haskell: getLine/getContents/readFile/writeFile + 0-arity builtin force (+12 tests, 587/587)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-on-sx.md

@@ -106,7 +106,7 @@ Key mappings:
 
 ### Phase 6 — real IO + Prelude completion
 - [x] Real `IO` monad backed by `perform`/`resume`
-- [ ] `putStrLn`, `getLine`, `readFile`, `writeFile`, `print`
+- [x] `putStrLn`, `getLine`, `readFile`, `writeFile`, `print`
 - [ ] Full-ish Prelude: `Maybe`, `Either`, `List` functions, `Map`-lite
 - [ ] Drive scoreboard toward 150+ passing
 
@@ -114,6 +114,17 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-06** — Phase 6 `getLine`/`getContents`/`readFile`/`writeFile`. `hk-force`
+  extended: 0-arity builtins (`arity=0` dicts) are called immediately when forced,
+  making `getLine`/`getContents` work naturally as IO actions (no arity-0 application
+  needed — `>>=` forces them and gets the `("IO" value)` result). `getLine` pops
+  from `hk-stdin-lines`; `getContents` drains it joining with `"\n"`; `readFile`
+  reads from `hk-vfs` (dict), errors on missing key; `writeFile` sets `hk-vfs` key.
+  `hk-run-io-with-input` resets both io-lines and stdin then runs. `>>=` and `>>`
+  added to `hk-binop` for infix operator path. Bug caught: `sx_replace_node` on the
+  thunk-force branch accidentally changed `"body"` → `"fn"` (key name); fixed.
+  11 new tests in `tests/io-input.sx`. 587/587 green.
+
 - **2026-05-06** — Phase 6 real IO monad. `eval.sx`: mutable `hk-io-lines` list
   buffer; `putStrLn` and `putStr` append the (forced) string arg; `print` appends
   `hk-show-val` of the arg; all three return `("IO" ("Tuple"))`. `hk-run-io`