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104 lines
3.6 KiB
Markdown
104 lines
3.6 KiB
Markdown
---
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layout: developer-doc
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title: Typing the Polyglot Bindings
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category: polyglot
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tags: [polyglot, types]
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order: 2
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---
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# Typing the Polyglot Bindings
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The polyglot bindings inherently provide a problem for the Enso type system.
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When many of the languages with which we can interoperate are highly dynamic and
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flexible, or have significant mismatches between their type system and Enso's,
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we can only make a best effort attempt to maintain type safety across this
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boundary.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Enso Values](#enso-values)
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- [Polyglot Values](#polyglot-values)
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- [Dynamic](#dynamic)
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- [The Enso Boundary](#the-enso-boundary)
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<!-- /MarkdownTOC -->
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## Enso Values
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The underlying nature of our runtime allows us to pass Enso values across the
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polyglot boundary while ensuring that they aren't modified. This means that the
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typing information known about a value `v` _before_ it is passed to a polyglot
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call is valid after the polyglot call, as long as the following properties hold:
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- The polyglot call does not modify the entity passed in.
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- The polyglot call returns an entity of the same type.
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However, there are sometimes cases where we _want_ to let an Enso value be used
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freely by the polyglot language. To that end, we have to have some way of
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distinguishing _safe_ usages of Enso values from _unsafe_ ones. In the latter
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case, the value needs to be treated as `Dynamic` after its use.
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> The actionables for this section are:
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>
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> - Think much more on this.
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## Polyglot Values
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In the presence of a polyglot value, however, there is very little that we can
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determine about a value with which we are working. This means that we need to
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have a principled way to assert properties on a polyglot object that can then be
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reflected in the Enso type system. This mechanism needs to deal with:
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- Concurrent access to polyglot objects.
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- Mutation and modification of polyglot objects.
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- Potentially taking _ownership_ of polyglot objects.
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> The actionables for this section are:
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>
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> - Reflect more on this problem and think about what principled approaches we
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> could take to it.
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## Dynamic
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As Enso can seamlessly interoperate with other programming languages, we need a
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principled way of handling dynamic types that we don't really know anything
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about. This mechanism needs:
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- A way to record what properties we _expect_ from the dynamic.
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- A way to turn a dynamic into a well-principled type-system member without
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having the dynamics pollute the whole type system. This may involve a 'trust
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me' function, and potentially dynamicness-polymorphic types.
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- A way to understand as much as possible about what a dynamic _does_ provide.
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- A way to try and refine information about dynamics where possible.
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```ruby
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obj.model =
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{ atom : Text
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, dict : Map Text Any
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, info :
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{ doc : Text
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, name : Text
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, code : Text
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, loc : Location
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}
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, arg : # used only when calling like a function
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{ doc : Text
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, default : Maybe Any
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}
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}
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```
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> The actionables for this section are:
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>
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> - Work out how to do dynamic properly, keeping in mind that in a dynamic value
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> could self-modify underneath us.
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### The Enso Boundary
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Fortunately, we can at least avoid foreign languages modifying memory owned by
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the Enso interpreter. As part of the interop library, Graal lets us mark memory
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as read-only. This means that the majority of data passed out (from a functional
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language like Enso) is not at risk. However, if we _do_ allow data to be worked
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with mutably, when control is returned to Enso it needs to be treated as a
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dynamic as it may have been modified.
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