<indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
<indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
- <sect1>
+ <sect1 id="ghci-introduction">
<title>Introduction to GHCi</title>
<para>Let's start with an example GHCi session. You can fire up
enter, GHCi will attempt to evaluate it.</para>
</sect1>
- <sect1>
+ <sect1 id="loading-source-files">
<title>Loading source files</title>
<para>Suppose we have the following Haskell source code, which we
</sect1>
- <sect1>
+ <sect1 id="interactive-evaluation">
<title>Interactive evaluation at the prompt</title>
<para>When you type an expression at the prompt, GHCi immediately
standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
a)</literal> for each type variable <literal>a</literal>, and defaults the
type variable if
- <itemizedlist>
- <listitem><para> The type variable <literal>a</literal>
- appears in no other constraints </para></listitem>
- <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
- <listitem><para> At least one of the classes <literal>Ci</literal> is
- numeric.</para></listitem>
- </itemizedlist>
- At the GHCi prompt, the second and third rules are relaxed as follows
- (differences italicised):
- <itemizedlist>
- <listitem><para> <emphasis>All</emphasis> of the classes
- <literal>Ci</literal> are single-parameter type classes.</para></listitem>
- <listitem><para> At least one of the classes <literal>Ci</literal> is
- numeric, <emphasis>or is <literal>Show</literal>,
- <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
- </itemizedlist>
- The same type-default behaviour can be enabled in an ordinary Haskell
- module, using the flag <literal>-fextended-default-rules</literal>.
+ <orderedlist>
+ <listitem>
+ <para>
+ The type variable <literal>a</literal> appears in no
+ other constraints
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ All the classes <literal>Ci</literal> are standard.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ At least one of the classes <literal>Ci</literal> is
+ numeric.
+ </para>
+ </listitem>
+ </orderedlist>
+ At the GHCi prompt, or with GHC if the
+ <literal>-fextended-default-rules</literal> flag is given,
+ the following additional differences apply:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Rule 2 above is relaxed thus:
+ <emphasis>All</emphasis> of the classes
+ <literal>Ci</literal> are single-parameter type classes.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Rule 3 above is relaxed this:
+ At least one of the classes <literal>Ci</literal> is
+ numeric, <emphasis>or is <literal>Show</literal>,
+ <literal>Eq</literal>, or
+ <literal>Ord</literal></emphasis>.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The unit type <literal>()</literal> is added to the
+ start of the standard list of types which are tried when
+ doing type defaulting.
+ </para>
+ </listitem>
+ </itemizedlist>
+ The last point means that, for example, this program:
+<programlisting>
+main :: IO ()
+main = print def
+
+instance Num ()
+
+def :: (Num a, Enum a) => a
+def = toEnum 0
+</programlisting>
+ prints <literal>()</literal> rather than <literal>0</literal> as the
+ type is defaulted to <literal>()</literal> rather than
+ <literal>Integer</literal>.
+ </para>
+ <para>
+ The motivation for the change is that it means <literal>IO a</literal>
+ actions default to <literal>IO ()</literal>, which in turn means that
+ ghci won't try to print a result when running them. This is
+ particularly important for <literal>printf</literal>, which has an
+ instance that returns <literal>IO a</literal>.
+ However, it is only able to return
+ <literal>undefined</literal>
+ (the reason for the instance having this type is to not require
+ extensions to the class system), so if the type defaults to
+ <literal>Integer</literal> then ghci gives an error when running a
+ printf.
</para>
</sect2>
</sect1>
</sect1>
+ <sect1 id="ghci-obj">
+ <title>Compiling to object code inside GHCi</title>
+
+ <para>By default, GHCi compiles Haskell source code into byte-code
+ that is interpreted by the runtime system. GHCi can also compile
+ Haskell code to object code: to turn on this feature, use the
+ <option>-fobject-code</option> flag either on the command line or
+ with <literal>:set</literal> (the option
+ <option>-fbyte-code</option> restores byte-code compilation
+ again). Compiling to object code takes longer, but typically the
+ code will execute 10-20 times faster than byte-code.</para>
+
+ <para>Compiling to object code inside GHCi is particularly useful
+ if you are developing a compiled application, because the
+ <literal>:reload</literal> command typically runs much faster than
+ restarting GHC with <option>--make</option> from the command-line,
+ because all the interface files are already cached in
+ memory.</para>
+
+ <para>There are disadvantages to compiling to object-code: you
+ can't set breakpoints in object-code modules, for example. Only
+ the exports of an object-code module will be visible in GHCi,
+ rather than all top-level bindings as in interpreted
+ modules.</para>
+ </sect1>
+
<sect1 id="ghci-faq">
<title>FAQ and Things To Watch Out For</title>
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