work in progress

index.js

@@ -345,6 +345,7 @@
   if (typeof module === 'object' && typeof module.exports === 'object') {
     module.exports = f (require ('sanctuary-def'),
                         require ('sanctuary-either'),
+                        require ('sanctuary-identity'),
                         require ('sanctuary-maybe'),
                         require ('sanctuary-pair'),
                         require ('sanctuary-show'),
@@ -353,6 +354,7 @@
   } else if (typeof define === 'function' && define.amd != null) {
     define (['sanctuary-def',
              'sanctuary-either',
+             'sanctuary-identity',
              'sanctuary-maybe',
              'sanctuary-pair',
              'sanctuary-show',
@@ -362,14 +364,15 @@
   } else {
     self.sanctuary = f (self.sanctuaryDef,
                         self.sanctuaryEither,
+                        self.sanctuaryIdentity,
                         self.sanctuaryMaybe,
                         self.sanctuaryPair,
                         self.sanctuaryShow,
                         self.sanctuaryTypeClasses,
                         self.sanctuaryTypeIdentifiers);
   }
 
-} (function($, Either, Maybe, Pair, show, Z, type) {
+} (function($, Either, Identity, Maybe, Pair, show, Z, type) {
 
   'use strict';
 
@@ -4748,6 +4751,62 @@
     impl: splitOnRegex
   };
 
+  //. ### Lens
+
+  //# lens :: TK
+  //.
+  //. TK.
+  //.
+  //. ```javascript
+  //. > S.toUpper ('ABC def 123')
+  //. 'ABC DEF 123'
+  //. ```
+  function lens(getter) {
+    return function(setter) {
+      return function(f) {
+        return function(s) {
+          return Z.map (
+            function(v) { return setter (v) (s); },
+            f (getter (s))
+          );
+        };
+      };
+    };
+  }
+  _.lens = {
+    consts: {},
+    types: function(s) {
+      return [$.Fn (s) (a), $.Fn (a) ($.Fn (s) (s)), $.Any];
+    } ($.TypeVariable ('s')),
+    impl: lens
+  };
+
+  //# view :: TK
+  function view(lens) {
+    return function(x) {
+      return (lens (Left) (x)).value;
+    };
+  }
+  _.view = {
+    consts: {},
+    types: [$.Any, $.Any, $.Any],
+    impl: view
+  };
+
+  //# over :: TK
+  function over(lens) {
+    return function(f) {
+      return function(x) {
+        return Z.extract (lens (function(y) { return Identity (f (y)); }) (x));
+      };
+    };
+  }
+  _.over = {
+    consts: {},
+    types: [$.Any, $.Any, $.Any, $.Any],
+    impl: over
+  };
+
   return create ({
     checkTypes: typeof process === 'undefined'
                 || process == null

test/lens.js

@@ -0,0 +1,12 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('lens', () => {
+
+  eq (S.show (S.lens)) ('lens :: (s -> a) -> (a -> s -> s) -> Any');
+
+});

test/over.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('over', () => {
+
+  eq (S.show (S.over)) ('over :: Any -> Any -> Any -> Any');
+
+  const snd = S.lens (S.snd) (x => S.map (S.K (x)));
+  const fahrenheit = S.lens (c => c * 9 / 5 + 32) (f => c => (f - 32) * 5 / 9);
+
+  // Increase the temperature in San Francisco by 1 °C
+  eq (S.over (snd) (S.add (1)) (S.Pair ('San Francisco') (20)))
+     (S.Pair ('San Francisco') (21));
+
+  // 1 °F above freezing, in °C
+  eq (S.over (fahrenheit) (S.add (1)) (0)) (0.5555555555555556);
+
+  // 1 °F above freezing, in °F
+  eq (S.view (fahrenheit) (S.over (fahrenheit) (S.add (1)) (0))) (33);
+
+  // Decrease the temperature in San Francisco by 4 °F
+  eq (S.over (S.compose (snd) (fahrenheit))
+             (S.sub (4))
+             (S.Pair ('San Francisco') (20)))
+     (S.Pair ('San Francisco') (17.77777777777778));
+
+});

test/set.js

@@ -0,0 +1,12 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('set', () => {
+
+  eq (S.show (S.set)) ('set :: TK');
+
+});

test/view.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const S = require ('..');
+
+const eq = require ('./internal/eq');
+
+
+test ('view', () => {
+
+  eq (S.show (S.view)) ('view :: Any -> Any -> Any');
+
+  const snd = S.lens (S.snd) (x => S.map (S.K (x)));
+  const fahrenheit = S.lens (c => c * 9 / 5 + 32) (f => c => (f - 32) * 5 / 9);
+
+  eq (S.view (snd) (S.Pair ('San Francisco') (20))) (20);
+
+  eq (S.view (fahrenheit) (0)) (32);
+  eq (S.view (fahrenheit) (50)) (122);
+  eq (S.view (fahrenheit) (100)) (212);
+
+  // Temperature in San Francisco, converted to °F
+  eq (S.view (S.compose (snd) (fahrenheit))
+             (S.Pair ('San Francisco') (20)))
+     (68);
+
+  // Lenses compose via function composition
+  eq (S.view (x => snd (fahrenheit (x)))
+             (S.Pair ('San Francisco') (20)))
+     (68);
+
+});