Section on type lambdas

Author: odersky

docs/docs/reference/type-lambdas.md

@@ -0,0 +1,22 @@
+---
+layout: doc-page
+title: "Type Lambdas"
+---
+
+A _type lambda_ lets one express a higher-kinded type directly, without
+a type definition.
+
+    [+X, Y] => Map[Y, X]
+
+For instance, the type above defines a binary type constructor, with a
+covariant parameter `X` and a non-variant parameter `Y`. The
+constructor maps arguments `S` and `T` to `Map[T, S]`. Type parameters
+of type lambdas can have variances and bounds. A parameterized type
+definition or declaration such as
+
+    type T[X] = (X, X)
+
+is a shorthand for a plain type definition with a type-lambda as its
+right-hand side:
+
+    type T = [X] => (X, X)

docs/sidebar.yml

@@ -7,6 +7,8 @@ sidebar:
           url: docs/reference/intersection-types.html
         - title: Union types
           url: docs/reference/union-types.html
+        - title: Type lambdas
+          url: docs/reference/type-lambdas.html
         - title: Trait Parameters
           url: docs/reference/trait-parameters.html
         - title: Enumerations

tests/pos/reference/type-lambdas.scala

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+package typeLambdas
+
+object Test {
+
+  type T =  [+X, Y] => Map[Y, X]
+
+}