trait Listen[F[_], L] extends Tell[F, L] with Serializable
Listen[F, L] is a function F[A] => F[(A, L)] which exposes some state
that is contained in all F[A] values, and can be modified using tell.
Listen has two external laws:
def listenRespectsTell(l: L) = { listen(tell(l)) <-> tell(l).as(((), l)) } def listenAddsNoEffects(fa: F[A]) = { listen(fa).map(_._1) <-> fa }
Listen has one internal law:
def listensIsListenThenMap(fa: F[A], f: L => B) = { listens(fa)(f) <-> listen(fa).map { case (a, l) => (a, f(l)) } }
- Annotations
- @implicitNotFound("Could not find an implicit instance of Listen[${F}, ${L}]. If you wish\nto capture side-channel output of type ${L} at this location, you may want\nto construct a value of type WriterT for this call-site, rather than ${F}.\nAn example type:\n\n WriterT[${F}, ${L}, *]\n\nOne use-case for this would be if ${L} represents an accumulation of values\nwhich are produced by this function *in addition to* its normal results.\nThis can be used to implement some forms of pure logging.\n\nIf you do not wish to capture a side-channel of type ${L} at this location,\nyou should add an implicit parameter of this type to your function. For\nexample:\n\n (implicit flisten: Listen[${F}, ${L}])\n")
- Source
- Listen.scala
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