Make custom operator without subscription for Combine.

CustomOperatorSample.swift

import Combine

public struct Double<Upstream,Output>: Publisher where Upstream:Publisher, Output : Numeric, Upstream.Output == Output {
    public typealias Failure = Upstream.Failure

    /// The upstream publisher.
    public let upstream: Upstream
    
    public init(upstream: Upstream) {
        self.upstream = upstream
    }

    public func receive<S>(subscriber: S) where S:Subscriber, S.Input==Output, S.Failure==Failure {
        self.upstream
            .map { $0 * 2 }
            .subscribe(subscriber)
    }
}

public struct Triple<Upstream,Output>: Publisher where Upstream:Publisher, Output : Numeric, Upstream.Output == Output {
    public typealias Failure = Upstream.Failure

    /// The upstream publisher.
    public let upstream: Upstream
    public init(upstream: Upstream) {
        self.upstream = upstream
    }

    public func receive<S>(subscriber: S) where S:Subscriber, S.Input==Output, S.Failure==Failure {
        self.upstream
            .map { $0 * 3 }
            .subscribe(subscriber)
    }
    
}

extension Publisher where Output : Numeric {
    
    func double() -> Double<Self, Output> {
        .init(upstream: self)
    }
    
    func triple() -> Triple<Self, Output> {
        .init(upstream: self)
    }
    
}

[1, 2, 3]
    .publisher
    .double()
    .triple()
    .sink { print($0) }

// print: 6
// print: 12
// print: 18