FYI, you can achieve the same thing with F# pipelines + partial application proposal [1]

  input |> func(?, 2)
  input |> func(1, ?)

While hack's placeholders work exclusively in pipelines, the partial application proposal can be used with any function

  "123"
    .split("")
    .map(parseInt(?)) // make sure radix param is not set

[1] https://github.com/tc39/proposal-partial-application