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I've never written OCaml, just SML and F#, but I thought that OCaml didn't have the |> operator.

OCaml, like SML, have the syntax for defining their own operators. |> is defined, to my knowledge, both in the standard prelude and in core.

Since Ocaml 4 if I remember correctly

4.01 to be exact. Along with @@, which is kinda comparable to $ in Haskell.