Cons v.2.3.0


Cons is a Software Construction System.Cons is a system for constructing, primarily, software, but is quite different from previous software construction systems. Cons was designed from the ground up to deal easily with the construction of software spread over multiple source directories. Cons makes it easy to create build scripts that are simple, understandable and maintainable. Cons ensures that complex software is easily and accurately reproducible.Cons uses a number of techniques to accomplish all of this. Construction scripts are just Perl scripts, making them both easy to comprehend and very flexible. Global scoping of variables is replaced with an import/export mechanism for sharing information between scripts, significantly improving the readability and maintainability of each script.Construction environments are introduced: these are Perl objects that capture the information required for controlling the build process. Multiple environments are used when different semantics are required for generating products in the build tree. Cons implements automatic dependency analysis and uses this to globally sequence the entire build. Variant builds are easily produced from a single source tree. Intelligent build subsetting is possible, when working on localized changes. Overrides can be setup to easily override build instructions without modifying any scripts. MD5 cryptographic signatures are associated with derived files, and are used to accurately determine whether a given file needs to be rebuilt.While offering all of the above, and more, Cons remains simple and easy to use. This will, hopefully, become clear as you read the remainder of this document.Why Cons? Why not Make?Cons is a make replacement. In the following paragraphs, we look at a few of the undesirable characteristics of make--and typical build environments based on make--that motivated the development of Cons.Build complexityTraditional make-based systems of any size tend to become quite complex. The original make utility and its derivatives have contributed to this tendency in a number of ways. Make is not good at dealing with systems that are spread over multiple directories. Various work-arounds are used to overcome this difficulty; the usual choice is for make to invoke itself recursively for each sub-directory of a build. This leads to complicated code, in which it is often unclear how a variable is set, or what effect the setting of a variable will have on the build as a whole. The make scripting language has gradually been extended to provide more possibilities, but these have largely served to clutter an already overextended language. Often, builds are done in multiple passes in order to provide appropriate products from one directory to another directory. This represents a further increase in build complexity.Build reproducibilityThe bane of all makes has always been the correct handling of dependencies. Most often, an attempt is made to do a reasonable job of dependencies within a single directory, but no serious attempt is made to do the job between directories. Even when dependencies are working correctly, make's reliance on a simple time stamp comparison to determine whether a file is out of date with respect to its dependents is not, in general, adequate for determining when a file should be rederived. If an external library, for example, is rebuilt and then ``snapped'' into place, the timestamps on its newly created files may well be earlier than the last local build, since it was built before it became visible.Variant buildsMake provides only limited facilities for handling variant builds. With the proliferation of hardware platforms and the need for debuggable vs. optimized code, the ability to easily create these variants is essential. More importantly, if variants are created, it is important to either be able to separate the variants or to be able to reproduce the original or variant at will. With make it is very difficult to separate the builds into multiple build directories, separate from the source. And if this technique isn't used, it's also virtually impossible to guarantee at any given time which variant is present in the tree, without resorting to a complete rebuild.RepositoriesMake provides only limited support for building software from code that exists in a central repository directory structure. The VPATH feature of GNU make (and some other make implementations) is intended to provide this, but doesn't work as expected: it changes the path of target file to the VPATH name too early in its analysis, and therefore searches for all dependencies in the VPATH directory. To ensure correct development builds, it is important to be able to create a file in a local build directory and have any files in a code repository (a VPATH directory, in make terms) that depend on the local file get rebuilt properly. This isn't possible with VPATH, without coding a lot of complex repository knowledge directly into the makefiles.

Cons is a Software Construction System.Cons is ...

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