<indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
<indexterm><primary>Foreign Function
Interface</primary><secondary>GHCi support</secondary></indexterm>.
- GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>)..</para>
+ GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
<sect1 id="ghci-introduction">
<title>Introduction to GHCi</title>
<screen>
$ ghci
- ___ ___ _
- / _ \ /\ /\/ __(_)
- / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
-/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
-\____/\/ /_/\____/|_| Type :? for help.
-
+GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
Prelude>
</screen>
<para>There may be a short pause while GHCi loads the prelude and
- standard libraries, after which the prompt is shown. If we follow
- the instructions and type <literal>:?</literal> for help, we
- get:</para>
-
-<screen>
- Commands available from the prompt:
-
- <stmt> evaluate/run <stmt>
- :add <filename> ... add module(s) to the current target set
- :browse [*]<module> display the names defined by <module>
- :cd <dir> change directory to <dir>
- :def <cmd> <expr> define a command :<cmd>
- :edit <file> edit file
- :edit edit last module
- :help, :? display this list of commands
- :info [<name> ...] display information about the given names
- :load <filename> ... load module(s) and their dependents
- :module [+/-] [*]<mod> ... set the context for expression evaluation
- :main [<arguments> ...] run the main function with the given arguments
- :reload reload the current module set
-
- :set <option> ... set options
- :set args <arg> ... set the arguments returned by System.getArgs
- :set prog <progname> set the value returned by System.getProgName
- :set prompt <prompt> set the prompt used in GHCi
- :set editor <cmd> set the command used for :edit
-
- :show modules show the currently loaded modules
- :show bindings show the current bindings made at the prompt
-
- :ctags [<file>] create tags file for Vi (default: "tags")
- :etags [<file>] create tags file for Emacs (default: "TAGS")
- :type <expr> show the type of <expr>
- :kind <type> show the kind of <type>
- :undef <cmd> undefine user-defined command :<cmd>
- :unset <option> ... unset options
- :quit exit GHCi
- :!<command> run the shell command <command>
-
- Options for ':set' and ':unset':
-
- +r revert top-level expressions after each evaluation
- +s print timing/memory stats after each evaluation
- +t print type after evaluation
- -<flags> most GHC command line flags can also be set here
- (eg. -v2, -fglasgow-exts, etc.)
-</screen>
+ standard libraries, after which the prompt is shown. As the banner
+ says, you can type <literal>:?</literal> to see the list of commands
+ available, and a half line description of each of them.</para>
<para>We'll explain most of these commands as we go along. For
Hugs users: many things work the same as in Hugs, so you should be
<screen>
Prelude> :! ghc -c D.hs
Prelude> :load A
-Skipping D ( D.hs, D.o )
-Compiling C ( C.hs, interpreted )
Compiling B ( B.hs, interpreted )
+Compiling C ( C.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>
</screen>
- <para>In the messages from the compiler, we see that it skipped D,
- and used the object file <filename>D.o</filename>. The message
- <literal>Skipping</literal> <replaceable>module</replaceable>
- indicates that compilation for <replaceable>module</replaceable>
- isn't necessary, because the source and everything it depends on
+ <para>In the messages from the compiler, we see that there is no line
+ for <literal>D</literal>. This is because
+ it isn't necessary to compile <literal>D</literal>,
+ because the source and everything it depends on
is unchanged since the last compilation.</para>
<para>At any time you can use the command
A ( A.hs, interpreted )
*Main></screen>
- <para>If we now modify the source of D (or pretend to: using Unix
+ <para>If we now modify the source of D (or pretend to: using the Unix
command <literal>touch</literal> on the source file is handy for
this), the compiler will no longer be able to use the object file,
because it might be out of date:</para>
*Main> :! touch D.hs
*Main> :reload
Compiling D ( D.hs, interpreted )
-Skipping C ( C.hs, interpreted )
-Skipping B ( B.hs, interpreted )
-Skipping A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>
</screen>
*Main> :! ghc -c C.hs
*Main> :load A
Compiling D ( D.hs, interpreted )
-Compiling C ( C.hs, interpreted )
Compiling B ( B.hs, interpreted )
+Compiling C ( C.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
</screen>
<screen>
*Main> :load A
-Skipping D ( D.hs, D.o )
-Skipping C ( C.hs, C.o )
Compiling B ( B.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
when working on a large program is to occasionally run
<literal>ghc ––make</literal> to compile the whole project (say
before you go for lunch :-), then continue working in the
- interpreter. As you modify code, the new modules will be
+ interpreter. As you modify code, the changed modules will be
interpreted, but the rest of the project will remain
compiled.</para>
Prelude IO>
</screen>
- <para>(Note: <literal>:module</literal> can be shortened to
+ <para>(Note: you can use <literal>import M</literal> as an
+ alternative to <literal>:module +M</literal>, and
+ <literal>:module</literal> can also be shortened to
<literal>:m</literal>). The full syntax of the
<literal>:module</literal> command is:</para>
<para>
Hint: GHCi will tab-complete names that are in scope; for
example, if you run GHCi and type <literal>J<tab></literal>
- then GHCi will expand it to <literal>Just </literal>.
+ then GHCi will expand it to “<literal>Just </literal>”.
</para>
<sect3>
</listitem>
</orderedlist>
At the GHCi prompt, or with GHC if the
- <literal>-fextended-default-rules</literal> flag is given,
+ <literal>-XExtendedDefaultRules</literal> flag is given,
the following additional differences apply:
<itemizedlist>
<listitem>
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
+ (the reason for the instance having this type is so that printf
+ doesn't 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>
<para>The debugger provides the following:
<itemizedlist>
<listitem>
- <para>The abilty to set a <firstterm>breakpoint</firstterm> on a
+ <para>The ability to set a <firstterm>breakpoint</firstterm> on a
function definition or expression in the program. When the function
is called, or the expression evaluated, GHCi suspends
execution and returns to the prompt, where you can inspect the
have no context information attached to them. Finding which
particular call to <literal>head</literal> in your program resulted in
the error can be a painstaking process, usually involving
- <literal>Debug.Trace.trace</literal>.</para>
+ <literal>Debug.Trace.trace</literal>, or compiling with
+ profiling and using <literal>+RTS -xc</literal> (see <xref
+ linkend="prof-time-options" />).</para>
<para>The GHCi debugger offers a way to hopefully shed some light on
these errors quickly and without modifying or recompiling the source
<variablelist>
<varlistentry>
<term>
+ <literal>:abandon</literal>
+ <indexterm><primary><literal>:abandon</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Abandons the current evaluation (only available when stopped at
+ a breakpoint).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:add</literal> <replaceable>module</replaceable> ...
<indexterm><primary><literal>:add</literal></primary></indexterm>
</term>
<varlistentry>
<term>
- <literal>:breakpoint</literal> <replaceable>list|add|continue|del|stop|step</replaceable> ...
- <indexterm><primary><literal>:breakpoint</literal></primary></indexterm>
+ <literal>:back</literal>
+ <indexterm><primary><literal>:back</literal></primary></indexterm>
</term>
<listitem>
- <para>Permits to add, delete or list the breakpoints in a debugging session.
- </para>
+ <para>Travel back one step in the history. See <xref
+ linkend="tracing" />. See also:
+ <literal>:trace</literal>, <literal>:history</literal>,
+ <literal>:forward</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:break [<replaceable>identifier</replaceable> |
+ [<replaceable>module</replaceable>] <replaceable>line</replaceable>
+ [<replaceable>column</replaceable>]]</literal>
+ </term>
+ <indexterm><primary><literal>:break</literal></primary></indexterm>
+ <listitem>
+ <para>Set a breakpoint on the specified function or line and
+ column. See <xref linkend="setting-breakpoints" />.</para>
</listitem>
</varlistentry>
<literal>:continue</literal>
<indexterm><primary><literal>:continue</literal></primary></indexterm>
</term>
- <listitem><para>Shortcut to <literal>:breakpoint continue</literal></para>
+ <listitem><para>Continue the current evaluation, when stopped at a
+ breakpoint.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:cmd</literal> <replaceable>expr</replaceable>
+ <indexterm><primary><literal>:cmd</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Executes <replaceable>expr</replaceable> as a computation of
+ type <literal>IO String</literal>, and then executes the resulting
+ string as a list of GHCi commands. Multiple commands are separated
+ by newlines. The <literal>:cmd</literal> command is useful with
+ <literal>:def</literal> and <literal>:set stop</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
+ <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
+ <indexterm><primary><literal>:etags</literal></primary>
+ </indexterm>
+ <indexterm><primary><literal>:etags</literal></primary>
+ </indexterm>
+ </term>
+ <listitem>
+ <para>Generates a “tags” file for Vi-style editors
+ (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
+ no filename is specified, the defaulit <filename>tags</filename> or
+ <filename>TAGS</filename> is
+ used, respectively. Tags for all the functions, constructors and
+ types in the currently loaded modules are created. All modules must
+ be interpreted for these commands to work.</para>
+ <para>See also <xref linkend="hasktags" />.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
+ <literal>:delete * | <replaceable>num</replaceable> ...</literal>
+ <indexterm><primary><literal>:delete</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Delete one or more breakpoints by number (use <literal>:show
+ breaks</literal> to see the number of each breakpoint). The
+ <literal>*</literal> form deletes all the breakpoints.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:edit <optional><replaceable>file</replaceable></optional></literal>
<indexterm><primary><literal>:edit</literal></primary></indexterm>
</term>
<varlistentry>
<term>
+ <literal>:force <replaceable>identifier</replaceable> ...</literal>
+ <indexterm><primary><literal>:force</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Prints the value of <replaceable>identifier</replaceable> in
+ the same way as <literal>:print</literal>. Unlike
+ <literal>:print</literal>, <literal>:force</literal> evaluates each
+ thunk that it encounters while traversing the value. This may
+ cause exceptions or infinite loops, or further breakpoints (which
+ are ignored, but displayed).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:forward</literal>
+ <indexterm><primary><literal>:forward</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Move forward in the history. See <xref
+ linkend="tracing" />. See also:
+ <literal>:trace</literal>, <literal>:history</literal>,
+ <literal>:back</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:help</literal>
<indexterm><primary><literal>:help</literal></primary></indexterm>
</term>
<varlistentry>
<term>
+ <literal>:history [<replaceable>num</replaceable>]</literal>
+ <indexterm><primary><literal>:history</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Display the history of evaluation steps. With a number,
+ displays that many steps (default: 20). For use with
+ <literal>:trace</literal>; see <xref
+ linkend="tracing" />.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:info</literal> <replaceable>name</replaceable> ...
<indexterm><primary><literal>:info</literal></primary></indexterm>
</term>
<varlistentry>
<term>
+ <literal>:kind</literal> <replaceable>type</replaceable>
+ <indexterm><primary><literal>:kind</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Infers and prints the kind of
+ <replaceable>type</replaceable>. The latter can be an arbitrary
+ type expression, including a partial application of a type constructor,
+ such as <literal>Either Int</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:load</literal> <replaceable>module</replaceable> ...
<indexterm><primary><literal>:load</literal></primary></indexterm>
</term>
<literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
<indexterm><primary><literal>:module</literal></primary></indexterm>
</term>
+ <term>
+ <literal>import <replaceable>mod</replaceable></literal>
+ </term>
<listitem>
<para>Sets or modifies the current context for statements
- typed at the prompt. See <xref linkend="ghci-scope"/> for
+ typed at the prompt. The form <literal>import
+ <replaceable>mod</replaceable></literal> is equivalent to
+ <literal>:module +<replaceable>mod</replaceable></literal>.
+ See <xref linkend="ghci-scope"/> for
more details.</para>
</listitem>
</varlistentry>
<indexterm><primary><literal>:print</literal></primary></indexterm>
</term>
<listitem>
- <para> Prints a semievaluated value without forcing its evaluation.
- <literal>:print </literal> works just like <literal>:sprint</literal> but additionally,
- <literal>:print</literal> binds the unevaluated parts -called
- <quote>suspensions</quote>-
- to names which you can play with. For example:
-<screen>
-Prelude> let li = map Just [1..5]
-Prelude> :sp li
-li - _
-Prelude> :p li
-li - (_t1::[Maybe Integer])
-Prelude> head li
-Just 1
-Prelude> :sp li
-li - Just 1 : _
-Prelude> :p li
-li - Just 1 : (_t2::[Maybe Integer])
-Prelude> last li
-Just 5
-Prelude> :sp li
-li - [Just 1,_,_,_,Just 5]
-Prelude> :p li
-li - [Just 1,(_t3::Maybe Integer),(_t4::Maybe Integer),(_t5::Maybe Integer),Just 4]
-Prelude> _t4
-Just 3
-Prelude> :p li
-li - [Just 1,(_t6::Maybe Integer),Just 3,(_t7::Maybe Integer),Just 4]
-</screen>
- The example uses <literal>:print</literal> and <literal>:sprint</literal>
- to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
- with it. Note for instance how <quote>last</quote> traverses all the elements of
- the list to compute its result, but without evaluating the individual elements.
- </para>
+ <para>Prints a value without forcing its evaluation.
+ <literal>:print</literal> may be used on values whose types are
+ unkonwn or partially known, which might be the case for local
+ variables with polymorphic types at a breakpoint. While inspecting
+ the runtime value, <literal>:print</literal> attempts to
+ reconstruct the type of the value, and will elaborate the type in
+ GHCi's environment if possible. If any unevaluated components
+ (thunks) are encountered, then <literal>:print</literal> binds
+ a fresh variable with a name beginning with <literal>_t</literal>
+ to each thunk. See <xref linkend="breakpoints" /> for more
+ information. See also the <literal>:sprint</literal> command,
+ which works like <literal>:print</literal> but does not bind new
+ variables.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
+ <literal>:set</literal> <literal>stop</literal>
+ [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
+ </term>
+ <listitem>
+ <para>Set a command to be executed when a breakpoint is hit, or a new
+ item in the history is selected. The most common use of
+ <literal>:set stop</literal> is to display the source code at the
+ current location, e.g. <literal>:set stop :list</literal>.</para>
+
+ <para>If a number is given before the command, then the commands are
+ run when the specified breakpoint (only) is hit. This can be quite
+ useful: for example, <literal>:set stop 1 :continue</literal>
+ effectively disables breakpoint 1, by running
+ <literal>:continue</literal> whenever it is hit (although GHCi will
+ still emit a message to say the breakpoint was hit). What's more,
+ with cunning use of <literal>:def</literal> and
+ <literal>:cmd</literal> you can use <literal>:set stop</literal> to
+ implement conditional breakpoints:</para>
+<screen>
+*Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
+*Main> :set stop 0 :cond (x < 3)
+</screen>
+ <para>Ignoring breakpoints for a specified number of iterations is
+ also possible using similar techniques.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:show bindings</literal>
<indexterm><primary><literal>:show bindings</literal></primary></indexterm>
</term>
<varlistentry>
<term>
+ <literal>:show breaks</literal>
+ <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>List the active breakpoints.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:show context</literal>
+ <indexterm><primary><literal>:show context</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>List the active evaluations that are stopped at breakpoints.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<literal>:show modules</literal>
<indexterm><primary><literal>:show modules</literal></primary></indexterm>
</term>
<para>Show the list of modules currently load.</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term>
+ <literal>:show [args|prog|prompt|editor|stop]</literal>
+ <indexterm><primary><literal>:show</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Displays the specified setting (see
+ <literal>:set</literal>).</para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term>
<literal>:sprint</literal>
<indexterm><primary><literal>:sprint</literal></primary></indexterm>
</term>
<listitem>
- <para>Prints a semievaluated value without forcing its evaluation.
- <literal>:sprint</literal> and its sibling <literal>:print</literal>
- are very useful to observe how lazy evaluation works in your code. For example:
-<screen>
-Prelude> let li = map Just [1..5]
-Prelude> :sp li
-li - _
-Prelude> head li
-Just 1
-Prelude> :sp li
-li - Just 1 : _
-Prelude> last li
-Just 5
-Prelude> :sp li
-li - [Just 1,_,_,_,Just 5]
-</screen>
- The example uses <literal>:sprint</literal> to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
- with it. Note for instance how <quote>last</quote> traverses all the elements of
- the list to compute its result, but without evaluating the individual elements.
- </para>
+ <para>Prints a value without forcing its evaluation.
+ <literal>:sprint</literal> is similar to <literal>:print</literal>,
+ with the difference that unevaluated subterms are not bound to new
+ variables, they are simply denoted by ‘_’.</para>
</listitem>
</varlistentry>
+
<varlistentry>
<term>
- <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
- <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
- <indexterm><primary><literal>:etags</literal></primary>
- </indexterm>
- <indexterm><primary><literal>:etags</literal></primary>
- </indexterm>
- </term>
+ <literal>:step [<replaceable>expr</replaceable>]</literal>
+ <indexterm><primary><literal>:step</literal></primary></indexterm>
+ </term>
<listitem>
- <para>Generates a “tags” file for Vi-style editors
- (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
- no filename is specified, the defaulit <filename>tags</filename> or
- <filename>TAGS</filename> is
- used, respectively. Tags for all the functions, constructors and
- types in the currently loaded modules are created. All modules must
- be interpreted for these commands to work.</para>
- <para>See also <xref linkend="hasktags" />.</para>
+ <para>Single-step from the last breakpoint. With an expression
+ argument, begins evaluation of the expression with a
+ single-step.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>:type</literal> <replaceable>expression</replaceable>
- <indexterm><primary><literal>:type</literal></primary></indexterm>
+ <literal>:trace [<replaceable>expr</replaceable>]</literal>
+ <indexterm><primary><literal>:trace</literal></primary></indexterm>
</term>
<listitem>
- <para>Infers and prints the type of
- <replaceable>expression</replaceable>, including explicit
- forall quantifiers for polymorphic types. The monomorphism
- restriction is <emphasis>not</emphasis> applied to the
- expression during type inference.</para>
+ <para>Evaluates the given expression (or from the last breakpoint if
+ no expression is given), and additionally logs the evaluation
+ steps for later inspection using <literal>:history</literal>. See
+ <xref linkend="tracing" />.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>:kind</literal> <replaceable>type</replaceable>
- <indexterm><primary><literal>:kind</literal></primary></indexterm>
+ <literal>:type</literal> <replaceable>expression</replaceable>
+ <indexterm><primary><literal>:type</literal></primary></indexterm>
</term>
<listitem>
- <para>Infers and prints the kind of
- <replaceable>type</replaceable>. The latter can be an arbitrary
- type expression, including a partial application of a type constructor,
- such as <literal>Either Int</literal>.</para>
+ <para>Infers and prints the type of
+ <replaceable>expression</replaceable>, including explicit
+ forall quantifiers for polymorphic types. The monomorphism
+ restriction is <emphasis>not</emphasis> applied to the
+ expression during type inference.</para>
</listitem>
</varlistentry>