD(String),
+    /// Invalid unicode codepoint
+    BadUnicode(u32),
+    /// Error came from [std::io]
+    IoFailure(std::io::Error),
+    /// End of input
+    EndOfInput,
+}
+
+impl std::error::Error for Error {}
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Error::CtrlC(_) => write!(f, "Ctrl+C"),
+            Error::CtrlD(_) => write!(f, "Ctrl+D"),
+            Error::BadUnicode(u) => write!(f, "\\u{{{u:x}}} is not a valid unicode codepoint"),
+            Error::IoFailure(s) => write!(f, "{s}"),
+            Error::EndOfInput => write!(f, "End of input"),
+        }
+    }
+}
+impl From<std::io::Error> for Error {
+    fn from(value: std::io::Error) -> Self {
+        Self::IoFailure(value)
+    }
+}
+impl From<BadUnicode> for Error {
+    fn from(value: BadUnicode) -> Self {
+        let BadUnicode(code) = value;
+        Self::BadUnicode(code)
+    }
+}
diff --git a/src/iter.rs b/src/iter.rs
new file mode 100644
index 0000000..4af50bc
--- /dev/null
+++ b/src/iter.rs
@@ -0,0 +1,68 @@
+//! Shmancy iterator adapters
+
+pub use chars::Chars;
+pub use flatten::Flatten;
+
+pub mod chars {
+    //! Converts an <code>[Iterator]<Item = [u8]></code> into an
+    //! <code>[Iterator]<Item = [Result]<[char], [BadUnicode]>></code>
+
+    /// Invalid unicode codepoint found when iterating over [Chars]
+    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+    pub struct BadUnicode(pub u32);
+    impl std::error::Error for BadUnicode {}
+    impl std::fmt::Display for BadUnicode {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            let Self(code) = self;
+            write!(f, "Bad unicode: {code}")
+        }
+    }
+
+    /// Converts an <code>[Iterator]<Item = [u8]></code> into an
+    /// <code>[Iterator]<Item = [char]></code>
+    #[derive(Clone, Debug)]
+    pub struct Chars<I: Iterator<Item = u8>>(pub I);
+    impl<I: Iterator<Item = u8>> Iterator for Chars<I> {
+        type Item = Result<char, BadUnicode>;
+        fn next(&mut self) -> Option<Self::Item> {
+            let Self(bytes) = self;
+            let start = bytes.next()? as u32;
+            let (mut out, count) = match start {
+                start if start & 0x80 == 0x00 => (start, 0), // ASCII valid range
+                start if start & 0xe0 == 0xc0 => (start & 0x1f, 1), // 1 continuation byte
+                start if start & 0xf0 == 0xe0 => (start & 0x0f, 2), // 2 continuation bytes
+                start if start & 0xf8 == 0xf0 => (start & 0x07, 3), // 3 continuation bytes
+                _ => return None,
+            };
+            for _ in 0..count {
+                let cont = bytes.next()? as u32;
+                if cont & 0xc0 != 0x80 {
+                    return None;
+                }
+                out = out << 6 | (cont & 0x3f);
+            }
+            Some(char::from_u32(out).ok_or(BadUnicode(out)))
+        }
+    }
+}
+pub mod flatten {
+    //! Flattens an [Iterator] returning [`Result<T, E>`](Result) or [`Option<T>`](Option)
+    //! into a *non-[FusedIterator](std::iter::FusedIterator)* over `T`
+
+    /// Flattens an [Iterator] returning [`Result<T, E>`](Result) or [`Option<T>`](Option)
+    /// into a *non-[FusedIterator](std::iter::FusedIterator)* over `T`
+    #[derive(Clone, Debug)]
+    pub struct Flatten<T, I: Iterator<Item = T>>(pub I);
+    impl<T, E, I: Iterator<Item = Result<T, E>>> Iterator for Flatten<Result<T, E>, I> {
+        type Item = T;
+        fn next(&mut self) -> Option<Self::Item> {
+            self.0.next()?.ok()
+        }
+    }
+    impl<T, I: Iterator<Item = Option<T>>> Iterator for Flatten<Option<T>, I> {
+        type Item = T;
+        fn next(&mut self) -> Option<Self::Item> {
+            self.0.next()?
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..d779cef
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,13 @@
+//! A small pseudo-multiline editing library
+
+mod editor;
+mod iter;
+mod raw;
+
+pub mod error;
+pub mod prebaked;
+pub mod repline;
+
+pub use error::Error;
+pub use prebaked::{read_and, Response};
+pub use repline::Repline;
diff --git a/src/prebaked.rs b/src/prebaked.rs
new file mode 100644
index 0000000..ba2aa15
--- /dev/null
+++ b/src/prebaked.rs
@@ -0,0 +1,56 @@
+//! Here's a menu I prepared earlier!
+//!
+//! Constructs a [Repline] and repeatedly runs the provided closure on the input strings,
+//! obeying the closure's [Response].
+
+use std::error::Error;
+
+use crate::{error::Error as RlError, repline::Repline};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+/// Control codes for the [prebaked menu](read_and)
+pub enum Response {
+    /// Accept the line, and save it to history
+    Accept,
+    /// Reject the line, and clear the buffer
+    Deny,
+    /// End the loop
+    Break,
+    /// Gather more input and try again
+    Continue,
+}
+
+/// Implements a basic menu loop using an embedded [Repline].
+///
+/// Repeatedly runs the provided closure on the input strings,
+/// obeying the closure's [Response].
+///
+/// Captures and displays all user [Error]s.
+///
+/// # Keybinds
+/// - `Ctrl+C` exits the loop
+/// - `Ctrl+D` clears the input, but *runs the closure* with the old input
+pub fn read_and<F>(color: &str, begin: &str, again: &str, mut f: F) -> Result<(), RlError>
+where F: FnMut(&str) -> Result<Response, Box<dyn Error>> {
+    let mut rl = Repline::new(color, begin, again);
+    loop {
+        let line = match rl.read() {
+            Err(RlError::CtrlC(_)) => break,
+            Err(RlError::CtrlD(line)) => {
+                rl.deny();
+                line
+            }
+            Ok(line) => line,
+            Err(e) => Err(e)?,
+        };
+        print!("\x1b[G\x1b[J");
+        match f(&line) {
+            Ok(Response::Accept) => rl.accept(),
+            Ok(Response::Deny) => rl.deny(),
+            Ok(Response::Break) => break,
+            Ok(Response::Continue) => continue,
+            Err(e) => print!("\x1b[40G\x1b[A\x1bJ\x1b[91m{e}\x1b[0m\x1b[B"),
+        }
+    }
+    Ok(())
+}
diff --git a/src/raw.rs b/src/raw.rs
new file mode 100644
index 0000000..1b66d3f
--- /dev/null
+++ b/src/raw.rs
@@ -0,0 +1,21 @@
+//! Sets the terminal to [`raw`] mode for the duration of the returned object's lifetime.
+
+/// Sets the terminal to raw mode for the duration of the returned object's lifetime.
+pub fn raw() -> impl Drop {
+    Raw::default()
+}
+struct Raw();
+impl Default for Raw {
+    fn default() -> Self {
+        std::thread::yield_now();
+        crossterm::terminal::enable_raw_mode()
+            .expect("should be able to transition into raw mode");
+        Raw()
+    }
+}
+impl Drop for Raw {
+    fn drop(&mut self) {
+        crossterm::terminal::disable_raw_mode()
+            .expect("should be able to transition out of raw mode");
+    }
+}
diff --git a/src/repline.rs b/src/repline.rs
new file mode 100644
index 0000000..4dc19e7
--- /dev/null
+++ b/src/repline.rs
@@ -0,0 +1,175 @@
+//! Prompts the user, reads the lines. Not much more to it than that.
+//!
+//! This module is in charge of parsing keyboard input and interpreting it for the line editor.
+
+use crate::{editor::Editor, error::*, iter::*, raw::raw};
+use std::{
+    collections::VecDeque,
+    io::{stdout, Bytes, Read, Result, Write},
+};
+
+/// Prompts the user, reads the lines. Not much more to it than that.
+#[derive(Debug)]
+pub struct Repline<'a, R: Read> {
+    input: Chars<Flatten<Result<u8>, Bytes<R>>>,
+
+    history: VecDeque<String>, // previous lines
+    hindex: usize,             // current index into the history buffer
+
+    ed: Editor<'a>, // the current line buffer
+}
+
+impl<'a> Repline<'a, std::io::Stdin> {
+    pub fn new(color: &'a str, begin: &'a str, again: &'a str) -> Self {
+        Self::with_input(std::io::stdin(), color, begin, again)
+    }
+}
+
+impl<'a, R: Read> Repline<'a, R> {
+    /// Constructs a [Repline] with the given [Reader](Read), color, begin, and again prompts.
+    pub fn with_input(input: R, color: &'a str, begin: &'a str, again: &'a str) -> Self {
+        Self {
+            input: Chars(Flatten(input.bytes())),
+            history: Default::default(),
+            hindex: 0,
+            ed: Editor::new(color, begin, again),
+        }
+    }
+    /// Set the terminal prompt color
+    pub fn set_color(&mut self, color: &'a str) {
+        self.ed.color = color
+    }
+    /// Append line to history and clear it
+    pub fn accept(&mut self) {
+        self.history_append(self.ed.to_string());
+        self.ed.clear();
+        self.hindex = self.history.len();
+    }
+    /// Clear the line
+    pub fn deny(&mut self) {
+        self.ed.clear()
+    }
+    /// Reads in a line, and returns it for validation
+    pub fn read(&mut self) -> ReplResult<String> {
+        const INDENT: &str = "    ";
+        let mut stdout = stdout().lock();
+        let stdout = &mut stdout;
+        let _make_raw = raw();
+        // self.ed.begin_frame(stdout)?;
+        // self.ed.redraw_frame(stdout)?;
+        self.ed.print_head(stdout)?;
+        loop {
+            stdout.flush()?;
+            match self.input.next().ok_or(Error::EndOfInput)?? {
+                // Ctrl+C: End of Text. Immediately exits.
+                '\x03' => {
+                    drop(_make_raw);
+                    writeln!(stdout)?;
+                    return Err(Error::CtrlC(self.ed.to_string()));
+                }
+                // Ctrl+D: End of Transmission. Ends the current line.
+                '\x04' => {
+                    drop(_make_raw);
+                    writeln!(stdout)?;
+                    return Err(Error::CtrlD(self.ed.to_string()));
+                }
+                // Tab: extend line by 4 spaces
+                '\t' => {
+                    self.ed.extend(INDENT.chars(), stdout)?;
+                }
+                // ignore newlines, process line feeds. Not sure how cross-platform this is.
+                '\n' => {}
+                '\r' => {
+                    self.ed.push('\n', stdout)?;
+                    return Ok(self.ed.to_string());
+                }
+                // Ctrl+Backspace in my terminal
+                '\x17' => {
+                    self.ed.erase_word(stdout)?;
+                }
+                // Escape sequence
+                '\x1b' => self.escape(stdout)?,
+                // backspace
+                '\x08' | '\x7f' => {
+                    let ed = &mut self.ed;
+                    if ed.ends_with(INDENT.chars()) {
+                        for _ in 0..INDENT.len() {
+                            ed.pop(stdout)?;
+                        }
+                    } else {
+                        ed.pop(stdout)?;
+                    }
+                }
+                c if c.is_ascii_control() => {
+                    if cfg!(debug_assertions) {
+                        self.ed.extend(c.escape_debug(), stdout)?;
+                    }
+                }
+                c => {
+                    self.ed.push(c, stdout)?;
+                }
+            }
+        }
+    }
+    /// Handle ANSI Escape
+    fn escape<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        match self.input.next().ok_or(Error::EndOfInput)?? {
+            '[' => self.csi(w)?,
+            'O' => todo!("Process alternate character mode"),
+            other => self.ed.extend(other.escape_debug(), w)?,
+        }
+        Ok(())
+    }
+    /// Handle ANSI Control Sequence Introducer
+    fn csi<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        match self.input.next().ok_or(Error::EndOfInput)?? {
+            'A' => {
+                self.hindex = self.hindex.saturating_sub(1);
+                self.restore_history(w)?
+            }
+            'B' => {
+                self.hindex = self.hindex.saturating_add(1).min(self.history.len());
+                self.restore_history(w)?
+            }
+            'C' => self.ed.cursor_forward(1, w)?,
+            'D' => self.ed.cursor_back(1, w)?,
+            'H' => self.ed.home(w)?,
+            'F' => self.ed.end(w)?,
+            '3' => {
+                if let '~' = self.input.next().ok_or(Error::EndOfInput)?? {
+                    let _ = self.ed.delete(w);
+                }
+            }
+            other => {
+                if cfg!(debug_assertions) {
+                    self.ed.extend(other.escape_debug(), w)?;
+                }
+            }
+        }
+        Ok(())
+    }
+    /// Restores the currently selected history
+    fn restore_history<W: Write>(&mut self, w: &mut W) -> ReplResult<()> {
+        let Self { history, hindex, ed, .. } = self;
+        ed.undraw(w)?;
+        ed.clear();
+        ed.print_head(w)?;
+        if let Some(history) = history.get(*hindex) {
+            ed.extend(history.chars(), w)?
+        }
+        Ok(())
+    }
+
+    /// Append line to history
+    fn history_append(&mut self, mut buf: String) {
+        while buf.ends_with(char::is_whitespace) {
+            buf.pop();
+        }
+        if !self.history.contains(&buf) {
+            self.history.push_back(buf)
+        }
+        while self.history.len() > 20 {
+            self.history.pop_front();
+        }
+    }
+}