Differences

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--- language:namespaces [2020/05/01 20:44] – [Importing namespaces] rajit
+++ language:namespaces [2024/07/27 16:52] (current) – [Renaming namespaces] rajit
@@ Line 12: / Line 12: @@
 A namespace is created by using the ''namespace'' construct.
-<code>
+<code act>
 namespace lib {
 ...
@@ Line 38: / Line 38: @@
 namespaces is shown below.
-<code>
+<code act>
 namespace datapath {
     export defproc bus_interface(...) { ... }
@@ Line 64: / Line 64: @@
 the entire namespace ''adder'' can be exported as follows:
-<code>
+<code act>
 namespace datapath {
     export defproc bus_interface(...) { ... }
@@ Line 93: / Line 93: @@
 The basic form of import statement is shown below:
-<code>
+<code act>
 import "channel.act";
 ...
@@ Line 107: / Line 107: @@
-There are a few things that might create issues in such a situation.
+A second version of import uses namespaces directly, but requires that
-First, duplicate namespaces might exist, especially when re-using old
+ACT files be placed in locations that match the namespace
-files. For instance, suppose we have two files: ''lib1.act'' and
+hierarchy. The import statement
-''lib2.act'' both containing namespace ''lib'', but having
-definitions that are useful. Importing both would result in the union of
-the namespaces, and could create naming conflicts (e.g. multiple
-definition of types having the same name---an error). To solve this
-problem, one can do the following:
-<code>
+<code act>
-import "lib1.act";
+import processor::lib;
-open lib -> lib1;
-import "lib2.act";
-open lib -> lib2;
 </code>
-The ''open'' construct enables one to rename a namespace. Once this
+is equivalent to the following:
-has occured, there cannot be any naming conflicts. This version of
-''open'' is a renaming construct. The old name for the namespace is
+<code act>
-eliminated.
+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 act>
+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 =====
-A second issue is one that is more about convenience. Consider a
+If  a project has many different people working on it, it can be convenient
-project that has many different people working on it, each in their
+to have each component/module 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:
+because not all types might be exported!
-<code>
+As a syntactic convenience, we can say:
+<code act>
 import "lib.act";
 open processor::lib;
@@ Line 148: / Line 161: @@
 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.
+all types cannot be uniquely resolved. Note that the sequence of ''open'' and ''import'' statements can only be
-The sequence of ''open'' and ''import'' statements can only be
 at the beginning of an ACT file.
-A second version of import uses namespaces directly, but requires that
+===== Renaming namespaces =====
-ACT files be placed in locations that match the namespace
-hierarchy. The import statement
-<code>
-import processor::lib;
-</code>
-is equivalent to the following:
+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.
-<code>
+To resolve this issue, ACT provides a way to rename a namespace that has been imported.
-import "processor/lib/_all_.act";
+<code act>
+import "lib1.act";
+open lib -> lib1;
+import "lib2.act";
+open lib -> lib2;
 </code>
-It assumes that the file ''_all_.act'' in the directory
+In this example, the ''open'' construct enables one to rename a namespace. Once this
-''processor/lib'' contains all the definitions corresponding to the
+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.
-''processor::lib'' namespace.
-===== Opening namespaces =====
+Another renaming scenario that can be useful is to move a namespace into another one.
+<code act>
+import lib;
+import lib => priv;
+</code>
+The first import statement above loads in the ''lib'' namespace. The second moves the namespace //into// ''priv''. If ''priv'' doesn't exist as a namespace, it is created. With this sequence, the original ''lib'' is now accessed as ''priv::lib''.