<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>
<varlistentry>
<term>
- <literal>:breakpoint</literal> <replaceable>list|add|del|stop|step</replaceable> ...
+ <literal>:breakpoint</literal> <replaceable>list|add|continue|del|stop|step</replaceable> ...
<indexterm><primary><literal>:breakpoint</literal></primary></indexterm>
</term>
<listitem>
<varlistentry>
<term>
+ <literal>:continue</literal>
+ <indexterm><primary><literal>:continue</literal></primary></indexterm>
+ </term>
+ <listitem><para>Shortcut to <literal>:breakpoint continue</literal></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
<indexterm><primary><literal>:def</literal></primary></indexterm>
</term>
<indexterm><primary>debugger</primary></indexterm>
<para>GHCi embeds an utility debugger with a very basic set of operations. The debugger
is always available in ghci, you do not need to do anything to activate it. </para>
- <para>The following conditions must hold before the debugger can be used with a module:
+ <para>The following conditions must hold before a module can be debugged in GHCi:
<itemizedlist>
<listitem>
<para>The module must have been loaded interpreted, i.e. not loaded from an <filename>.o</filename> file compiled by ghc </para>
</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 type, and not by line as in traditional debuggers for imperative languages.
+ <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, and evaluate arbitrary Haskell expressions in
- a special interactive environment prompt.No matter what you do at the
- breakpoint context, you cannot change the value assigned to the bindings of
- your program. When you are done you issue the <literal>:quit</literal>
- command and the execution goes on. All the other GHCi commands are supported
- too, but the commands <literal>:load</literal> and <literal>:reload</literal>,
- will cause a breakpoint session to stop.
+ in the context of the event, as well as evaluate arbitrary Haskell expressions in
+ a special interactive prompt. </para><para>
+
+ When you are done you issue the <literal>:continue</literal>
+ command to leave the breakpoint and let the execution go on.
+ Note that not all the GHCi commands are supported in a breakpoint.
</para>
<sidebar><title>Events</title><?dbfo float-type="left"?>
<para>do notation statements</para>
</callout>
</calloutlist></para>
- <para>In reality however, we eliminate some redundant event sites.
+ <para>In reality however, ghci eliminates some redundant event sites.
For instance, sites with two co-located breakpoint events are coalesced into a single one,
- and sites with no bindings in scope are assumed to be uninteresting and eliminated
- too.</para>
+ and sites with no bindings in scope are assumed to be uninteresting and no breakpoint can be set in them.</para>
</sidebar>
<para>
<programlisting>
*main:Main> :break add Main 2
Breakpoint set at (2,15)
+
*main:Main> qsort [10,9..1]
Local bindings in scope:
x :: a, xs :: [a], left :: [a], right :: [a]
+
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!
- This is due to the type of the <code>qsort</code> function, 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.
things. The <literal>:print</literal> command in ghci allows you to
explore its contents and see if it is evaluated or not.
This is useful because you cannot just type <literal>x</literal> in the
- prompt and expect GHCi to return you the value of a. Perhaps you know for
+ 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
- type of <literal>x</literal> to do its work. </para>
- <para> Let's go on with the debugging session of the <code>qsort</code>
+ <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 <literal>qsort</literal>
example:
+<example id="debuggingEx"><title>A short debugging session</title>
<programlisting>
qsort2.hs:2:15-46> x
This is an untyped, unevaluated computation. You can use seq to
qsort2.hs:2:15-46> x <co id="seq3"/>
This is an untyped, unevaluated computation. You can use seq to
force its evaluation and then :print to recover its type
+
qsort2.hs:2:15-46> :t x
x :: GHC.Base.Unknown
qsort2.hs:2:15-46> :p x <co id="seq4"/>
x - 10
+qsort2.hs:2:15-46> :t x <co id="seq5"/>
+x :: Int
</programlisting>
-
+</example>
<calloutlist>
<callout arearefs="seq1">
<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 is a
- static property and cannot change during a session.
- Thus, the binding <code>x</code> still has type Unknown,
- even if in fact the value it is bound to already got a concrete type.</para>
+ This is a bit counterintuitive, but currently in GHCi the type of a binding
+ 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">
+ <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. In the
- future, the restriction of static types will be removed and <code>x</code> will get a proper
- type after step (2) in the example.
+ The example shows the standard way to proceeed with polymorphic values in a breakpoint.
</para>
</sect2>
<sect2><title>Commands</title>
at line <literal>line</literal> in <literal>module</literal>, if any.
</para></listitem>
</varlistentry>
+
+<varlistentry>
+ <term>
+ <literal>:breakpoint continue</literal>
+ </term>
+ <listitem><para>
+ When at a breakpoint, continue execution up to the next breakpoint
+ or end of evaluation.
+ </para></listitem>
+</varlistentry>
+
+<varlistentry>
+ <term>
+ <literal>:continue</literal>
+ </term>
+ <listitem><para>
+ Shortcut for <literal>:breakpoint continue</literal>
+ </para></listitem>
+</varlistentry>
+
<varlistentry>
<term>
<literal>:breakpoint list</literal>
<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>
</itemizedlist>
</para>
</sect2>
- </sect1>
+ <sect2><title>Tips</title>
+ <variablelist>
+ <varlistentry><term>* Use PRAGMAs to fine tune which modules are loaded under debugging mode</term>
+ <listitem>
+ <programlisting>{-# OPTIONS_GHC -fdebugging #-}</programlisting>
+ </listitem>
+ </varlistentry>
+ <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> * <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]]
+
+main:Main> :m +GHC.Exts
+main:Main> main
+Local bindings in scope:
+ x :: a
+Main.hs:15> let x' = unsafeCoerce x :: MyLongType Bool
+Main.hs:15> x'
+[Just [Just False, Just True]]
+ </programlisting>
+ Note that a wrong coercion will likely result in your debugging session being interrupted by a segmentation fault
+ </para></listitem>
+ </varlistentry>
+ <varlistentry> <term> * The undocumented (and unsupported) :force command </term>
+ <listitem><para>
+ equivalent to <literal> :print</literal> with automatic
+ <literal>seq</literal> forcing,
+ may prove useful to replace sequences of <literal>seq</literal> and
+ <literal>:print</literal> in some situations.
+ </para></listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2></sect1>
<sect1 id="ghci-dot-files">
<title>The <filename>.ghci</filename> file</title>
<indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
</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>
<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>
projects / ghc-hetmet.git / blobdiff