go/types: add examples/contracts.go2, extended examples/functions.go2

Change-Id: Icb1c0dfc6e752598a51d8039b5df77cde9a115ca

src/go/types/README

@@ -1,20 +1,21 @@
-This version of go/types (and related go/*) libraries contains the changes
-to type-check generic code as outlined in the latest contracts proposal, as
-presented by Ian Taylor at GopherCon 2019.
+This version of go/types (and related go/*) libraries contains changes
+to type-check generic code as outlined in the latest contracts proposal,
+as presented by Ian Taylor at GopherCon 2019.
 
-NOTE: THIS IS A PROTOTYPE. NOT EVERYTHING IS IMPLEMENTED. THERE ARE BUGS.
+CAUTUION: EARLY PROTOTYPE. A LOT IS STILL MISSING. THERE ARE BUGS.
 
-The code is not very well tested, some parts were hacked up quickly,
-and no code review has happened. Read and use the code at your own risk.
+The code is not tested enough, many parts need a rewrite/cleanup
+and no code review has happened. Read and use at your own risk.
 
-Specifically, the following pieces are missing from type-checking or lead
-to unexpected behavior:
+Specifically, the following pieces (and more) are missing from type-
+checking or lead to unexpected behavior:
 
 - Importing of packages with type parameters or contracts.
 - Type-checking of contracts and type parameter lists with contracts.
 - Alias type names with type parameters.
+- gofmt (and any other tools) don't work for parameterized code.
 
-The following is "working" (as in passes simple tests):
+The following is "working" (as in "passes simple tests"):
 
 - Parsing of parameterized types and functions.
 - Parsing of contract declarations.
@@ -22,6 +23,9 @@ The following is "working" (as in passes simple tests):
 - Declaration and use (calls) of parameterized functions without
   contracts, including type inference from function arguments.
 
+Error messages, where present, are in usable condition but expect
+them to be significantly better in a real implementation.
+
 To play with this prototype:
 
 - Cherry-pick this CL on top of tip:
@@ -37,6 +41,10 @@ To play with this prototype:
 - Run it against a test case (assuming gotype was installed in a location that
   is in your $PATH):
 
-  gotype example.go2
+  gotype $GOROOT/src/go/types/examples/functions.go2
 
 See also `gotype -h` for more information.
+
+Note: Currently, files containing parameterized code have extension ".go2".
+This is solely to distinguish them from regular Go code and to prevent gofmt
+from touching them. We expect a proper implementation to keep using ".go".

src/go/types/examples/contracts.go2

@@ -0,0 +1,50 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows examples of contract declarations.
+// They are not type-checked at the moment.
+
+package p
+
+// A contract declaration, like any other declaration,
+// starts with a keyword, followed by the contract name
+// and the contract type parameters, and then the contract
+// specification.
+contract C(T) {
+	T m()
+}
+
+// Contracts may be empty.
+contract Empty() {}
+
+// Contracts may be grouped.
+contract (
+	C1(T) {}
+	C2(T) {}
+	C3(T) {}
+)
+
+// A contract specifies methods and types for each of the
+// type parameters it constrains.
+contract Stringer(T) {
+	T String() string
+}
+
+contract Sequence(T) {
+	T string, []byte
+}
+
+// Contracts may constrain multiple type parameters
+// in mutually recursive ways.
+contract G(Node, Edge) {
+	Node Edges() []Edge
+	Edge Nodes() (from Node, to Node)
+}
+
+type Graph (type Node, Edge G) struct { /* ... */ }
+
+func New (type Node, Edge G) (nodes []Node) *Graph(Node, Edge) { panic("unimplemented") }
+
+func (g *Graph(Node, Edge)) ShortestPath(from, to Node) []Edge { panic("unimplemented") }
+

src/go/types/examples/functions.go2

@@ -62,3 +62,11 @@ var s = foo(int, string, float64)(1, []string{"first"}, new(float64)())
 // Or we can use type inference.
 var _ float64 = foo(42, []float64{1.0}, &s)
 
+// Type inference works in a straight-forward manner even
+// for variadic functions.
+func variadic(type A, B)(A, B, ...B) int { panic("unimplemented") }
+
+// var _ = variadic(1) // ERROR not enough arguments
+var _ = variadic(1, 2.3)
+var _ = variadic(1, 2.3, 3.4, 4.5)
+var _ = variadic(int, float64)(1, 2.3, 3.4, 4)

src/go/types/testdata/contracts.go2

@@ -11,7 +11,7 @@ contract _(A, B, C){}
 contract _(A, B, A /* ERROR A redeclared */ ){}
 
 // For now we also allow this form of contract declaration.
-// TODO(gri) probably not a good idea. Disallow a local level for sure.
+// TODO(gri) probably not a good idea. Disallow at local level for sure.
 type _ contract(){}
 type _ contract(A, B, A /* ERROR A redeclared */ ){}