Before you get too carried away working at the lowest level (e.g.,
sloshing <literal>MutableByteArray#</literal>s around your
program), you may wish to check if there are libraries that provide a
-“Haskellised veneer” over the features you want. See
-<xref linkend="book-hslibs">.
+“Haskellised veneer” over the features you want. The
+separate libraries documentation describes all the libraries that come
+with GHC.
</para>
<!-- LANGUAGE OPTIONS -->
<para>With the <option>-fglasgow-exts</option> flag, GHC lets you declare
a data type with no constructors. For example:</para>
+
<programlisting>
data S -- S :: *
data T a -- T :: * -> *
</programlisting>
+
<para>Syntactically, the declaration lacks the "= constrs" part. The
-type can be parameterised, but only over ordinary types, of kind *; since
-Haskell does not have kind signatures, you cannot parameterise over higher-kinded
-types.</para>
+type can be parameterised over types of any kind, but if the kind is
+not <literal>*</literal> then an explicit kind annotation must be used
+(see <xref linkend="sec-kinding">).</para>
<para>Such data types have only one value, namely bottom.
Nevertheless, they can be useful when defining "phantom types".</para>
class constraints.
</para>
<para>
-An implicit parameter is bound using an expression of the form
-<emphasis>expr</emphasis> <literal>with</literal> <emphasis>binds</emphasis>,
-where <literal>with</literal> is a new keyword. This form binds the implicit
-parameters arising in the body, not the free variables as a <literal>let</literal> or
-<literal>where</literal> would do. For example, we define the <literal>min</literal> function by binding
-<literal>cmp</literal>.
+An implicit parameter is bound using the standard
+<literal>let</literal> binding form, where the bindings must be a
+collection of simple bindings to implicit-style variables (no
+function-style bindings, and no type signatures); these bindings are
+neither polymorphic or recursive. This form binds the implicit
+parameters arising in the body, not the free variables as a
+<literal>let</literal> or <literal>where</literal> would do. For
+example, we define the <literal>min</literal> function by binding
+<literal>cmp</literal>.</para>
<programlisting>
min :: [a] -> a
- min = least with ?cmp = (<=)
+ min = let ?cmp = (<=) in least
</programlisting>
-Syntactically, the <emphasis>binds</emphasis> part of a <literal>with</literal> construct must be a
-collection of simple bindings to variables (no function-style
-bindings, and no type signatures); these bindings are neither
-polymorphic or recursive.
-</para>
<para>
Note the following additional constraints:
<itemizedlist>
</sect3>
</sect2>
-<sect2>
+<sect2 id="type-synonyms">
<title>Liberalised type synonyms
</title>
<para>
The rewrite is only performed by the compiler when it spots
-applications of <function>Exception.assert</function>, so you can still define and
-use your own versions of <function>assert</function>, should you so wish. If not,
-import <literal>Exception</literal> to make use <function>assert</function> in your code.
+applications of <function>Control.Exception.assert</function>, so you
+can still define and use your own versions of
+<function>assert</function>, should you so wish. If not, import
+<literal>Control.Exception</literal> to make use
+<function>assert</function> in your code.
</para>
<para>
To have the compiler ignore uses of assert, use the compiler option
-<option>-fignore-asserts</option>. <indexterm><primary>-fignore-asserts option</primary></indexterm> That is,
-expressions of the form <literal>assert pred e</literal> will be rewritten to <literal>e</literal>.
+<option>-fignore-asserts</option>. <indexterm><primary>-fignore-asserts
+option</primary></indexterm> That is, expressions of the form
+<literal>assert pred e</literal> will be rewritten to
+<literal>e</literal>.
</para>
<para>
Assertion failures can be caught, see the documentation for the
-<literal>Exception</literal> library (<xref linkend="sec-Exception">)
-for the details.
+<literal>Control.Exception</literal> library for the details.
</para>
</sect1>
<sect1 id="syntax-extns">
<title>Syntactic extensions</title>
+<!-- ====================== HIERARCHICAL MODULES ======================= -->
+
+ <sect2 id="hierarchical-modules">
+ <title>Hierarchical Modules</title>
+
+ <para>GHC supports a small extension to the syntax of module
+ names: a module name is allowed to contain a dot
+ <literal>‘.’</literal>. This is also known as the
+ “hierarchical module namespace” extension, because
+ it extends the normally flat Haskell module namespace into a
+ more flexible hierarchy of modules.</para>
+
+ <para>A module name in the extended syntax consists of a
+ sequence of components, each separated by a dot. When searching
+ for an interface file (or a source file, in the case of GHCi or
+ when using <option>--make</option>) for an imported module, GHC
+ interprets the dot as a path separator. So for example, if a
+ module <literal>A.B.C</literal> is imported, then for each
+ directory <literal>D</literal> on the search path (see the
+ <option>-i</option> option, <xref
+ linkend="options-finding-imports">), GHC will look in the
+ directory <literal>D/A/B</literal><footnote><para>On Windows,
+ this would be <literal>D\A\B</literal>.</para></footnote> for an
+ interface file called <filename>C.hi</filename> or a source file
+ <filename>C.hs</filename> or <filename>C.lhs</filename>.</para>
+
+ <para>Note that as far as the compiler is concerned, module
+ names are always fully qualified; the hierarchy only has a
+ special meaning when searching for interface files and source
+ files in the filesystem. In particular, this means that the
+ full module name must be given after the
+ <literal>module</literal> keyword at the beginning of the
+ module; for example, the module <literal>A.B.C</literal> must
+ begin</para>
+
+<programlisting>module A.B.C</programlisting>
+
+ <para>GHC comes with a large collection of libraries arranged
+ hierarchically; see the accompanying library documentation.
+ There is an ongoing project to create and maintain a stable set
+ of <quote>core</quote> libraries used by several Haskell
+ compilers, and the libraries that GHC comes with represent the
+ current status of that project. For more details, see <ulink
+ url="http://www.haskell.org/~simonmar/libraries/libraries.html">.</ulink></para>
+
+ </sect2>
+
<!-- ====================== PATTERN GUARDS ======================= -->
<sect2 id="pattern-guards">
<title>Rebindable syntax</title>
- <para> Your may want to
- define your own numeric class hierarchy. It completely
- defeats that purpose if the literal "1" means
- "<literal>Prelude.fromInteger 1</literal>", which is what
- the Haskell Report specifies. So the
- <option>-fno-implicit-prelude</option> flag causes the
- following pieces of built-in syntax to refer to <emphasis>whatever
- is in scope</emphasis>, not the Prelude versions:</para>
+ <para>GHC allows most kinds of built-in syntax to be rebound by
+ the user, to facilitate replacing the <literal>Prelude</literal>
+ with a home-grown version, for example.</para>
+
+ <para>You may want to define your own numeric class
+ hierarchy. It completely defeats that purpose if the
+ literal "1" means "<literal>Prelude.fromInteger
+ 1</literal>", which is what the Haskell Report specifies.
+ So the <option>-fno-implicit-prelude</option> flag causes
+ the following pieces of built-in syntax to refer to
+ <emphasis>whatever is in scope</emphasis>, not the Prelude
+ versions:</para>
<itemizedlist>
<listitem>
</listitem>
<listitem>
- <para>"Do" notation is translated using whatever functions
- <literal>(>>=)</literal>, <literal>(>>)</literal>, <literal>fail</literal>,
- and <literal>return</literal>, are in scope (not the Prelude versions).
- List comprehensions, and parallel array comprehensions, are unaffected.
- </para></listitem>
+ <para>"Do" notation is translated using whatever
+ functions <literal>(>>=)</literal>,
+ <literal>(>>)</literal>, <literal>fail</literal>, and
+ <literal>return</literal>, are in scope (not the Prelude
+ versions). List comprehensions, and parallel array
+ comprehensions, are unaffected. </para></listitem>
</itemizedlist>
<para>Be warned: this is an experimental facility, with fewer checks than
projects / ghc-hetmet.git / blobdiff