What is the << (double left arrow) syntax in YAML called, and where's it specced?
It is called the Merge Key Language-Independent Type for YAML version 1.1. and specced here
It is something that parsers can optionally support, it is essentially an interpretation of the key-value pair with the special key <<
, where the value is either a mapping (usually indicated via an alias as in the spec, and although that doesn't seem to be required, it makes little sense not to use an alias) or a list of mappings (i.e. aliases of mappings), and gets interpreted in a special way.
To add on to other answers:
IMO, the example from "Learn yaml in Y Minutes" is incomplete because it doesn't show what happens when the keys are the same. For example:
base: &base
name: Everyone has same name
age: 5
foo: &foo
<<: *base
bar: &bar
<<: *base
age: 20
For the bottom two items yields:
foo:
name: Everyone has same name
age: 5
bar:
name: Everyone has same name
age: 20
bar
overrides the age while foo
does not. According to the spec the entries of the object merging in have a lower priority than those on the object receiving them.