<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Restricted type synonyms</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="UP"
TITLE="Hugs-specific language extensions"
HREF="hugs-only.html"><LINK
REL="PREVIOUS"
TITLE="Hugs-specific language extensions"
HREF="hugs-only.html"><LINK
REL="NEXT"
TITLE="Here documents"
HREF="here-documents.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="hugs-ug.css"></HEAD
><BODY
CLASS="SECT1"
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="hugs-only.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 7. Hugs-specific language extensions</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="here-documents.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RESTRICTED-SYNONYMS"
>7.2. Restricted type synonyms</A
></H1
><P
>Restricted type synonyms are a mechanism for defining abstract datatypes.
You can achieve similar effects, and more portably,
using the Haskell 98 module system.</P
><P
>The idea is that you can say that a type synonym is transparent
in the definitions of certain functions (the operations on the type),
but opaque elsewhere, by writing
<PRE
CLASS="PROGRAMLISTING"
>type Table a b = [(a,b)] in
empty :: Table a b,
isEmpty :: Table a b -> Bool,
add :: a -> b -> Table a b -> Table a b,
search :: a -> Table a b -> Maybe b
empty = []
isEmpty = null
add a b t = (a,b):t
search = lookup</PRE
>
or equivalently
<PRE
CLASS="PROGRAMLISTING"
>type Table a b = [(a,b)] in empty, isEmpty, add, search
empty :: Table a b
empty = []
...</PRE
>
See
<A
HREF="http://cvs.haskell.org/Hugs/pages/hugsman/exts.html#sect7.3.5"
TARGET="_top"
>Section 7.3.5 of the <I
CLASS="CITETITLE"
>Hugs 98 User Manual</I
></A
>
for details.</P
></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="hugs-only.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="here-documents.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Hugs-specific language extensions</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="hugs-only.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Here documents</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
