lush/issues/22-implicit-interactive-commands.md

22 — Implicit interactive commands (drop ! prefix)

Status: in progress

Goal

In the REPL, treat unrecognized input as shell commands so users can type git status or ls -la without the ! prefix.

Implementation

REPL fallback chain (loadline() in lua.c)

The standard Lua REPL tries two compilations. We add a third:

1. Try as expression (return <line>)
2. Try as statement (with continuation for incomplete input)
3. Try as shell command (!<line>)

This is done by adding addshellcmd() which prepends ! to the line and compiles it, triggering the existing lexer/parser path for interactive commands.

The bare identifier problem

Multi-word input like git status fails both Lua paths and correctly falls through to shell. But single-word commands like ls compile as return ls; — a valid Lua expression that returns nil — so they never reach the shell fallback.

Worse: if addreturn is bypassed, ls as a statement is incomplete Lua (the parser expects ls(...) or ls = ...), so multiline() enters continuation mode and the REPL hangs waiting for more input.

Fix: check _G before the expression path

Before trying return <line>, check if the line is a bare identifier (single word matching [a-zA-Z_][a-zA-Z0-9_]*). If it is, look it up in _G:

• In _G → proceed normally (return print shows the function, return x shows its value)
• Not in _G → skip expression and statement paths entirely, go straight to shell

This is a compile-time check — no runtime error interception, no flags, no special subroutines. ~10 lines of C in loadline():

line = lua_tostring(L, 1);
bare = isbareid(line);
if (bare && lua_getglobal(L, line) == LUA_TNIL) {
    lua_pop(L, 1);
    status = addshellcmd(L);  /* straight to shell */
}
else {
    if (bare) lua_pop(L, 1);
    /* normal expression → statement → shell fallback chain */
}

Summary of behavior

Input	Path taken	Result
print("hi")	expression	Lua expression
x = 42	statement	Lua statement
if true then print("x") end	statement	Lua statement
print	expression (in _G)	shows function value
x (after x=42)	expression (in _G)	shows 42
git status	expression fails → statement fails → shell	runs git
ls -la	expression fails → statement fails → shell	runs ls
echo hello	expression fails → statement fails → shell	runs echo
ls	bare id, not in _G → shell	runs ls
!pwd	expression (already valid ! syntax)	runs pwd

Edge cases

• A global explicitly set to nil (x = nil) would still try the expression path — return x compiles and returns nil. This is correct: the user defined it, Lua should handle it.
• Lua keywords (if, for, while) are not valid identifiers in isbareid terms — they fail addreturn, enter multiline for continuation, and behave normally.
• !cmd still works — it compiles as a valid expression in the first path.

Files

File	Changes
lua.c	isbareid(), addshellcmd(), modified loadline()