PEP 646: tweaks, clarifications, and move summary example up (#1828)

Readability changes:
* Tweak framing to also acknowledge that variadic generics have been wanted for other things too (though as far as I've been able to tell there isn't a consistent enough pattern to the other use cases to justify including them in the PEP)
* Move the Array example much earlier (both because reading a detailed example is going to be a faster way of getting the gist for people in a hurry, and because we want to make it clear to numerical computing folks from the start that we can also specify a datatype here). (We still have a more detailed Array example later; I'm leaving that there for the time being pending some discussion with Pradeep, but eventually I guess it'll be removed and absorbed into this earlier example)
* Make it clear that other types can appear in `Union` alongside `TypeVarTuple` instances.

Semantic changes:
* Disallow a `Union` of more than one type variable tuple (I thought this would be easier than concatenation of more than one type variable tuple, but Pradeep points out it could still get hairy for type checkers, so disallow it for now to be on the safe safe)

Also some *more* type fixes...

pep-0646.rst

@@ -18,7 +18,8 @@ Abstract
 PEP 484 introduced ``TypeVar``, enabling creation of generics parameterised
 with a single type. In this PEP, we introduce ``TypeVarTuple``, enabling parameterisation
 with an *arbitrary* number of types - that is, a *variadic* type variable,
-enabling *variadic* generics. This allows the type of array-like structures
+enabling *variadic* generics. This enables a wide variety of use cases.
+In particular, it allows the type of array-like structures
 in numerical computing libraries such as NumPy and TensorFlow to be
 parameterised with the array *shape*, enabling static type checkers
 to catch shape-related bugs in code that uses these libraries.
@@ -26,10 +27,15 @@ to catch shape-related bugs in code that uses these libraries.
 Motivation
 ==========
 
-In the context of numerical computation with libraries such as NumPy and
-TensorFlow, the *shape* of arguments is often just as important as the
-argument *type*. For example, consider the following function which converts a
-batch [#batch]_ of videos to grayscale:
+Variadic generics have long been a requested feature, for a myriad of
+use cases [#typing193]_. One particular use case - a use case with potentially
+large impact, and the main case this PEP targets - concerns typing in
+numerical libraries.
+
+In the context of numerical computation with libraries such as NumPy and TensorFlow,
+the *shape* of arguments is often just as important as the argument *type*.
+For example, consider the following function which converts a batch [#batch]_
+of videos to grayscale:
 
 ::
 
@@ -73,6 +79,54 @@ and so on throughout their code) and for the authors of array libraries (who wou
 Variadic generics are necessary for an ``Array`` that is generic in an arbitrary
 number of axes to be cleanly defined as a single class.
 
+Summary Examples
+================
+
+Cutting right to the chase, this PEP allows an ``Array`` class that is generic
+in its shape (and datatype) to be defined using a newly-introduced
+arbitrary-length type variable, ``TypeVarTuple``, as follows:
+
+::
+
+    from typing import TypeVar, TypeVarTuple
+
+    DType = TypeVar('DType')
+    Shape = TypeVarTuple('Shape')
+
+    class Array(Generic[DType, *Shape]):
+
+        def __abs__(self) -> Array[DType, *Shape]: ...
+
+        def __add__(self, other: Array[DType, *Shape]) -> Array[DType, *Shape]: ...
+
+Such an ``Array`` can be used to support a number of different kinds of
+shape annotations. For example, we can add labels describing the
+semantic meaning of each axis:
+
+::
+
+    from typing import NewType
+
+    Height = NewType('Height', int)
+    Width = NewType('Width', int)
+
+    x: Array[float, Height, Width] = Array()
+
+We could also add annotations describing the actual size of each axis:
+
+::
+
+    from typing import Literal
+
+    L640 = Literal[640]
+    L480 = Literal[480]
+
+    x: Array[int, L480, L640] = Array()
+
+For consistency, we use semantic axis annotations as the basis of the examples
+in this PEP, but this PEP is agnostic about which of these two (or possibly other)
+ways of using ``Array`` is preferable; that decision is left to library authors.
+
 Specification
 =============
 
@@ -141,10 +195,6 @@ signatures and variable annotations:
         def get_shape(self) -> Tuple[*Shape]:
             return self._shape
 
-        def __abs__(self) -> Array[*Shape]: ...
-
-        def __add__(self, other: Array[*Shape]) -> Array[*Shape]: ...
-
     shape = (Height(480), Width(640))
     x: Array[Height, Width] = Array(shape)
     y = abs(x)  # Inferred type is Array[Height, Width]
@@ -306,8 +356,6 @@ As of this PEP, only a single type variable tuple may appear in a type parameter
 
     class Array(Generic[*Ts1, *Ts2]): ...  # Error
 
-(``Union`` is the one exception to this rule; see `Type Variable Tuples with Union`_.)
-
 Type Concatenation
 ------------------
 
@@ -326,7 +374,7 @@ prefixed and/or suffixed:
 
     a: Array[Height, Width]
     b = add_batch_axis(a)      # Inferred type is Array[Batch, Height, Width]
-    c = add_batch_axis(b)      # Array[Height, Width]
+    c = del_batch_axis(b)      # Array[Height, Width]
     d = add_batch_channels(a)  # Array[Batch, Height, Width, Channels]
     
 
@@ -447,29 +495,20 @@ Type variable tuples can also be used with ``Union``:
 
     f(1, 'foo')  # Inferred type is Union[int, str]
 
-More than one type variable tuple may appear in the the parameter list
-to ``Union``:
-
-::
-
-  def cond_random_choice(
-      cond: bool,
-      cond_true: Tuple[*Ts1],
-      cond_false: Tuple[*Ts2]
-  ) -> Union[*Ts1, *Ts2]:
-    if cond:
-      return random.choice(cond_true)
-    else:
-      return random.choice(cond_false)
-
-  # Inferred type is Union[int, str, float]
-  cond_random_choice(True, (1, 'foo'), (0.0, 'bar')) 
-
-If the type variable tuple is empty (e.g. if we had ``*args: Tuple[*Ts]``
+Here, if the type variable tuple is empty (e.g. if we had ``*args: *Ts``
 and didn't pass any arguments), the type checker should
 raise an error on the ``Union`` (matching the behaviour of ``Union``
 at runtime, which requires at least one type argument).
 
+Other types can also be included in the ``Union``:
+
+::
+
+    def f(*args :*Ts) -> Union[int, str, *Ts]: ...
+
+However, note that as elsewhere, only a single type variable tuple
+may occur within a ``Union``.
+
 Aliases
 -------
 
@@ -704,6 +743,8 @@ Footnotes
 References
 ==========
 
+.. [#typing193] Python typing issue #193: https://github.com/python/typing/issues/193
+
 .. [#pep-612] PEP 612, "Parameter Specification Variables":
    https://www.python.org/dev/peps/pep-0612