diff --git a/index.js b/index.js
index 6ce1c53..0d898a3 100644
--- a/index.js
+++ b/index.js
@@ -220,6 +220,9 @@
   //  Right :: b -> Either a b
   var Right = Either.Right;
 
+  //  lefts :: (Filterable f, Functor f) => f (Either a b) -> f a
+  var lefts = Z.compose (map (prop ('value')), filter (prop ('isLeft')));
+
   //  B :: (b -> c) -> (a -> b) -> a -> c
   function B(f) {
     return function(g) {
@@ -251,6 +254,13 @@
     };
   }
 
+  //  map :: Functor f => (a -⁠> b) -⁠> f a -⁠> f b
+  function map(f) {
+    return function(xs) {
+      return Z.map (f, xs);
+    };
+  }
+
   //  init :: Array a -> Array a
   function init(xs) { return xs.slice (0, -1); }
 
@@ -1499,6 +1509,95 @@
     };
   }
 
+  //# validate :: Type -> a -> Either (Array ValidationError) a
+  //.
+  //. Takes a type, and any value. Returns `Right a` if
+  //. the value is a member of the type;
+  //. `Left (Array ValidationError)` for each property
+  //. that is invalid. The first index in a `Left` array
+  //. is always named `$$`, which refers to the entire value.
+  function validate(t) {
+    return function(x) {
+      //  $$Result :: {value, propPath} e => Either e a
+      var $$Result = t.validate ([]) (x);
+
+      //  props :: Array (Either ValidationError TestObject)
+      var props = t.keys.map (function(p) {
+        return x == null
+          ? Left ({
+              error: 'MissingValue',
+              type: t.name || t.type,
+              name: p,
+              value: x
+            })
+          : Right ({
+              name: p,
+              type: t.types[p],
+              value: x[p]
+            });
+      });
+
+      //  validateTestObject :: TestObject -> Either ValidationError TestObject
+      var validateTestObject = Z.compose (function(p) {
+        if (p.result.isRight) {
+          return Right (p);
+        } else if (p.name in x) {
+          return Left ({
+            error: 'WrongValue',
+            // TODO: figure out what propPath really is
+            type: p.result.value.propPath.length > 0
+              ? p.type.types[p.result.value.propPath[0]].name
+              : p.type.name,
+            name: p.name,
+            value: p.value
+          });
+        } else {
+          return Left ({
+            error: 'MissingValue',
+            type: p.type.name,
+            name: p.name,
+            value: p.value
+          });
+        }
+      }, function(p) {
+        return {
+          name: p.name,
+          result: p.type.validate ([]) (p.value),
+          type: p.type,
+          value: p.value
+        };
+      });
+
+      if ($$Result.isLeft) {
+        //  tmp0 :: Array (ValidationError)
+        var tmp0 = lefts (Z.map (function(prop) {
+          return Z.chain (validateTestObject, prop);
+        }, props));
+
+        //  tmp1 :: Array (ValidationError)
+        var tmp1 = Z.prepend ({
+          error: 'WrongValue',
+          type: t.name || t.type,
+          name: '$$',
+          value: x
+        }, tmp0);
+
+        // return :: Left (Array ValidationError)
+        return Left (tmp1);
+      } else {
+        // return :: Right a
+        return $$Result;
+      }
+
+      // return Z.concat (
+      //   returnValue,
+      //   Z.filter (
+      //     either => either.isLeft,
+      //     Z.map (prop => Z.map (validateRights, prop), props))
+      // );
+    };
+  }
+
   //. ### Type constructors
   //.
   //. sanctuary-def provides several functions for defining types.
@@ -2906,6 +3005,11 @@
           ({})
           ([Array_ (Type), Type, Any, Boolean_])
           (test),
+    validate:
+      def ('validate')
+          ({})
+          ([Type, Any, Either_ (Array_ (Object_)) (Any)])
+          (validate),
     NullaryType:
       def ('NullaryType')
           ({})
diff --git a/test/index.js b/test/index.js
index 71ed57e..fb9e0f5 100644
--- a/test/index.js
+++ b/test/index.js
@@ -3877,3 +3877,140 @@ suite ('interoperability', () => {
   });
 
 });
+
+suite ('validate', () => {
+
+  test ('Undefined', () => {
+
+    eq ($.validate ($.Undefined) (undefined))
+       (Right (undefined));
+
+  });
+
+  test ('NamedRecordType', () => {
+    //    FooBar :: Type
+    const FooBar = $.NamedRecordType
+      ('FooBar')
+      ('')
+      ([])
+      ({foo: $.String,
+        bar: $.Number});
+
+    // null is not a member of ‘FooBar’
+    eq ($.validate (FooBar) (null))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': null},
+         {'error': 'MissingValue', 'name': 'bar', 'type': 'FooBar', 'value': null},
+         {'error': 'MissingValue', 'name': 'foo', 'type': 'FooBar', 'value': null},
+       ]));
+
+    // undefined is not a member of ‘FooBar’
+    eq ($.validate (FooBar) (undefined))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': undefined},
+         {'error': 'MissingValue', 'name': 'bar', 'type': 'FooBar', 'value': undefined},
+         {'error': 'MissingValue', 'name': 'foo', 'type': 'FooBar', 'value': undefined},
+       ]));
+
+    // ''bar' field is missing', ''foo' field is missing'
+    eq ($.validate (FooBar) ({}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': {}},
+         {'error': 'MissingValue', 'name': 'bar', 'type': 'Number', 'value': undefined},
+         {'error': 'MissingValue', 'name': 'foo', 'type': 'String', 'value': undefined},
+       ]));
+
+    // 'bar' field is missing
+    eq ($.validate (FooBar) ({foo: null}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': {'foo': null}},
+         {'error': 'MissingValue', 'name': 'bar', 'type': 'Number', 'value': undefined},
+         {'error': 'WrongValue', 'name': 'foo', 'type': 'String', 'value': null},
+       ]));
+
+    // Value of 'bar' field, null, is not a member of ‘Number’
+    eq ($.validate (FooBar) ({foo: null, bar: null}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': {'bar': null, 'foo': null}},
+         {'error': 'WrongValue', 'name': 'bar', 'type': 'Number', 'value': null},
+         {'error': 'WrongValue', 'name': 'foo', 'type': 'String', 'value': null},
+       ]));
+
+      // Value of 'foo' field, null, is not a member of ‘String’
+    eq ($.validate (FooBar) ({foo: null, bar: 42}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'FooBar', 'value': {'bar': 42, 'foo': null}},
+         {'error': 'WrongValue', 'name': 'foo', 'type': 'String', 'value': null},
+       ]));
+
+    eq ($.validate (FooBar) ({foo: 'blue', bar: 42}))
+       (Right ({foo: 'blue', bar: 42}));
+
+  });
+
+  test ('Custom Type', () => {
+
+    //    $DateIso :: NullaryType
+    const $DateIso = (
+      $.NullaryType ('DateIso')
+                    ('https://www.w3.org/QA/Tips/iso-date')
+                    ([$.String])
+                    (x => /^\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[1-2]\d|3[0-1])$/.test (x))
+    );
+
+    const model1 = $.RecordType ({
+      date: $DateIso,
+    });
+
+    const model2 = $.RecordType ({
+      date: $.NonEmpty ($DateIso),
+      bool: $.Boolean,
+    });
+
+    eq ($.validate (model1) ({date: '2020-04-10'}))
+       (Right ({date: '2020-04-10'}));
+
+    eq ($.validate (model1) ({date: '2020-04-100'}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': {'date': '2020-04-100'}},
+         {'error': 'WrongValue', 'name': 'date', 'type': 'DateIso', 'value': '2020-04-100'},
+       ]));
+
+    eq ($.validate (model2) (undefined))
+       (Left ([
+          {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': undefined},
+          {'error': 'MissingValue', 'name': 'date', 'type': 'RECORD', 'value': undefined},
+          {'error': 'MissingValue', 'name': 'bool', 'type': 'RECORD', 'value': undefined},
+       ]));
+
+    eq ($.validate (model2) ({bool: 'foobar', date: '2020-04-100'}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': {'bool': 'foobar', 'date': '2020-04-100'}},
+         {'error': 'WrongValue', 'name': 'date', 'type': 'DateIso', 'value': '2020-04-100'},
+         {'error': 'WrongValue', 'name': 'bool', 'type': 'Boolean', 'value': 'foobar'},
+       ]));
+
+    eq ($.validate (model2) ({date: '2020-04-10', bool: 'foobar'}))
+       (Left ([
+          {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': {'bool': 'foobar', 'date': '2020-04-10'}},
+          {'error': 'WrongValue', 'name': 'bool', 'type': 'Boolean', 'value': 'foobar'},
+       ]));
+
+    eq ($.validate (model2) ({date: '2020-04-100', bool: true}))
+       (Left ([
+          {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': {'bool': true, 'date': '2020-04-100'}},
+          {'error': 'WrongValue', 'name': 'date', 'type': 'DateIso', 'value': '2020-04-100'},
+       ]));
+
+    eq ($.validate (model2) ({date: [], bool: false}))
+       (Left ([
+         {'error': 'WrongValue', 'name': '$$', 'type': 'RECORD', 'value': {'bool': false, 'date': []}},
+         {'error': 'WrongValue', 'name': 'date', 'type': 'NonEmpty', 'value': []},
+       ]));
+
+    eq ($.validate (model2) ({bool: false, date: '2020-04-10'}))
+       (Right ({date: '2020-04-10', bool: false}));
+
+  });
+
+});