Edit errors.md using inpage editor

Summary:
This diff has been automatically generated by the inpage editor.
                        If you want to update this diff, go through the preview link that would be attached to the test plan.

Reviewed By: dkgi

Differential Revision: D30354087

fbshipit-source-id: f4f01df4eee6075b7233de21882b4a6051993f13

Author: pradeep90

documentation/website/docs/errors.md

@@ -70,6 +70,42 @@ def zeroes(number_of_elements: int) -> List[float]:
   return a # Type checks!
 ```
 
+#### Contravariance
+
+`Callable`, on the other hand, is [contravariant](https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Contravariant_method_parameter_type) in its parameter types. This means that, when checking if `Callable[[A], None]` is compatible with `Callable[[B], None]`, we check if `B` is compatible with `A`, not the other way around. This is because the former should be capable of accepting any arguments accepted by the latter.
+
+For example, a function of type `Callable[[Base], int]` may be given an argument of type `Child2`. But if we passed in a function of type `Callable[[Child1], int]`, this could fail at runtime:
+
+```python
+class Base: pass
+
+class Child1(Base):
+    size: int = 42
+
+# No size field.
+class Child2(Base): pass
+
+def print_child2_size(get_size: Callable[[Base], int]) -> None:
+    child2 = Child2()
+    size = get_size(child2)
+    print(size)
+
+def size_of_child1(child1: Child1) -> int:
+    return child1.size
+
+print_child2_size(size_of_child1) # BAD!
+
+# At runtime:
+# AttributeError: 'Child2' object has no attribute 'size'
+```
+
+To prevent such errors, Pyre raises a type error when violating contravariance:
+
+```
+$ pyre
+Incompatible parameter type [6]: Expected `typing.Callable[[Base], int]` for 1st positional only parameter to call `print_child2_size` but got `typing.Callable(size_of_child1)[[Named(child1, Child1)], int]`.
+```
+
 ### Optional Attributes
 A common pattern in Python is to check whether an attribute is `None` before accessing its value. E.g.