This class allows to add support for Isabelle ML types in Scala in a quick ad-hoc way.

Let something be an ML type. To support values of ML type type in Scala, write

object Something extends AdHocConverter("something")

(Here Something is an arbitrary identifier that should preferably remind of the ML type something.) Then ML values of type something stored in the Isabelle process can be used in Scala as objects of type Something.T. That is, an Something.T is a thin wrapper around an MLValue that references values of type something. The actual ML value is never transferred from Isabelle.

An MLValueConverter is automatically available for type Something.T, thus, e.g., MLValue.apply and MLValue.retrieveNow and related functions work properly to translate between Something.T and MLValue[Something.T].

Note: This is only intended for local use. That is, if some Scala code needs to handle ML values for which there is no proper support, and that just need to be passed around between different ML functions, then it is possible to use this class for supporting them with minimal boilerplate. However, if the Scala types representing the ML values are supposed to be used on a larger scale or exported from a library, or are supposed to be extended, then it is recommended to use a more flexible mechanism such as MLValueWrapper for creating wrapper classes for ML values.

Note: object ... extends AdHocConverter(...) creates a new exception type in ML when it is created. Thus this declaration should only be used in a static context (toplevel/inside an object) or in classes that are instantiated very rarely (or in OperationCollection.Ops in a static object).

Usage example: Say we want to use the ML function Simplifier.pretty_simpset: bool -> Proof.context -> Pretty.T. This function returns a Pretty.T which is not (currently) directly supported by this library. But we simply want to pass it on to Pretty.unformatted_string_of: Pretty.T -> string. So we can declare

object Pretty extends AdHocConverter("Pretty.T")
val pretty_simpset = MLValue.compileFunction[Boolean, Context, Pretty.T]("Simplifier.pretty_simpset")
val unformatted_string_of = MLValue.compileFunction[Pretty.T, String]("Pretty.unformatted_string_of")

and then pretty print the simpset of a context ctxt by

val string : String = unformatted_string_of(pretty_simpset(false, ctxt).retrieveNow).retrieveNow

(Or without the first .retrieveNow.)