fix: forE iterates the half-open range [lo, hi)

PureScript/JS `forE lo hi f` runs over `[lo, hi)` — lo inclusive, hi
exclusive (Effect.purs doc + JS `for (i = lo; i < hi; i++)`). Lua's
numeric `for i = lo, hi` is inclusive of the upper bound, so the port ran
one extra iteration at i = hi and never produced an empty range
(`forE n n` ran once instead of zero times). Use `for i = lo, hi - 1`.

Adds a Lua 5.1 regression guard (test/regression/effect.lua).

Fixes purescript-lua/purescript-lua#78

Author: Unisay

scripts/test

@@ -4,3 +4,4 @@ set -euo pipefail
 echo "Running regression tests..."
 
 lua test/regression/uncurried.lua
+lua test/regression/effect.lua

src/Effect.lua

@@ -3,6 +3,6 @@ return {
   bindE = (function(a) return function(f) return function() return f(a())() end end end),
   untilE = (function(f) return function() while not f() do end end end),
   whileE = (function(f) return function(a) return function() while f() do a() end end end end),
-  forE = (function(lo) return function(hi) return function(f) return function() for i = lo, hi do f(i)() end end end end end),
+  forE = (function(lo) return function(hi) return function(f) return function() for i = lo, hi - 1 do f(i)() end end end end end),
   foreachE = (function(as) return function(f) return function() for i = 1, #as do f(as[i])() end end end end)
 }

test/regression/effect.lua

@@ -0,0 +1,81 @@
+-- Regression guard for the Lua 5.1 FFI of Effect (control flow).
+--
+-- #78 forE lo hi f iterates over the half-open range [lo, hi) — lo inclusive,
+-- hi EXCLUSIVE (per Effect.purs and the JS `for (i = lo; i < hi; i++)`). Lua's
+-- numeric `for i = lo, hi` is inclusive of the upper bound, so the naive port
+-- ran one extra iteration and never produced an empty range. The fix uses
+-- `for i = lo, hi - 1`.
+--
+-- `Effect a` here is a zero-arg thunk; `f :: Int -> Effect Unit` is a curried
+-- `function(i) return function() ... end end`.
+-- Run from the repo root: `lua test/regression/effect.lua`.
+local E = dofile("src/Effect.lua")
+
+local failures = 0
+
+local function check(name, cond, detail)
+  if cond then
+    print("ok   - " .. name)
+  else
+    failures = failures + 1
+    print("FAIL - " .. name .. ": " .. tostring(detail))
+  end
+end
+
+-- Run forE over [lo, hi) and return the list of indices it visited.
+local function visited(lo, hi)
+  local seen = {}
+  local function f(i) return function() seen[#seen + 1] = i end end
+  E.forE(lo)(hi)(f)()
+  return seen
+end
+
+local function showList(t)
+  local parts = {}
+  for i = 1, #t do parts[i] = tostring(t[i]) end
+  return "{" .. table.concat(parts, ", ") .. "}"
+end
+
+local function eqList(a, b)
+  if #a ~= #b then return false end
+  for i = 1, #a do if a[i] ~= b[i] then return false end end
+  return true
+end
+
+--------------------------------------------------------------------------------
+-- #78 forE iterates over [lo, hi) (hi exclusive) -----------------------------
+
+do
+  local got = visited(0, 3)
+  check("forE 0 3 visits 0,1,2 (hi exclusive)", eqList(got, {0, 1, 2}), "got " .. showList(got))
+end
+
+do
+  local got = visited(1, 5)
+  check("forE 1 5 visits 1,2,3,4", eqList(got, {1, 2, 3, 4}), "got " .. showList(got))
+end
+
+do
+  local got = visited(2, 2)
+  check("forE n n is an empty range (zero iterations)", #got == 0, "got " .. showList(got))
+end
+
+do
+  local got = visited(5, 3)
+  check("forE lo>hi is empty", #got == 0, "got " .. showList(got))
+end
+
+--------------------------------------------------------------------------------
+-- foreachE iterates the whole array 1..#as (sanity: NOT off-by-one) ----------
+
+do
+  local seen = {}
+  local function f(x) return function() seen[#seen + 1] = x end end
+  E.foreachE({10, 20, 30})(f)()
+  check("foreachE visits every element", eqList(seen, {10, 20, 30}), "got " .. showList(seen))
+end
+
+--------------------------------------------------------------------------------
+
+if failures > 0 then error(failures .. " regression check(s) failed") end
+print("purescript-lua-effect: all FFI regression checks passed")