Add `at_cleanup()` function

3 files changed, 77 insertions, 3 deletions

diff --git a/api/api.md b/api/api.md
index 02ad054e..5c9dc9c4 100644
--- a/api/api.md
+++ b/api/api.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.
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.
diff --git a/api/builtins.yaml b/api/builtins.yaml
index 2eae5340..764a1bd4 100644
--- a/api/builtins.yaml
+++ b/api/builtins.yaml
@@ -38,7 +38,7 @@ exit:
   description: >
     Exits the program with a given status and optionally prints a message.
   return:
-    type: 'Void'
+    type: 'Abort'
     description: >
       This function never returns.
   args:
@@ -56,6 +56,32 @@ exit:
   example: |
     exit(status=1, "Goodbye forever!")
 
+at_cleanup:
+  short: register a cleanup function
+  description: >
+    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.
+  note: >
+    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.
+  args:
+    fn:
+      type: 'func()'
+      description: >
+        A function to run at cleanup time.
+  return:
+    type: 'Void'
+    description: >
+      Nothing.
+  example: |
+    at_cleanup(func()
+        (/tmp/file.txt).remove(ignore_missing=yes)
+    )
+
 getenv:
   short: get an environment variable
   description: >