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josh11bandjonmeow
e0e684067a
Update terminology around expression properties: - "value phase" -> "expression phase" - "symbolic value" -> "symbolic constant" - "constant" -> "template constant" - "constant or symbolic value" -> "constant" Implements the decision in #1391 . --------- Co-authored-by: jonmeow <jperkins@google.com>
94 lines
3.5 KiB
Markdown
94 lines
3.5 KiB
Markdown
# Expression phase terminology
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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/2964)
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## Table of contents
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- [Abstract](#abstract)
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- [Problem](#problem)
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- [Background](#background)
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- [Proposal](#proposal)
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- [Rationale](#rationale)
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- [Alternatives considered](#alternatives-considered)
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## Abstract
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Update terminology around expression properties:
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- "value phase" -> "expression phase"
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- "symbolic value" -> "symbolic constant"
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- "constant" -> "template constant"
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- "constant or symbolic value" -> "constant"
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Implements the decision in
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[#1391](https://github.com/carbon-language/carbon-lang/issues/1391).
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## Problem
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There are a few concerns with the current terminology:
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- We were not happy with it at the time it was introduced in
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[proposal #1378](https://github.com/carbon-language/carbon-lang/pull/1378/files/198c7bd152bc4e223f1fc2484455e17ba31f19f9#r921592116),
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but didn't want to block that proposal on finding better names.
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- It is a property of expressions, not values. We are switching from "value
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categories" to "expression categories" for the same reason. See
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[proposal #2006](https://github.com/carbon-language/carbon-lang/pull/2006)
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and [PR #2744](https://github.com/carbon-language/carbon-lang/pull/2744).
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- No good term for the bindings that use `:!`, which share a number of
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properties, and often we want to refer to those together.
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The concerns about the names of the individual phases led to
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[questions-for-leads issue #1391: New name for "constant" value phase](https://github.com/carbon-language/carbon-lang/issues/1391).
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This proposal implements the resolution from that issue.
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## Background
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The current names were introduced in
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[proposal #1378: Design overview update part 7: values](https://github.com/carbon-language/carbon-lang/pull/1378)
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based on a discussion in
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[#typesystem on Discord](https://discord.com/channels/655572317891461132/708431657849585705/996547601451204630).
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I am not aware of corresponding terminology in other programming languages.
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## Proposal
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We update terminology around expression properties, specifically what we
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previously referred to as "value phase" and values it can have, as follows:
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- "value phase" -> "expression phase": since this is a property of
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expressions, not values.
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- "symbolic value" -> "symbolic constant": for symbolic compile-time values
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like checked-generic parameters.
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- "constant" -> "template constant": for compile-time values where the value
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is available during type checking, like literals and `template` parameters.
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By making these last two terms both end in "constant," we allow their
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combination (including all compile-time values) to be collectively referred to
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using the term "constant." For example, either kind of constant may be passed to
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a function with either kind of constant binding.
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## Rationale
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The goals of this proposal are:
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- clearer and more concise communication, and
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- making it easier for people to learn Carbon by using more consistent
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terminology.
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This supports Carbon's goal of having
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[Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write).
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## Alternatives considered
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We were happy with this option and did not spend time coming up with more
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alternatives.
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