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Editorial Team
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Asked: 2026-05-14T22:41:07+00:00

I remember reading about the double pipe operators — ||> and <|| — somewhere

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I remember reading about the double pipe operators — ||> and <|| — somewhere and now I can’t remember where. I can’t find them on MSDN or in the language spec. Are they documented anywhere?

Example

let print a b = sprintf "%O %O" a b
(1, 2) ||> print
// val it : string = "1 2"
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1 Answer

  1. Editorial Team

    Double (forward/backward) pipe operators are documented in the list of F# operators on MSDN and are also documented as a function exported from the Core.Operators module.

    This is probably automatically generated from the XML documentation in the F# sources, so the pages have somewhat cryptic names:

    As a side-note, finding the operator using search engines is a bit of a problem, so I looked in the F# sources (distributed with CTP release) and the prim-types.fs includes the following:

    /// <summary>Apply a function to two values, the 
///   values being a pair on the left, the function on the right</summary>
/// <param name="arg1">The first argument.</param>
/// <param name="arg2">The second argument.</param>
/// <param name="func">The function.</param>
/// <returns>The function result.</returns>
val inline (||>): arg1:'T1 * arg2:'T2 -> func:('T1 -> 'T2 -> 'U) -> 'U

    I was going to recommend the F# sources as a good documentation for this kind of thing (which they certainly are), but then I pasted a part of the <summary> tag to google and found the pages mentioned above :-).

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