Generic functions, first baby step (#658)

* generic functions: progress on parser and AST

* finished first baby step

* revisions based on reviews

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Symbol => VariableType

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/generic_function1.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn id[T:! Type](x: T) -> T {
+  return x;
+}
+
+fn main() -> Int {
+  return id(0);
+}

executable_semantics/testdata/generic_function1.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/generic_function2.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn fst[T:! Type](x: T, y: T) -> T {
+  return x;
+}
+
+fn main() -> Int {
+  return fst(0, 1);
+}

executable_semantics/testdata/generic_function2.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/generic_function3.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn snd[T:! Type](x: Int, y: T) -> T {
+  return y;
+}
+
+fn main() -> Int {
+  return snd(0, 1);
+}

executable_semantics/testdata/generic_function3.golden

@@ -0,0 +1 @@
+result: 1

executable_semantics/testdata/generic_function_apply.carbon

@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn apply[T:! Type, U:! Type](f: fnty (T) -> U, x: T) -> U {
+  return f(x);
+}
+
+fn positive(x: Bool) -> Int {
+  if (x) {
+    return 2;
+  } else {
+    return -2;
+  }
+}
+
+fn main() -> Int {
+  return apply(positive, false);
+}

executable_semantics/testdata/generic_function_apply.golden

@@ -0,0 +1 @@
+result: -2

executable_semantics/testdata/generic_function_fail1.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn fst[T:! Type](x: T, y: T) -> T {
+  return x;
+}
+
+fn main() -> Int {
+  return fst(0, true);
+}

executable_semantics/testdata/generic_function_fail1.golden

@@ -0,0 +1,4 @@
+ERROR: 10: type error in argument deduction
+expected: Int
+actual: Bool
+EXIT CODE: 255

executable_semantics/testdata/generic_function_fail2.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn id[T:! Type](x: Int) -> Int {
+  return x;
+}
+
+fn main() -> Int {
+  return id(0);
+}

executable_semantics/testdata/generic_function_fail2.golden

@@ -0,0 +1,2 @@
+10: error, could not deduce type argument for type parameter T
+EXIT CODE: 255

executable_semantics/testdata/generic_function_swap.carbon

@@ -0,0 +1,11 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn swap[T:! Type, U:! Type](tuple: (T, U)) -> (U, T) {
+  return (tuple[1], tuple[0]);
+}
+
+fn main() -> Int {
+  return swap((0, true))[1];
+}

executable_semantics/testdata/generic_function_swap.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/generic_function_tuple_map.carbon

@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn map[T:! Type](f: fnty (T) -> T, tuple: (T, T)) -> (T, T) {
+  return (f(tuple[0]), f(tuple[1]));
+}
+
+fn inc(x: Int) -> Int { return x + 1; }
+
+fn main() -> Int {
+  return map(inc, (0, 2))[0];
+}

executable_semantics/testdata/generic_function_tuple_map.golden

@@ -0,0 +1 @@
+result: 1