Ruby's `[1, 2, 3].inject(:+)` alias for `reduce` (name borrowed from Smalltalk) nicely reinforces that very mnemonic: you inject the operator _between_ the elements: 1 + 2 + 3. :+ here is symbol meaning "send + message", more general form takes a block:

    [1, 2, 3].inject { |a,b| a + b }

The downside of the "between" mnemonic is encouraging not handling an empty collection! Ruby's separate syntax to pass (args) {block} at least offers natural place to add initial value, but with symbol shorthand the API is harder to guess:

    numbers.inject(0) { |a,b| a + b }
    numbers.inject(0, :+)

(Also, it's always a left fold ((a) OP b) OP c, even if you give a symbol like :* which parses right-assiciative without parens — making the "inject operator between values" mental model less accurate.)