# 09 — Programmable prompt

**Status:** done (superseded by #21)
**Blocked by:** #08

Users should be able to customize the shell prompt by defining a Lua function, similar to how other shells use `PS1`/`PROMPT_COMMAND`/`precmd`.

## Design

- The prompt is generated by calling a global Lua function (e.g. `__prompt()`)
- The function returns a string which is used as the prompt
- If the function is not defined or errors, fall back to a sensible default

## Default prompt

A built-in default prompt is provided (implemented on the C side) until the user overrides `__prompt()` in their config. The default could be something like:

```
lush>
```

or include the current directory:

```
~/Code/project>
```

## User customization

Users override the prompt in their config (`~/.config/lush/config`):

```lua
function __prompt()
  return $PWD .. "> "
end
```

Or with colours/git info/etc:

```lua
function __prompt()
  local cwd = $PWD or "?"
  local branch = `git branch --show-current 2>/dev/null`
  local git = (branch.code == 0) and " (" .. branch.stdout:gsub("\n","") .. ")" or ""
  return cwd .. git .. "> "
end
```

## Implementation considerations

- The REPL loop (in `lua.c`) calls `__prompt()` before each input line
- If `__prompt` is not a function or the call fails, use the default C-side prompt
- The function runs in the normal Lua state so it has access to all globals, env vars, commands, etc.

## Open questions

- Should there be a separate `__continuation_prompt()` for multiline input?
- Should the prompt function receive any arguments (e.g. exit code of last command)?