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dynamic-dispatch.md

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layout developer-doc
title Dynamic Dispatch
category types
tags
types
dispatch
order 6

Dynamic Dispatch

Enso is a language that supports pervasive dynamic dispatch. This is a big boon for usability, as users can write very flexible code that still plays nicely with the GUI.

The current implementation of Enso supports single dispatch (dispatch purely on the type of this), but there are broader visions afoot for the final implementation of dynamic dispatch in Enso.

The actionables for this section include:

  • Determining whether we want to support proper multiple dispatch in the future. This is important to know as it has implications for the type system, and the design of the dispatch algorithm.

Specificity

In order to determine which of the potential dispatch candidates is the correct one to select, the compiler needs to have a notion of specificity, which is effectively an algorithm for determining which candidate is more specific than another.

  • Always prefer a member function for both x.f y and f y x notations.
  • Only member functions, current module's functions, and imported functions are considered to be in scope. Local variable f could not be used in the x.f y syntax.
  • Selecting the matching function:
    1. Look up the member function. If it exists, select it.
    2. If not, find all functions with the matching name in the current module and all directly imported modules. These functions are the candidates.
    3. Eliminate any candidate X for which there is another candidate Y whose this argument type is strictly more specific. That is, Y this type is a substitution of X this type but not vice versa.
    4. If not all of the remaining candidates have the same this type, the search fails.
    5. Eliminate any candidate X for which there is another candidate Y which type signature is strictly more specific. That is, Y type signature is a substitution of X type signature.
    6. If exactly one candidate remains, select it. Otherwise, the search fails.

The actionables for this section are as follows:

  • THE ABOVE VERSION IS OLD. NEEDS UPDATING.
  • The definition of specificity for dispatch candidates (including how it interacts with the subsumption relationship on typesets and the ordering of arguments).

Multiple Dispatch

It is an open question as to whether we want to support proper multiple dispatch in Enso. Multiple dispatch refers to the dynamic dispatch target being determined based not only on the type of the this argument, but the types of the other arguments to the function.

To do multiple dispatch properly, it is very important to get a rigorous specification of the specificity algorithm. It must account for:

  • The typeset subsumption relationship.

  • The ordering of arguments.

  • How to handle defaulted and lazy arguments.

  • Constraints in types. This means that for two candidates f and g, being dispatched on a type t with constraint c, the more specific candidate is the one that explicitly matches the constraints. An example follows:

    type HasName
  name : String

greet : t -> Nothing in IO
greet _ = print "I have no name!"

greet : (t : HasName) -> Nothing in IO
greet t = print 'Hi, my name is `t.name`!'

type Person
  Pers (name : String)

main =
  p1 = Person.Pers "Joe"
  greet p1 # Hi, my name is Joe!
  greet 7  # I have no name

    Here, because Person conforms to the HasName interface, the second greet implementation is chosen because the constraints make it more specific.