Justices Alito and Kennedy on Monads vs Applicatives

From the oral arguments in Alleyne v United States (audio):

JUSTICE ALITO: Now, if you were defending a case involving drug weight and your client maintained that he or she had nothing to do with these drugs, how would you proceed? Your argument would be: They're not my drugs, but if they were my drugs, they weren't -- they didn't weigh more than one kilo.

MS. MAGUIRE: Well, Justice Alito, those are strategical questions that come up in every trial case that we have. ... But those -- those strategic decisions exist whether or not the Court adopts this rule or doesn't adopt the rule.

...

JUSTICE KENNEDY: But -- but isn't it difficult for you to say he had nothing to do with the drugs, plus the drugs didn't weigh more than a certain amount?

MS. MAGUIRE: I don't believe that that is difficult, and I believe that those are decisions that you make in every case. ...

...

JUSTICE KENNEDY: Well, we're not getting very far with this. But one answer you could say is that, in order to preserve the constitutional right, you want us to have a bifurcated trial. I thought you were -- might say that.

MS. MAGUIRE: No, we are not -- we are not asking for a bifurcated trial. We are just asking that if there's one --

JUSTICE KENNEDY: That's good because that's an extra problem.

A bifurcated trial is one in which the jury first decides on a subset of questions, then the trial continues, and the jury later decides the remaining questions.

Alito notices that the trial is set to proceed in applicative style, and sees that the defense will be in an awkward position, since they must prove facts about an incident they claim never took place. Kennedy then offers a bid for a monadic trial, where the result of the first verdict can drive the argument leading to the second. But Ms. Maguire resists, for the same reason Kennedy had in mind: in law as in programming, monadic style brings extra costs.

The cost of a bifurcated trial is that you must interrupt the trial, wait for the jury to deliberate, then wait for the lawyers to adapt their strategies to the verdict, wherein either the lawyers have planned for both outcomes (wasting effort) or the trial is delayed further. Analogously, if you have a monadic query library, every use of >>= aka flatMap:

val X : Column[Bool]
val Y : Column[Int]
val Z : Column[Int]

X flatMap { x =>
    if (x)
        Y
    else
        Z
}

requires the preliminary query to complete, return control and data back to the main program, execute some code, make a new query, send it back to the database again, and resume. And the query optimizer doesn't get the benefit of seeing the whole query at once. This is why LINQ is not strictly monadic (and can operate on expressions, not just higher order functions) and all monadic query languages have some really awkward spots where they try to avoid this problem.

The analogy to speculative planning by the lawyers would be speculative execution of the closure { x => ...} by the client environment, which is theoretically possible but I am aware of no software that actually does this.

(In this case it would seem that even if the court holds that the trial should proceed applicatively, Ms Maguire's client gets the benefit of a monadic trial because he started his appeal after the primary verdict had already been reached; but no other defendant to whom the ruling applies will get this accidental benefit. Perhaps this explains why Ms Maguire seems relatively unconcerned by the issue).