This version looks ok IMO:

"test"
|> then(&"shmest #{&1}")

Right, so "234#{five}" is actually <<"234", five::binary>>, which is invoking Kernel.SpecialForms.<<>>. Piping something into that prepends the value at the front of that binary, if it is a legal parameter to <<>>.

So in the end "1" |> "234#{five}" expands to <<"1", "234", five::binary>> which happens to be legal. I don’t think it’s intentional.

It turns out a |> "#{}" doesn’t mean we are actually interpolating a into the position of #{}

iex(71)> "a" |> "This is: #{}, and #{last}"
"aThis is: , and b"
iex(72)> "a" |> "This is: , and #{last}"
"aThis is: , and b"

Underneath, it’s

iex> quote do: "aa" |> "bb#{}" 
{:|>, [context: Elixir, import: Kernel],
 [
   "aa",
   {:<<>>, [],
    [
      "bb",
      {:::, [],
       [
         {{:., [], [Kernel, :to_string]}, [], [{:__block__, [], []}]},
         {:binary, [], Elixir}
       ]}
    ]}
 ]}

Looks totally OK, it’s relatively new though; there was no such syntax three years ago