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Jon MeowandRichard Smith
231264e0c0
Starting to apply #339 Co-authored-by: Richard Smith <richard@metafoo.co.uk>
112 lines
4.3 KiB
Markdown
112 lines
4.3 KiB
Markdown
# Templates
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<!--
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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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<!-- toc -->
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## Table of contents
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- [TODO](#todo)
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- [Overview](#overview)
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- [Types with template parameters](#types-with-template-parameters)
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- [Functions with template parameters](#functions-with-template-parameters)
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- [Overloading](#overloading)
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- [Constraining templates with interfaces](#constraining-templates-with-interfaces)
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<!-- tocstop -->
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## TODO
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This is a skeletal design, added to support [the overview](README.md). It should
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not be treated as accepted by the core team; rather, it is a placeholder until
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we have more time to examine this detail. Please feel welcome to rewrite and
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update as appropriate.
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## Overview
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Carbon templates follow the same fundamental paradigm as C++ templates: they are
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instantiated, resulting in late type checking, duck typing, and lazy binding.
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They both enable interoperability between Carbon and C++ and address some
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(hopefully limited) use cases where the type checking rigor imposed by generics
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isn't helpful.
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### Types with template parameters
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When parameterizing a user-defined type, the parameters can be marked as
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template parameters. The resulting type-function will instantiate the
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parameterized definition with the provided arguments to produce a complete type
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when used. Note that only the parameters marked as having this template behavior
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are subject to full instantiation -- other parameters will be type checked and
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bound early to the extent possible. For example:
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```
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struct Stack(Type$$ T) {
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var Array(T) storage;
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fn Push(T value);
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fn Pop() -> T;
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}
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```
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This both defines a parameterized type (`Stack`) and uses one (`Array`). Within
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the definition of the type, the template type parameter `T` can be used in all
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of the places a normal type would be used, and it will only by type checked on
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instantiation.
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### Functions with template parameters
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Both implicit and explicit function parameters in Carbon can be marked as
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template parameters. When called, the arguments to these parameters trigger
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instantiation of the function definition, fully type checking and resolving that
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definition after substituting in the provided (or computed if implicit)
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arguments. The runtime call then passes the remaining arguments to the resulting
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complete definition.
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```
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fn Convert[Type$$ T](T source, Type$$ U) -> U {
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var U converted = source;
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return converted;
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}
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fn Foo(Int i) -> Float {
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// Instantiates with the `T` implicit argument set to `Int` and the `U`
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// explicit argument set to `Float`, then calls with the runtime value `i`.
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return Convert(i, Float);
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}
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```
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Here we deduce one type parameter and explicitly pass another. It is not
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possible to explicitly pass a deduced type parameter, instead the call site
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should cast or convert the argument to control the deduction. The explicit type
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is passed after a runtime parameter. While this makes that type unavailable to
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the declaration of _that_ runtime parameter, it still is a template parameter
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and available to use as a type even within the remaining parts of the function
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declaration.
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### Overloading
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An important feature of templates in C++ is the ability to customize how they
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end up specialized for specific types. Because template parameters (whether as
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type parameters or function parameters) are pattern matched, we expect to
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leverage pattern matching techniques to provide "better match" definitions that
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are selected analogously to specializations in C++ templates. When expressed
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through pattern matching, this may enable things beyond just template parameter
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specialization, but that is an area that we want to explore cautiously.
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### Constraining templates with interfaces
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Because we consider only specific _parameters_ to be templated and they could be
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individually migrated to a constrained interface using the
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[generics system](README.md#generics), constraining templates themselves may be
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less critical. Instead, we expect parameterized types and functions may use a
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mixture of generic parameters and templated parameters based on where they are
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constrained.
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However, if there are still use cases, we would like to explore applying the
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interface constraints of the generics system directly to template parameters
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rather than create a new constraint system.
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