Docs about naming Primitives

Author: eregon

doc/contributor/primitives.md

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+# Primitives
+
+## Naming
+
+Primitives are generally named `#{module or class to which it belongs}_#{name of the operation}` such as `string_start_with?`.
+
+The `object_` prefix should only be used for instance variables-related operations, like `object_ivars`.
+
+For primitives which are not specific to a module or class, use no prefix, such as `Primitive.is_a?`/`Primitive.equal?`.
+
+For primitives used in many places it is nice to have a shorter name.
+OTOH for primitives used in a single place, e.g., to implement part of the logic in Ruby, then a longer name is fine.