<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Using Hugs</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REL="HOME" TITLE="The Hugs 98 User's Guide" HREF="index.html"><LINK REL="PREVIOUS" TITLE="Introduction" HREF="introduction.html"><LINK REL="NEXT" TITLE="Loading and editing Haskell module files" HREF="module-commands.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="hugs-ug.css"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >The Hugs 98 User's Guide</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="introduction.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="module-commands.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="USING-HUGS" ></A >Chapter 2. Using Hugs</H1 ><P >The interpreter may be started with a command line of the form <P ><B CLASS="COMMAND" >hugs</B > [<TT CLASS="REPLACEABLE" ><I >option</I ></TT >...] [<TT CLASS="REPLACEABLE" ><I >file</I ></TT >...]</P > On many systems it can also be found in the system menus, and may be started by (double) clicking on a file with a <SPAN CLASS="QUOTE" >"<TT CLASS="LITERAL" >.hs</TT >"</SPAN > or <SPAN CLASS="QUOTE" >"<TT CLASS="LITERAL" >.lhs</TT >"</SPAN > extension.</P ><P >Hugs takes options from the command line and elsewhere (see <A HREF="options.html#OPTIONS-LIST" >Section 3.1</A >), and then loads the Haskell <TT CLASS="LITERAL" >Prelude</TT > module, as well as as any modules specified on the command line.</P ><P >Hugs starts with a banner like <PRE CLASS="SCREEN" >__ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Bugs: http://hackage.haskell.org/trac/hugs || || Version: March 2005 _________________________________________ Haskell 98 mode: Restart with command line option -98 to enable extensions Type :? for help Hugs></PRE > The prompt string <SAMP CLASS="PROMPT" >Hugs></SAMP > indicates that the current module is an empty module called <TT CLASS="LITERAL" >Hugs</TT > (assuming no modules were specified on the command line). At this prompt, you can type Haskell expressions to be evaluated, and also enter commands of the form <SPAN CLASS="QUOTE" >"<TT CLASS="LITERAL" >:<TT CLASS="REPLACEABLE" ><I >cmd</I ></TT ></TT >"</SPAN >, where <TT CLASS="REPLACEABLE" ><I >cmd</I ></TT > may be abbreviated to a single letter.</P ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="BASIC-COMMANDS" >2.1. Basic operation</A ></H1 ><P ></P ><DIV CLASS="VARIABLELIST" ><DL ><DT ><A NAME="EVAL-COMMAND" ></A ><P > <TT CLASS="REPLACEABLE" ><I >expr</I ></TT > </P ></DT ><DD ><P >Evaluate a Haskell expression. The expression cannot be broken over multiple lines. Usually, the value is simply converted to a string (using <CODE CLASS="FUNCTION" >show</CODE >) and printed: <PRE CLASS="SCREEN" >Hugs> 1+2 3</PRE > The printing style can be changed with the <A HREF="options.html#OPTION-USE-SHOW" ><CODE CLASS="OPTION" >-u</CODE ></A > option (see <A HREF="options.html#OPTIONS-EVALUATION" >Section 3.1.4</A >).</P ><P >However, if <TT CLASS="REPLACEABLE" ><I >expr</I ></TT > has type <TT CLASS="LITERAL" >IO</TT > <TT CLASS="REPLACEABLE" ><I >t</I ></TT > for some type <TT CLASS="REPLACEABLE" ><I >t</I ></TT >, the resulting IO action is performed: <PRE CLASS="SCREEN" >Hugs> print (1+2) >> putStrLn "bye" 3 bye</PRE > Usually the value produced by this action is ignored, but this can be changed with the <A HREF="options.html#OPTION-DISPLAY-IO" ><CODE CLASS="OPTION" >+I</CODE ></A > option (see <A HREF="options.html#OPTIONS-EVALUATION" >Section 3.1.4</A >).</P ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >On ambiguous types: </B >If the type of <TT CLASS="REPLACEABLE" ><I >expr</I ></TT > is ambiguous, defaulting is applied to each ambiguous type variable <TT CLASS="REPLACEABLE" ><I >v</I ></TT > whose constraints all have the form <TT CLASS="REPLACEABLE" ><I >C</I ></TT > <TT CLASS="REPLACEABLE" ><I >v</I ></TT > where <TT CLASS="REPLACEABLE" ><I >C</I ></TT > is a standard class, and at least one of these classes is a numeric class, or is <TT CLASS="LITERAL" >Show</TT >, <TT CLASS="LITERAL" >Eq</TT > or <TT CLASS="LITERAL" >Ord</TT >. (This is an extension of the Haskell 98 rule applied to top-level definitions in modules, which requires a numeric class.) It is an error if any ambiguous type variables cannot be handled in this way. For example, consider <PRE CLASS="SCREEN" >Hugs> reverse [] []</PRE > Here a <TT CLASS="LITERAL" >Show</TT > constraint on the list elements arises from Hugs's use of <CODE CLASS="FUNCTION" >show</CODE > to display the result, so the type of the elements defaults to <TT CLASS="LITERAL" >Integer</TT >, removing the ambiguity.</P ></BLOCKQUOTE ></DIV ></DD ><DT ><A NAME="TYPE-COMMAND" ></A ><P ><B CLASS="COMMAND" >:type</B > <TT CLASS="REPLACEABLE" ><I >expr</I ></TT > </P ></DT ><DD ><P >Print the type of <TT CLASS="REPLACEABLE" ><I >expr</I ></TT >, without evaluating it. Usually the defaulting rules (discussed above) are not applied to the type before printing, but this can be changed with the <A HREF="options.html#OPTION-DEFAULT-TYPES" ><CODE CLASS="OPTION" >+T</CODE ></A > option (see <A HREF="options.html#OPTIONS-EVALUATION" >Section 3.1.4</A >).</P ></DD ><DT ><A NAME="SET-COMMAND" ></A ><P ><B CLASS="COMMAND" >:set</B > [<TT CLASS="REPLACEABLE" ><I >option</I ></TT >...]</P ></DT ><DD ><P >Set command line options. See <A HREF="options.html#OPTIONS-LIST" >Section 3.1</A > for a list of available options. On Win32, the new option settings are saved to the registry under the <TT CLASS="LITERAL" >HKEY_CURRENT_USER</TT > key, and so persist across Hugs sessions. To make settings persistent on other systems, put them in the <CODE CLASS="ENVAR" >HUGSFLAGS</CODE > environment variable.</P ><P >If no options are given, list the available options and their current settings.</P ></DD ><DT ><A NAME="MAIN-COMMAND" ></A ><P ><B CLASS="COMMAND" >:main</B > [<TT CLASS="REPLACEABLE" ><I >argument</I ></TT >...]</P ></DT ><DD ><P >Run the <CODE CLASS="FUNCTION" >main</CODE > function, with the <CODE CLASS="FUNCTION" >getArgs</CODE > function returning the values specified for the arguments. This is useful for testing programs from the interpreter.</P ><P >If your <TT CLASS="LITERAL" >Main</TT > module imports <TT CLASS="LITERAL" >System.Environment</TT >, you can get the same effect using <CODE CLASS="FUNCTION" >withArgs</CODE >, e.g.</P ><PRE CLASS="SCREEN" >withArgs ["Hello", "World"] main</PRE ></DD ><DT ><A NAME="QUIT-COMMAND" ></A ><P ><B CLASS="COMMAND" >:quit</B > </P ></DT ><DD ><P >Exit the interpreter.</P ></DD ></DL ></DIV ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="introduction.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="module-commands.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Introduction</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Loading and editing Haskell module files</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
