Merge pull request #456 from maxsnew/trampoline

Trampoline

libraries/Native/Trampoline.js

@@ -0,0 +1,25 @@
+Elm.Native.Trampoline = {};
+Elm.Native.Trampoline.make = function(elm) {
+    elm.Native = elm.Native || {};
+    elm.Native.Trampoline = elm.Native.Trampoline || {};
+    if (elm.Native.Trampoline.values) return elm.Native.Trampoline.values;
+
+    var _E = Elm.Native.Error.make(elm),
+
+    // trampoline : Trampoline a -> a
+    trampoline = function(t) {
+        var tramp = t;
+        while(true) {
+            switch(tramp.ctor) {
+            case "Done":
+                return tramp._0;
+            case "Continue":
+                tramp = tramp._0({ctor: "_Tuple0"});
+                continue;
+            }
+            _E.Case("Trampoline", "in Native.Trampoline.trampoline");
+        }
+    }
+
+    return elm.Native.Trampoline.values = { trampoline: trampoline };
+};

libraries/Trampoline.elm

@@ -0,0 +1,42 @@
+module Trampoline where
+
+{-| Trampolining loops for unbounded recursion.
+Since most javascript implementations lack tail-call elimination, deeply tail-recursive functions will result in a stack overflow.
+
+```haskell
+fac' : Int -> Int -> Int
+fac' n acc = if n <= 0 
+             then acc
+             else fac' (n - 1) (n * acc)
+
+fac : Int -> Int
+fac n = fac' n 1
+
+-- Stack overflow
+main = asText <| fac 1000000000000
+```
+
+Trampolining allows for long-running tail-recursive loops to be run without pushing calls onto the stack:
+```haskell
+facT : Int -> Int -> Trampoline Int
+facT n acc = if n <= 0 
+             then Done acc
+             else Continue <| \() -> facT (n - 1) (n * acc)
+fac : Int -> Int
+fac n = trampoline <| facT n 0
+
+-- Doesn't stack overflow
+main = asText <| fac 1000000000000
+```
+# Trampoline
+@docs Trampoline, trampoline
+ -}
+import Native.Trampoline
+
+{-| A computation that might loop. A trampoline is either the resulting value or a thunk that needs to be run more. -}
+data Trampoline a = Done a
+                  | Continue (() -> Trampoline a)
+
+{-| Run a trampolining loop in constant space. -}
+trampoline : Trampoline a -> a
+trampoline = Native.Trampoline.trampoline