Redesign prompt system: revert #09, use standard _PROMPT (issue #21)

Revert the bespoke __prompt()/__prompt2() function mechanism from issue
#09 and return to stock Lua's _PROMPT/_PROMPT2 globals (which already
support dynamic prompts via __tostring metatables). Set default
_PROMPT = "lush> " in doREPL if not already defined by config.

testes/lush/prompt.lua

@@ -1,7 +1,7 @@
 -- testes/lush/prompt.lua
--- Tests for programmable prompt (issue #09).
+-- Tests for prompt system (issue #21).
 
-print "testing programmable prompt"
+print "testing prompt system"
 
 -- helper: get a unique temp directory
 local tmpbase = os.tmpname()
@@ -39,95 +39,47 @@ local function run_lua(input, config_content)
 end
 
 
--- ===== TEST 1: Default __prompt is installed =====
--- After starting the REPL, __prompt should be a function
+-- ===== TEST 1: Default _PROMPT is "lush> " =====
 do
-  local out = run_lua('"print(type(__prompt))"')
-  assert(out:find("function"),
-    "default __prompt should be installed as function: " .. out)
+  local out = run_lua('"print(_PROMPT)"')
+  assert(out:find("lush> ", 1, true),
+    "default _PROMPT should be 'lush> ': " .. out)
 end
 
 
--- ===== TEST 2: Default __prompt returns cwd =====
+-- ===== TEST 2: _PROMPT can be overridden by config =====
 do
-  local cwd = os.getenv("PWD") or ""
-  local home = os.getenv("HOME") or ""
-  local expected
-  if home ~= "" and cwd:sub(1, #home) == home then
-    expected = "~" .. cwd:sub(#home + 1)
-  else
-    expected = cwd
-  end
-  local out = run_lua('"print(__prompt(0))"')
-  assert(out:find(expected, 1, true),
-    "default __prompt should return cwd: expected '" ..
-    expected .. "' in: " .. out)
-end
-
-
--- ===== TEST 3: Custom __prompt() overrides default =====
-do
-  local out = run_lua('"print(__prompt(0))"',
-    'function __prompt() return "custom> " end\n')
+  local out = run_lua('"print(_PROMPT)"',
+    '_PROMPT = "custom> "\n')
   assert(out:find("custom> ", 1, true),
-    "custom __prompt should be used: " .. out)
+    "config should override _PROMPT: " .. out)
 end
 
 
--- ===== TEST 4: __prompt() receives exit code =====
+-- ===== TEST 3: _PROMPT with __tostring metatable (dynamic prompt) =====
 do
-  local out = run_lua(
-    '"error()" "print(__prompt())" "os.exit()"',
-    'function __prompt(code)\n'
-    .. '  return "exit:" .. tostring(code) .. "> "\n'
-    .. 'end\n')
-  -- After error(), the next prompt call should receive non-zero code.
-  -- But since we're calling __prompt() manually via print, we test
-  -- the mechanism differently: call it with a code directly.
-  local out2 = run_lua('"print(__prompt(42))"',
-    'function __prompt(code)\n'
-    .. '  return "exit:" .. tostring(code) .. "> "\n'
-    .. 'end\n')
-  assert(out2:find("exit:42> ", 1, true),
-    "exit code should be passed to __prompt: " .. out2)
+  local out = run_lua('"print(_PROMPT)"',
+    '_PROMPT = setmetatable({}, { __tostring = function() return "dynamic> " end })\n')
+  assert(out:find("dynamic> ", 1, true),
+    "_PROMPT with __tostring should work: " .. out)
 end
 
 
--- ===== TEST 5: __prompt2 for continuation =====
+-- ===== TEST 4: _PROMPT2 works for continuation =====
 do
-  local out = run_lua('"print(type(__prompt2))"')
-  -- __prompt2 is not installed by default
-  assert(out:find("nil") or out:find("function"),
-    "__prompt2 should be nil or function: " .. out)
+  local out = run_lua('"print(_PROMPT2)"',
+    '_PROMPT2 = "...> "\n')
+  assert(out:find("...> ", 1, true),
+    "_PROMPT2 should be settable: " .. out)
 end
 
 
--- ===== TEST 6: Broken __prompt() falls back gracefully =====
+-- ===== TEST 5: _PROMPT = nil falls back gracefully =====
 do
-  local out = run_lua('"print(42)"',
-    'function __prompt() error("boom") end\n')
+  -- Set _PROMPT to nil during the REPL; get_prompt falls back to LUA_PROMPT.
+  local out = run_lua('"_PROMPT = nil" "print(42)"')
   assert(out:find("42"),
-    "broken __prompt should still allow REPL to work: " .. out)
-end
-
-
--- ===== TEST 7: _PROMPT still works (backward compat) =====
-do
-  -- When __prompt is defined (default), it takes priority.
-  -- To test _PROMPT fallback, unset __prompt.
-  local out = run_lua('"print(42)"',
-    '__prompt = nil\n_PROMPT = "old> "\n')
-  assert(out:find("42"),
-    "_PROMPT fallback should still work: " .. out)
-end
-
-
--- ===== TEST 8: Config-defined __prompt is not overwritten =====
-do
-  local out = run_lua('"print(__prompt(0))"',
-    'function __prompt() return "from_config> " end\n')
-  assert(out:find("from_config> ", 1, true),
-    "config __prompt should not be overwritten: " .. out)
+    "_PROMPT = nil should still allow REPL to work: " .. out)
 end