Add `at_cleanup()` function

1 files changed, 25 insertions, 1 deletions

diff --git a/api/builtins.md b/api/builtins.md
index 0b06a41b..6d042741 100644
--- a/api/builtins.md
+++ b/api/builtins.md
@@ -33,10 +33,34 @@ force_tty | `Bool` | Whether or not to force the use of /dev/tty.  | `yes`
 assert ask("What's your name? ") == "Arthur Dent"
 
 ```
+## at_cleanup
+
+```tomo
+at_cleanup : func(fn: func() -> Void)
+```
+
+Register a function that runs at cleanup time for Tomo programs. Cleanup time happens when a program exits (see `atexit()` in C), or immediately before printing error messages in a call to `fail()`. This allows for terminal cleanup so error messages can be visible as the program shuts down.
+
+Use this API very carefully, because errors that occur during cleanup functions may make it extremely hard to figure out what's going on. Cleanup functions should be designed to not error under any circumstances.
+
+Argument | Type | Description | Default
+---------|------|-------------|---------
+fn | `func()` | A function to run at cleanup time.  | -
+
+**Return:** Nothing.
+
+
+**Example:**
+```tomo
+at_cleanup(func()
+    (/tmp/file.txt).remove(ignore_missing=yes)
+)
+
+```
 ## exit
 
 ```tomo
-exit : func(message: Text? = none, status: Int32 = Int32(1) -> Void)
+exit : func(message: Text? = none, status: Int32 = Int32(1) -> Abort)
 ```
 
 Exits the program with a given status and optionally prints a message.