Add Async helper with tick-based interleaving

Author: Your Name

platform/Async.roc

@@ -0,0 +1,42 @@
+module [
+    Async,
+    after_ticks,
+    from_task!,
+    map!,
+    poll!,
+]
+
+Async a err : [Pending { remaining : U64, step! : {} => Result a err }, Ready a]
+
+poll! : Async a err => Result (Async a err) err
+poll! = |async|
+    when async is
+        Ready(val) -> Ok(Ready(val))
+        Pending({ remaining, step! }) ->
+            if remaining == 0 then
+                when step!({}) is
+                    Ok(val) -> Ok(Ready(val))
+                    Err(err) -> Err(err)
+            else
+                Ok(Pending({ remaining: remaining - 1, step! }))
+
+from_task! : ({} => Result a err) => Async a err
+from_task! = |task!|
+    Pending({ remaining: 0, step!: task! })
+
+map! : (a -> b), Async a err => Async b err
+map! = |f, async|
+    when async is
+        Ready(val) -> Ready(f(val))
+        Pending({ remaining, step! }) ->
+            Pending({
+                remaining,
+                step!: |{}|
+                    when step!({}) is
+                        Ok(val) -> Ok(f(val))
+                        Err(err) -> Err(err),
+            })
+
+after_ticks : U64 -> Async {} []
+after_ticks = |ticks|
+    Pending({ remaining: ticks, step!: |{}| Ok({}) })

platform/main.roc

@@ -21,6 +21,7 @@ platform "cli"
         Random,
         Gmp,
         Math,
+        Async,
     ]
     packages {}
     imports []