Author: Drew DeVault <firstname.lastname@example.org>
Date: Sat, 4 Dec 2021 21:05:35 +0100
docs: add modules.md
Signed-off-by: Drew DeVault <email@example.com>
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+# Hare Modules
+*This document is informative. It describes the behavior of the upstream Hare
+distribution's build driver, but other Hare implementations may differ, and we
+may revise this behavior in the future.*
+- Describe caching mechanism
+- hare.ini considerations and linking to static libraries
+The **host** is the machine which is running the build driver and Hare
+toolchain. The **target** is the machine which the completed program is expected
+to run on. This may not be the same as the host configuration, for example when
+**cross-compiling**. The **build driver**, located at `cmd/hare`, orchestrates
+this process by collecting the necessary source files to build a Hare program,
+resolving its dependencies, and executing the necessary parts of the toolchain
+in the appropriate order.
+## The build driver and the Hare specification
+The Hare language specification is defined at a layer of abstraction that does
+not include filesystems, leaving it to the implementation to define how Hare
+sources are organized. The upstream Hare distribution maps the concept of a
+"module" onto what the spec defines as a *unit*, and each Hare source file in
+the filesystem provides what the specification refers to as a *subunit*.
+The upstream Hare distribution provides the "hosted" translation environment.
+Hare programs prepared for the "freestanding" environment may also be compiled
+with the upstream distribution, but the standard library is not used in this
+## Build tags
+The upstream distribution defines the concept of a **build tag**, or "tag",
+which is an alphanumeric string and an "inclusive" or "exclusive" bit, which is
+used to control the list of source files considered for inclusion in a Hare
+The environment defines a number of default build tags depending on the target
+system it was configured for. For example, a Linux system running on an x86\_64
+processor defines +linux and +x86\_64 by default, which causes files tagged
++linux or +x86\_64 to be included, and files tagged -linux or -x86\_64 to be
+The host configuration defines a set of default build tags, which may be
+overridden by specifying an alternate target. The `hare version -v` command
+prints out the defaults.
+It is important to note that Hare namespaces and build tags are mutually
+exclusive grammars, thanks to the fact that the + and - symbols may not appear
+in a Hare identifier.
+## Locating modules on the filesystem
+Each module, identified by its namespace, is organized into "root" directory,
+where all of its source files may be found, either as members or descendants.
+This directory corresponds to a file path which is formed by replacing the
+namespace delimiters (`::`) with the path separator for the target host system
+(e.g. `/`). This forms a relative path, which is then applied to each of several
+possible **source roots**.
+A source root is a directory which forms the root of a hierarchy of Hare modules
+and their sources. This directory may also itself be a module, namely the **root
+module**: it provides the unit for the empty namespace, where, for example, the
+"main" function can be found. Generally speaking, there will be at least two
+source roots to choose from: the user's program, and the standard library.
+The current working directory (`.`) is always assigned the highest priority. If
+the `HAREPATH` environment variable is set, it specifies a colon-delimited (`:`)
+list of additional candidates in descending order of preference. If unset, a
+default value is used, which depends on the host configuration, generally
+providing at least the path to the standard library's installation location, as
+well the installation location of third-party Hare modules. The `hare version
+-v` command prints out the defaults configured for this host.
+Each of these source roots is considered in order of precedence by concatenating
+the source root path and the relative path of the desired module, and checking
+if a **valid** Hare module is present. A module is considered valid if it
+contains any regular files, or symlinks to regular files, whose names end in
+`.ha` or `.s`; or if it contains any directories, or symlinks to directories;
+whose names begin with `+` or `-` and which would also be considered valid under
+these criteria, applied recursively.
+The user's program, or any dependency, may *shadow* a module from the standard
+library (or another dependency) by providing a suitably named directory in a
+source root with a higher level of precedence.
+## Assembling the list of source files
+A source file is named with the following convention:
+The \< and \> symbols denote a required parameter, and \[ and \[ denote optional
+parameters. Some example names which follow this convention are:
+The build driver examines the list of files in a given module's root directory,
+eliminating those with incompatible build tags, and produces a list of
+applicable files. Once files with incompatible build tag have been eliminated,
+only one file for a given "name" may be provided, such that a module with the
+files `hello.ha` and `hello.s` is invalid. Only the "ha" and "s" extensions are
+used, respectively denoting Hare sources and assembly sources.
+If any sub-directories of the module's root directory begin with `-` or `+`,
+they are treated as a set of build tags and considered for their compatibility
+with the build driver's active set of build tags. If compatible, the process is
+repeated within that directory, treating its contents as members of the desired
+## Semantics of specific tools
+A summary of how the mechanisms documented above are applied by each tool is
+### hare build, hare run
+The input to this command is the location of the root module for the Hare
+program to be built or run. If the path provided identifies a file, that file is
+used as the sole input file for the root module. If the path identifies a
+directory, the directory is used as the root directory for the root module,
+whose source files are assembled according to the algorithm described above.
+### hare test
+`hare test` walks the current source root (i.e. the current working directory)
+by recursively checking if that directory, and every directory which is a
+descendant of it, is a valid Hare module. Each of these modules is compiled with
+the special +test build tag defined. Dependencies of these modules are also
+built, but with the +test tag unspecified, with the exception of the rt module,
+which provides a special test runner in this mode. The resulting executable is
+executed, which causes all of the `@test` functions in the current source root
+to be executed.
+The command line arguments for hare test, if given at all, are interpreted by
+rt+test as a list of namespace wildcards (see [fnmatch]) defining which subsets
+of the test suite to run.
+The `haredoc` command accepts a list of identifiers to fetch documentation for,
+using the same identifiers which the user might use in a Hare source file to
+utilize the corresponding module or declaration.
+The desired identifier is converted to a path. If this path refers to a
+directory which is a valid Hare module, documentation for that module is shown.
+If that path refers to a directory which is not a valid Hare module, it is
+walked to determine if any of its sub-directories are valid Hare modules; if so,
+a list of those sub-directories is shown. If the path does not exist, the most
+specific component of the identifier is removed, and looked up as a module,
+within which the least-significant component is looked up as a declaration
+exported from that module. If the module or this declaration still is not found,
+the identifier is deemed unresolvable and an error is shown.