Checked and template generic terminology (#2138)

Change terminology from "generics" and "templates" to "checked generics" and "template generics". Afterwards, "generics" will be an umbrella term that include templates.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

proposals/p2138.md

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+# Checked and template generic terminology
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/2138)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Abstract](#abstract)
+-   [Problem](#problem)
+-   [Proposal](#proposal)
+-   [Rationale](#rationale)
+-   [Alternatives considered](#alternatives-considered)
+    -   [Status quo: "generic" means "checked"](#status-quo-generic-means-checked)
+    -   [Use the name "templates" instead of "template generics"](#use-the-name-templates-instead-of-template-generics)
+    -   [Use the name "unchecked generics" instead of "template generics"](#use-the-name-unchecked-generics-instead-of-template-generics)
+
+<!-- tocstop -->
+
+## Abstract
+
+Change terminology from "generics" and "templates" to "checked generics" and
+"template generics". Afterwards, "generics" will be an umbrella term that
+include templates.
+
+## Problem
+
+The C++ community does "generic programming" using templates in C++, and so
+thinks of templates as a mechanism for implementing generics. In Carbon,
+templates and other generic features are more similar than they are different,
+so it is worthwhile to have an easy way to talk about things that apply to both.
+
+## Proposal
+
+Change terminology from "generics" and "templates" to "checked generics" and
+"template generics". Afterwards, "generics" will be an umbrella term that
+include templates.
+
+Similarly, for parameters, use "checked generic parameter" and "template generic
+parameter", with the umbrella term "generic parameter" describing both kinds.
+The word "generic" can be omitted from these terms if it's clear from context,
+which will usually be the case for template parameters.
+
+Some existing design docs give examples of the difference before and after this
+proposal:
+
+-   The [generics overview](/docs/design/generics/overview.md), for example,
+    currently demonstrates the current approach of using "generics" and
+    "templates".
+-   The
+    [generics section of the design overview](/docs/design/README.md#generics)
+    demonstrates the use of the proposed new terminology.
+
+## Rationale
+
+Using the terminology expected by the community supports these goals:
+
+-   [Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write)
+    since our documentation governing the interpretation of that code will be
+    more easily understood and with greater accuracy.
+-   [Interoperability with and migration from existing C++ code](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code)
+    since there will be a smaller terminology gap between Carbon and C++.
+
+## Alternatives considered
+
+### Status quo: "generic" means "checked"
+
+Use "generic" for checked generics, "template" for template generics, and
+"template or generic" when we mean both.
+
+Advantages:
+
+-   The terms "generic" and "template" are more concise than "checked generic"
+    and "template generic".
+-   The term "template" is more familiar than "template generic" to people
+    coming from C++.
+
+Disadvantages:
+
+-   Does not provide a unifying term for templates and generics, leading to our
+    needing to talk about both in various places, given that they are very
+    similar concepts in Carbon.
+-   Does not acknowledge that templates are used for generic programming and as
+    such are a kind of generic.
+-   Carbon's template generics aren't exactly the same as C++'s templates, so
+    this may give the wrong intuition for people coming from C++ in some cases.
+
+### Use the name "templates" instead of "template generics"
+
+Use "checked generic" for checked generics, "template" for template generics,
+and "generic" when we mean both.
+
+Advantages:
+
+-   More concise term than in this proposal.
+-   More familiar to people coming from C++.
+-   People will likely use this term anyway rather than saying "template
+    generics".
+
+Disadvantages:
+
+-   Does not parallel the term "checked generics".
+-   Loses the implication that a template generic is a kind of generic.
+-   Carbon's template generics aren't exactly the same as C++'s templates, so
+    this may give the wrong intuition for people coming from C++ in some cases.
+
+### Use the name "unchecked generics" instead of "template generics"
+
+Use "checked generic" for checked generics, "unchecked generic" for template
+generics, and "generic" when we mean both.
+
+Advantages:
+
+-   Creates a better parallel between "checked generics" and "unchecked
+    generics".
+-   Describes the semantics rather than the implementation strategy, especially
+    given that templating / monomorphization is also the implementation strategy
+    we intend to use for unchecked generics.
+
+Disadvantages:
+
+-   Unfamiliar to people coming from C++.
+-   Would either need to rename the `template` keyword or pick new non-keyword
+    syntax for it, or have a mismatch between keywords and terminology.
+-   Template generics still involve checking, it just can't complete until later
+    when the argument values are known.
+
+This was suggested in
+[#1443](https://github.com/carbon-language/carbon-lang/pull/1443).