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+ <title>The GHC Commentary - Primitives</title>
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+ <h1>The GHC Commentary - Primitives</h1>
+ <p>
+ Most user-level Haskell types and functions provided by GHC (in
+ particular those from the Prelude and GHC's Prelude extensions) are
+ internally constructed from even more elementary types and functions.
+ Most notably, GHC understands a notion of <em>unboxed types,</em> which
+ are the Haskell representation of primitive bit-level integer, float,
+ etc. types (as opposed to their boxed, heap allocated counterparts) -
+ cf. <a
+ href="http://research.microsoft.com/Users/simonpj/Papers/unboxed-values.ps.Z">"Unboxed
+ Values as First Class Citizens."</a>
+
+ <h4>The Ultimate Source of Primitives</h4>
+ <p>
+ The hardwired types of GHC are brought into scope by the module
+ <code>PrelGHC</code>. This modules only exists in the form of a
+ handwritten interface file <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/lib/std/PrelGHC.hi-boot"><code>PrelGHC.hi-boot</code>,</a>
+ which lists the type and function names, as well as instance
+ declarations. The actually types of these names as well as their
+ implementation is hardwired into GHC. Note that the names in this file
+ are z-encoded, and in particular, identifiers ending on <code>zh</code>
+ denote user-level identifiers ending in a hash mark (<code>#</code>),
+ which is used to flag unboxed values or functions operating on unboxed
+ values. For example, we have <code>Char#</code>, <code>ord#</code>, and
+ so on.
+
+ <h4>The New Primitive Definition Scheme</h4>
+ <p>
+ As of (about) the development version 4.11, the types and various
+ properties of primitive operations are defined in the file <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/primops.txt.pp"><code>primops.txt.pp</code></a>.
+ (Personally, I don't think that the <code>.txt</code> suffix is really
+ appropriate, as the file is used for automatic code generation; the
+ recent addition of <code>.pp</code> means that the file is now mangled
+ by cpp.)
+ <p>
+ The utility <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/utils/genprimopcode/"><code>genprimopcode</code></a>
+ generates a series of Haskell files from <code>primops.txt</code>, which
+ encode the types and various properties of the primitive operations as
+ compiler internal data structures. These Haskell files are not complete
+ modules, but program fragments, which are included into compiler modules
+ during the GHC build process. The generated include files can be found
+ in the directory <code>fptools/ghc/compiler/</code> and carry names
+ matching the pattern <code>primop-*.hs-incl</code>. They are generate
+ during the execution of the <code>boot</code> target in the
+ <code>fptools/ghc/</code> directory. This scheme significantly
+ simplifies the maintenance of primitive operations.
+ <p>
+ As of development version 5.02, the <code>primops.txt</code> file also allows the
+ recording of documentation about intended semantics of the primitives. This can
+ be extracted into a latex document (or rather, into latex document fragments)
+ via an appropriate switch to <code>genprimopcode</code>. In particular, see <code>primops.txt</code>
+ for full details of how GHC is configured to cope with different machine word sizes.
+ <p><small>
+<!-- hhmts start -->
+Last modified: Mon Nov 26 18:03:16 EST 2001
+<!-- hhmts end -->
+ </small>
+ </body>
+</html>
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