<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>2.4. Type Signatures</title><link rel="stylesheet" href="fptools.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.73.2"><link rel="start" href="index.html" title="Happy User Guide"><link rel="up" href="sec-using.html" title="Chapter 2. Using Happy"><link rel="prev" href="sec-Precedences.html" title="2.3. Using Precedences"><link rel="next" href="sec-monads.html" title="2.5. Monadic Parsers"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">2.4. Type Signatures</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="sec-Precedences.html">Prev</a> </td><th width="60%" align="center">Chapter 2. Using <span class="application">Happy</span></th><td width="20%" align="right"> <a accesskey="n" href="sec-monads.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sec-type-signatures"></a>2.4. Type Signatures</h2></div></div></div><a class="indexterm" name="id2496734"></a><p><span class="application">Happy</span> allows you to include type signatures
in the grammar file itself, to indicate the type of each
production. This has several benefits:</p><div class="itemizedlist"><ul type="disc"><li><p> Documentation: including types in the grammar helps
to document the grammar for someone else (and indeed
yourself) reading the code.</p></li><li><p> Fixing type errors in the generated module can become
slightly easier if <span class="application">Happy</span> has inserted type
signatures for you. This is a slightly dubious benefit,
since type errors in the generated module are still somewhat
difficult to find. </p></li><li><p> Type signatures generally help the Haskell compiler
to compile the parser faster. This is important when really
large grammar files are being used.</p></li></ul></div><p>The syntax for type signatures in the grammar file is as
follows:</p><pre class="programlisting">
stmts :: { [ Stmt ] }
stmts : stmts stmt { $2 : $1 }
| stmt { [$1] }
</pre><p>In fact, you can leave out the superfluous occurrence of
<code class="literal">stmts</code>:</p><pre class="programlisting">
stmts :: { [ Stmt ] }
: stmts stmt { $2 : $1 }
| stmt { [$1] }
</pre><p>Note that currently, you have to include type signatures
for <span class="emphasis"><em>all</em></span> the productions in the grammar to benefit
from the second and third points above. This is due to boring
technical reasons, but it is hoped that this restriction can be
removed in the future.</p><p>It is possible to have productions with polymorphic or overloaded
types. However, because the type of each production becomes the
argument type of a constructor in an algebraic datatype in the
generated source file, compiling the generated file requires a compiler
that supports local universal quantification. GHC (with the
<code class="option">-fglasgow-exts</code> option) and Hugs are known to support
this.</p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sec-Precedences.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sec-using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sec-monads.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">2.3. Using Precedences </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 2.5. Monadic Parsers</td></tr></table></div></body></html>
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