Commit f5a6b456 authored by chak's avatar chak
Browse files

[project @ 2001-11-26 08:31:05 by chak]

Added a new section that describes how GHC defines its hardwired knowledge
about primitives and special prelude definitions.
parent 2b8262a3
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<h1>The Glasgow Haskell Compiler (GHC) Commentary [v0.5]</h1>
<h1>The Glasgow Haskell Compiler (GHC) Commentary [v0.6]</h1>
<p>
<!-- Contributors: Whoever makes substantial additions or changes to the
document, please add your name and keep the order alphabetic. Moreover,
......@@ -46,6 +46,7 @@
<h2>The Beast Dissected</h2>
<ul>
<li><a href="the-beast/driver.html">The Glorious Driver</a>
<li><a href="the-beast/prelude.html">Primitives and the Prelude</a>
<li><a href="the-beast/syntax.html">Just Syntax</a>
<li><a href="the-beast/basicTypes.html">The Basics</a>
<li><a href="the-beast/vars.html">The Real Story about Variables, Ids, TyVars, and the like</a>
......@@ -79,7 +80,7 @@
<p><small>
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Last modified: Sat Nov 17 14:10:48 EST 2001
Last modified: Mon Nov 26 19:23:12 EST 2001
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</small>
</body>
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<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"><code>primops.txt</code></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).
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>
......@@ -61,7 +63,7 @@
for full details of how GHC is configured to cope with different machine word sizes.
<p><small>
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Last modified: Wed Aug 8 19:29:12 EST 2001
Last modified: Mon Nov 26 18:03:16 EST 2001
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<title>The GHC Commentary - Primitives and the Prelude</title>
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<h1>The GHC Commentary - Primitives and the Prelude</h1>
<p>
Most of what the compiler has to have wired in about primitives and
prelude definitions is in
<a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/"><code>fptools/ghc/compiler/prelude/</code></a>.
</p>
<h2>Primitives</h2>
<p>
Some types and functions have to be hardwired into the compiler as they
are atomic; all other code is essentially built around this primitive
functionality. This includes basic arithmetic types, such as integers,
and their elementary operations as well as pointer types. Primitive
types and functions often receive special treatment in the code
generator, which means that these entities have to be explicitly
represented in the compiler. Moreover, many of these types receive some
explicit treatment in the runtime system, and so, there is some further
information about <a href="../rts-libs/primitives.html">primitives in
the RTS section</a> of this document.
<p>
The module <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/TysPrim.lhs"><code>TysPrim</code></a>
exports a list of all primitive type constructors as <code>primTyCons ::
[TyCon]</code>. All of these type constructors (of type
<code>TyCon</code>) are also exported as <code>intPrimTyCon</code>,
<code>stablePtrPrimTyCon</code>, and so on. In addition, for each
nullary type constructor the corresponding type (of type
<code>Type</code>) is also exported; for example, we have
<code>intPrimTy :: Type</code>. For all other type constructors, a
function is exported that constructs the type obtained by applying the
type constructors to an argument type (of type <code>Type</code>); for
example, we have <code>mkStablePtrPrimTy :: Type -> Type</code>.
<p>
As it is inconvenient to identify type that receive a special treatment
by the code generator by looking at their name, the module <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/PrimRep.lhs"><code>PrimRep</code></a>
exports a data type <code>PrimRep</code>, which lists all
machine-manipulable implementation types. The module also exports a set
of query functions on <code>PrimRep</code> that define properties, such
as a type's byte size or whether a primitive type is a pointer type.
Moreover, the function <code>TysPrim.primRepTyCon :: PrimRep ->
TyCon</code> converts <code>PrimRep</code> values into the corresponding
type constructor.
<h2>The Prelude</h2>
<p>
In addition to entities that are primitive, as the compiler has to treat
them specially in the backend, there is a set of types, functions,
etc. that the Haskell language definition flags as essential to the
language by placing them into the special module <code>Prelude</code>
that is implicitly imported into each Haskell module. For some of these
entities it suffices to define them (by standard Haskell definitions) in
a <code>Prelude</code> module and ensuring that this module is treated
specially by being always imported .
<p>
However, there is a set of entities (such as, for example, the list type
and the corresponding data constructors) that have an inbetween status:
They are not truly primitive (lists, for example, can easily be defined
by a <code>data</code> declaration), but the compiler has to have extra
knowledge about them, as they are associated with some particular
features of the language (in the case of lists, there is special syntax,
such as list comprehensions, associated with the type). Another
example, for a special kind of entity are type classes that can be used
in a <code>deriving</code> clause. All types that are not-primitive,
but about which the compiler nonetheless has to have some extra
knowledge are defined in the module <a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/TysWiredIn.lhs"><code>TysWiredIn</code></a>.
<p>
All wired in type constructors are contained in <code>wiredInTyCons ::
[TyCon]</code>. In addition to that list, <code>TysWiredIn</code>
exports variables bound to representations of all listed type
constructors and their data constructors. So, for example, we have
<code>listTyCon</code> together with <code>nilDataCon</cons> and
</code>consDataCon</code>. There are also convenience functions, such
as <code>mkListTy</code> and <code>mkTupleTy</code>, which construct
compound types.
<p>
All names of types, functions, etc. known to the compiler are defined in
<a
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/PrelNames.lhs"><code>PrelNames</code></a>.
This includes the names of types and functions exported from
<code>TysWiredIn</code>, but also others. In particular, this module
also fixes the names of all prelude modules; i.e., of the modules whose
name starts with <code>Prel</code>, which GHC's library uses to bring
some structure into the quite large number of <code>Prelude</code>
definitions.
<p>
<code>PrelNames.knownKeyNames :: [Name]</code> contains all names known
to the compiler, but the elements of the list are also exported
individually as variables, such as <code>floatTyConName</code> (having
the lexeme <code>Float</code>) and <code>floatDataConName</code> (having
the lexeme <code>F#</code>). For each of these names,
<code>PrelNames</code> derfines a unique key with a definition, such as
<p>
<blockquote><pre>
floatPrimTyConKey = mkPreludeTyConUnique 11</pre>
</blockquote>
<p>
that is, all unique keys for known prelude names are hardcoded into
<code>PrelNames</code> (and uniqueness has to be manually ensured in
that module). To simplify matching the types of important groups of
type constructors, <code>PrelNames</code> also exports lists, such as
<code>numericTyKeys</code> (keys of all numeric types), that contain the
unique keys of all names in that group. In addition, derivable type
classes and their structure is defined by
<code>derivableClassKeys</code> and related definitions.
<p>
In addition to names that have unique keys, <code>PrelNames</code> also
defines a set of names without uniqueness information. These names end
on the suffix <code>_RDR</code> and are of type <code>RdrName</code> (an
example, is <code>times_RDR</code>, which represents the lexeme
<code>*</code>). The names are used in locations where they pass
through the renamer anyway (e.g., code generated from deriving clauses),
which will take care of adding uniqueness information.
<p>
The module
<a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/PrelInfo.lhs"><code>PrelInfo</code></a>
in some sense ties all the above together and provides a reasonably
restricted interface to these definition to the rest of the compiler.
However, from what I have seen, this doesn't quite work out and the
earlier mentioned modules are directly imported in many places.
<p><small>
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Last modified: Mon Nov 26 19:29:33 EST 2001
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</small>
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