make function composition stack safe

Author: safareli

package.json

@@ -7,8 +7,8 @@
   },
   "devDependencies": {
     "eslint": "^3.17.1",
-    "purescript-psa": "^0.5.0-rc.1",
-    "pulp": "^10.0.4",
+    "purescript-psa": "^0.5.1",
+    "pulp": "^11.0.0",
     "rimraf": "^2.6.1"
   }
 }

src/Control/Semigroupoid.js

@@ -0,0 +1,31 @@
+"use strict";
+
+// https://medium.com/@safareli/stack-safe-function-composition-85d61feee37e
+var runComposition = function(composition, x) {
+  var root = composition;
+  var val = x;
+  var stack = [];
+  for (;;) {
+    if (root._0 !== undefined){
+      stack.push(root._1);
+      root = root._0;
+    } else {
+      val = root(val);
+      if (stack.length === 0) {
+        return val;
+      }
+      root = stack.shift();
+    }
+  }
+};
+
+exports.functionCompose = function(f) {
+  return function(g) {
+    var res = function composition(x) {
+      return runComposition(composition, x);
+    };
+    res._0 = g;
+    res._1 = f;
+    return res;
+  };
+};

src/Control/Semigroupoid.purs

@@ -13,7 +13,9 @@ class Semigroupoid a where
   compose :: forall b c d. a c d -> a b c -> a b d
 
 instance semigroupoidFn :: Semigroupoid (->) where
-  compose f g x = f (g x)
+  compose = functionCompose
+
+foreign import functionCompose :: forall b c d. (c -> d) -> (b -> c) -> (b -> d)
 
 infixr 9 compose as <<<
 

test/Test/Main.purs

@@ -6,11 +6,21 @@ type AlmostEff = Unit -> Unit
 
 main :: AlmostEff
 main = do
+    functionComposition
     testNumberShow show
     testOrderings
     testOrdUtils
     testIntDegree
 
+functionComposition :: AlmostEff
+functionComposition =
+    assert
+        ("composition is stack safe")
+        (composeGo (_ + 1) id 0 0 == 100000)
+
+composeGo :: forall x a. Semigroupoid x => x a a -> x a a -> Int -> x a a
+composeGo f acc n = if n == 100000 then acc else composeGo f (compose acc f) (n + 1)
+
 foreign import testNumberShow :: (Number -> String) -> AlmostEff
 foreign import throwErr :: String -> AlmostEff