<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>
<literal>:continue</literal>
<indexterm><primary><literal>:continue</literal></primary></indexterm>
</term>
- <listitem> Shortcut to <literal>:breakpoint continue </literal>
+ <listitem><para>Shortcut to <literal>:breakpoint continue</literal></para>
</listitem>
</varlistentry>
</para></listitem>
</itemizedlist></para>
<sect2><title>Using the debugger</title>
- <para>The debugger allows the insertion of breakpoints at specific locations in the source code. These locations are goberned by event sites, and not by line as in traditional debuggers such as gdb. </para> <para>
+ <para>The debugger allows the insertion of breakpoints at specific locations in the source code. These locations are governed by event sites, and not by line as in traditional debuggers such as gdb. </para> <para>
Once a breakpointed event is hit, the debugger stops the execution and you can examine the local variables in scope
in the context of the event, as well as evaluate arbitrary Haskell expressions in
a special interactive prompt. </para><para>
qsort2.hs:2:15-46>
</programlisting>
What is happening here is that GHCi has interrupted the evaluation of
- <code>qsort</code> at the breakpoint set in line 2, as the prompt indicates.
+ <literal>qsort</literal> at the breakpoint set in line 2, as the prompt indicates.
At this point you can freely explore the contents of the bindings in scope,
but with two catches. </para><para>
First, take into account that due to the lazy nature of Haskell, some of
trigger a computation. </para><para>
Second: look at the types of the things in scope.
GHCi has left its types parameterised by a variable!
- Look at the type of <code>qsort</code>, which is
+ Look at the type of <literal>qsort</literal>, which is
polymorphic on the type of its argument. It does not
- tell us really what the types of <code>x</code> and <code>xs</code> can be.
+ tell us really what the types of <literal>x</literal> and <literal>xs</literal> can be.
In general, polymorphic programs deal with polymorphic values,
and this means that some of the bindings available in a breakpoint site
will be parametrically typed.
This is useful because you cannot just type <literal>x</literal> in the
prompt and expect GHCi to return you its value. Perhaps you know for
sure that
- <literal>x</literal> is of type <code>Int</code>, which is an instance of
- <code>Show</code>, but GHCi does not have this information.
- <code>:print</code> however is fine, because it does not need to know the
+ <literal>x</literal> is of type <literal>Int</literal>, which is an instance of
+ <literal>Show</literal>, but GHCi does not have this information.
+ <literal>:print</literal> however is fine, because it does not need to know the
type to do its work. </para>
- <para> Let's go on with the debugging session of the <code>qsort</code>
+ <para> Let's go on with the debugging session of the <literal>qsort</literal>
example:
<example id="debuggingEx"><title>A short debugging session</title>
<programlisting>
<para>GHCi reminds us that this value is untyped, and instructs us to force its evaluation </para>
</callout>
<callout arearefs="seq2">
- <para>This line forces the evaluation of <code>x</code> </para>
+ <para>This line forces the evaluation of <literal>x</literal> </para>
</callout>
<callout arearefs="seq3">
<para>Even though x has been evaluated, we cannot simply use its name to see its value!
This is a bit counterintuitive, but currently in GHCi the type of a binding
- cannot be a type variable <code>a</code>.
- Thus, the binding <code>x</code> gets assigned the concrete type Unknown.</para>
+ cannot be a type variable <literal>a</literal>.
+ Thus, the binding <literal>x</literal> gets assigned the concrete type Unknown.</para>
</callout>
<callout arearefs="seq4">
- <para>We can explore <code>x</code> using the <literal>:print</literal>
- command, which does find out that <code>x</code> is of type Int and prints
+ <para>We can explore <literal>x</literal> using the <literal>:print</literal>
+ command, which does find out that <literal>x</literal> is of type Int and prints
its value accordingly.</para>
</callout>
<callout arearefs="seq5">
- <literal>:print</literal> also updates the type of <code>x</code> with
- the most concrete type information available.
+ <para><literal>:print</literal> also updates the type of <literal>x</literal> with
+ the most concrete type information available.</para>
</callout>
</calloutlist>
The example shows the standard way to proceeed with polymorphic values in a breakpoint.
<para>
<itemizedlist>
<listitem><para>
- <xref linkend="implicit-parameters" xrefstyle="select: title"/> are only available
+ Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
at the scope of a breakpoint if there is a explicit type signature.
</para></listitem>
</itemizedlist>
<programlisting>{-# OPTIONS_GHC -fdebugging #-}</programlisting>
</listitem>
</varlistentry>
- <varlistentry> <term>* Repeated use of <code>seq</code> and
+ <varlistentry> <term>* Repeated use of <literal>seq</literal> and
<literal>:print</literal> may be necessary to observe unevaluated
untyped bindings</term>
<listitem><para>see <xref linkend="debuggingEx"/>
</para></listitem>
</varlistentry>
- <varlistentry> <term> * <code>GHC.Exts.unsafeCoerce</code> can help if you are positive about the type of a binding</term>
+ <varlistentry> <term> * <literal>GHC.Exts.unsafeCoerce</literal> can help if you are positive about the type of a binding</term>
<listitem><para><programlisting>
type MyLongType a = [Maybe [Maybe a]]
<varlistentry> <term> * The undocumented (and unsupported) :force command </term>
<listitem><para>
equivalent to <literal> :print</literal> with automatic
- <code>seq</code> forcing,
- may prove useful to replace sequences of <code>seq</code> and
+ <literal>seq</literal> forcing,
+ may prove useful to replace sequences of <literal>seq</literal> and
<literal>:print</literal> in some situations.
</para></listitem>
</varlistentry>
<term>I can't use Control-C to interrupt computations in
GHCi on Windows.</term>
<listitem>
- <para>See <xref linkend="ghci-windows">.</xref></para>
+ <para>See <xref linkend="ghci-windows"/></para>
</listitem>
</varlistentry>
</variablelist>
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