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--- language:namespaces [2019/04/18 14:30]
rajit [Creating a namespace]
+++ language:namespaces [2020/12/02 01:00]
127.0.0.1 external edit
@@ Line 83: / Line 83: @@
 or other constructs used to construct circuits, other namespaces can
 only have global data types or channels---i.e. no circuits.
+===== Importing namespaces =====
+There are two mechanisms to import other files in ACT. These
+imports have to be at the beginning of an ACT file. If the same
+file is imported twice within the same scope, ACT automatically
+skips the second import.
+The basic form of import statement is shown below:
+<code>
+import "channel.act";
+...
+</code>
+This import searches for a file called ''channel.act'' to be read as
+part of the ACT design. ACT has multiple mechanisms that can be used
+to specify search paths for the files that an import statement can access:
+   * the current directory: if the file is located in the current working directory, then it is used.
+   * the colon-separated path specified by ''$ACT_PATH'': The ACT_PATH environment variable can be used to specify a colon-separated list of directories. These are searched for the file specified in the import statement after the current working directory. This is typically used to specify project-specific configuration.
+   * ''$ACT_HOME/act'': This path is used for system ACT files, shared across multiple projects.
+A second version of import uses namespaces directly, but requires that
+ACT files be placed in locations that match the namespace
+hierarchy. The import statement
+<code>
+import processor::lib;
+</code>
+is equivalent to the following:
+<code>
+import "processor/lib/_all_.act";
+</code>
+It assumes that the file ''_all_.act'' in the directory
+''processor/lib'' contains all the definitions corresponding to the
+''processor::lib'' namespace. More precisely, an import statement
+<code>
+import foo;
+</code>
+would do the following:
+  * Look for ''foo/_all_.act'' using the search paths specified earlier;
+  * If unsuccessful, then look for ''foo.act''
+  * If unsuccessful, report an error
+After the import, the specified namespace is checked to see if it exists. If not, an error will be reported.
+===== Opening namespaces =====
+If there are two different files that define the same namespace (say defined in multiple projects), importing both the files may result in type conflicts. Consider a scenario where we have two ''lib'' namespaces defined for two different projects, but  both have useful circuits that we would like to re-use in a third project. If ''lib1.act'' and ''lib2.act'' both contain namespace ''lib'', importing both would take the union of the definitions in the two files, potentially resulting in errors.
+To resolve this issue, ACT provides a way to rename a namespace that has been imported.
+<code>
+import "lib1.act";
+open lib -> lib1;
+import "lib2.act";
+open lib -> lib2;
+</code>
+In this example, the ''open'' construct enables one to rename a namespace. Once this
+has occured, there cannot be any naming conflicts. This version of ''open'' is a renaming construct; after the open statement, the old name for the namespace is eliminated.
+A second issue is one that is more about convenience. Consider a
+project that has many different people working on it, each in their
+own namespace to avoid naming conflicts. This situation can result in
+very long type names. Plus it would be more bookkeeping to have to
+create a test environment for the types within, say,
+''processor::lib''---not just because of the long type names, but
+because not all types might be exported! In this case we can say:
+<code>
+import "lib.act";
+open processor::lib;
+a1of2 d;
+</code>
+This version of the ''open'' directive has two
+functions: (i) it adds ''processor::lib'' to the search path for
+types, and (ii) it allows one to access all types within the
+namespace, not just the ones that are exported (including types within
+nested namespaces). Note that this ''open'' statement will fail if
+all types cannot be uniquely resolved. Note that the sequence of ''open'' and ''import'' statements can only be
+at the beginning of an ACT file.