Ecto the pin operator

This is simply a choice of how Ecto wanted to make its api. It helps make it clear when a term refers to a table, and when it refers to an injected value. For example consider the following example:

u = %User{id: 1}
from u in User, where: u.id == ^u.id

The pin operator makes it clear that the right hand side of the == is referring to the external u.

Think of it as a safe interpolation operator: the Ecto query doesn’t see your variables outside the from macro call unless you tell it to look outside itself. You using the ^ is marking that the variable is outside the DSL language of the macro to separate the variables like p that represent a table. This always the macro to detect errors and typos at compile time as it can be sure if a variable is in the DSL or from the scope.