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Geoff RomerandRichard Smith
44ba541f1f
This proposal updates the design of [p0157](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p0157.md) to reflect subsequent evolution of the language. Resolves #1805 Co-authored-by: Richard Smith <richard@metafoo.co.uk>
74 lines
3.0 KiB
Markdown
74 lines
3.0 KiB
Markdown
# Update sum types design
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Part of the Carbon Language project, under the Apache License v2.0 with LLVM
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Exceptions. See /LICENSE for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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[Pull request](https://github.com/carbon-language/carbon-lang/pull/2187)
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## Table of contents
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- [Abstract](#abstract)
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- [Problem](#problem)
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- [Proposal](#proposal)
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- [Alternatives considered](#alternatives-considered)
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- [Keep the continuation interface as a parameter](#keep-the-continuation-interface-as-a-parameter)
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- [Require the continuation to execute the whole `case` body](#require-the-continuation-to-execute-the-whole-case-body)
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## Abstract
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This proposal updates the design of [#157](p0157.md) to reflect subsequent
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evolution of the language.
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## Problem
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Some aspects of the design of #157 have become inconsistent with the rest of the
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language, or can be made more precise in light of subsequent language
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development.
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## Proposal
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- Make the continuation interface an associated type rather than an interface
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parameter.
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- For patterns that can be matched using either `Match` or `==`, require both
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implementations to be well-formed.
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- Use `name: Type` order instead of `Type: name` order in alternative
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declarations.
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- Use current syntax and semantics for generics and class types.
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- Rename `Matchable.Match` to `Match.Op`, following the resolution of
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[#1058](https://github.com/carbon-language/carbon-lang/issues/1058).
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## Alternatives considered
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### Keep the continuation interface as a parameter
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Making the continuation interface a parameter of `Match` could in principle
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allow a single type to support pattern matching in multiple ways, by
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implementing `Match` for multiple continuation interfaces. However, that would
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require something like overload resolution on interfaces, to choose the
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implementation of `Match(C)` on the sum type for which `C` best matches the
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continuation constructed by the compiler. No such overloading mechanism is
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planned, and we don't have sufficiently compelling use cases to motivate it.
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### Require the continuation to execute the whole `case` body
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We will probably want to support in-place mutation of alternative parameters
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(for example so you can call a mutable method on the value stored in an
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`Optional(Foo)`), and we might even want to extend that to cases where the
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underlying parameter isn't represented as an lvalue of that type, but has to be
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unpacked by `Match.Op`. The only way I see to make that work is to have
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`Match.Op` unpack the parameter to a local lvalue, pass it to the continuation,
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and then pack the possibly-mutated value back into the sum object after the
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continuation returns. That would mean the compiler has to execute the case body
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inside the continuation, not after `Match.Op` returns.
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However, that's fairly speculative, and wouldn't apply to the read-only cases
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that we currently support, so we need not preemptively constrain the compiler
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here.
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