Added function S.on

index.js

@@ -750,6 +750,30 @@
   }
   S.pipe = def('pipe', {}, [$.Array($.Function), a, b], pipe);
 
+  //# on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
+  //.
+  //. Takes a binary function `f`, a unary function `g`, and two
+  //. values `x` and `y`. Returns `f(g(x))(g(y))`.
+  //.
+  //. See also [`on_`](#on_).
+  //.
+  //. ```javascript
+  //. > S.on(S.concat, S.reverse, [1, 2, 3], [4, 5, 6])
+  //. [3, 2, 1, 6, 5, 4]
+  //. ```
+  function on(f, g, x, y) {
+    return on_(uncurry2(f), g, x, y);
+  }
+  S.on = def('on', {}, [$.Function, $.Function, a, a, c], on);
+
+  //# on_ :: ((b, b) -> c) -> (a -> b) -> a -> a -> c
+  //.
+  //. Version of [`on`](#on) accepting uncurried functions.
+  function on_(f, g, x, y) {
+    return f(g(x), g(y));
+  }
+  S.on_ = def('on_', {}, [$.Function, $.Function, a, a, c], on_);
+
   //. ### Maybe type
   //.
   //. The Maybe type represents optional values: a value of type `Maybe a` is

test/on.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var S = require('..');
+
+var eq = require('./internal/eq');
+var rem = require('./internal/rem');
+
+
+describe('on', function() {
+
+  it('is a quaternary function', function() {
+    eq(typeof S.on, 'function');
+    eq(S.on.length, 4);
+  });
+
+  it('passes the last two arguments through the modifying function into the converging function', function() {
+    eq(S.on(rem, S.prop('x'), {x: 5, y: 5}, {x: 3, y: 3}), 2);
+    eq(S.on(S.concat, S.reverse)([1, 2, 3], [4, 5, 6]), [3, 2, 1, 6, 5, 4]);
+  });
+
+});

test/on_.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var S = require('..');
+
+var eq = require('./internal/eq');
+var rem_ = require('./internal/rem_');
+
+
+describe('on_', function() {
+
+  it('is a quaternary function', function() {
+    eq(typeof S.on_, 'function');
+    eq(S.on_.length, 4);
+  });
+
+  it('passes the last two arguments through the modifying function into the converging function', function() {
+    eq(S.on_(rem_, S.prop('x'), {x: 5, y: 5}, {x: 3, y: 3}), 2);
+  });
+
+});