mapchange is not defined in xml

Your code works (apart from the printf). Maybe check your F# or .NET (Core) version. The Collections namespace doesn’t have to be open.

let testMap = Map.empty.Add("test", true).Add("otherTest", false)

let newTestMap = 
    testMap 
    |> Map.change // Here's the error!
            "otherTest" 
            (fun v -> 
                match v with
                | Some b -> Some (not b)
                | None -> None )

printf "%A" newTestMap // map [("otherTest", true); ("test", true)]val it : unit = ()

Map.change

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