📄 sec-type-signatures.html
字号:
<HTML
><HEAD
><TITLE
>Type Signatures</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.74b"><LINK
REL="HOME"
TITLE="Happy User Guide"
HREF="happy.html"><LINK
REL="UP"
TITLE="Using Happy"
HREF="sec-using.html"><LINK
REL="PREVIOUS"
TITLE="Using Precedences"
HREF="sec-precedences.html"><LINK
REL="NEXT"
TITLE="Monadic Parsers"
HREF="sec-monads.html"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Happy User Guide</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="sec-precedences.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 2. Using <SPAN
CLASS="APPLICATION"
>Happy</SPAN
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="sec-monads.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SEC-TYPE-SIGNATURES"
>2.4. Type Signatures</A
></H1
><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
><P
></P
><UL
><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
><P
>The syntax for type signatures in the grammar file is as
follows:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>stmts :: { [ Stmt ] }
stmts : stmts stmt { $2 : $1 }
| stmt { [$1] }</PRE
></TD
></TR
></TABLE
><P
>In fact, you can leave out the superfluous occurrence of
<TT
CLASS="LITERAL"
>stmts</TT
>:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>stmts :: { [ Stmt ] }
: stmts stmt { $2 : $1 }
| stmt { [$1] }</PRE
></TD
></TR
></TABLE
><P
>Note that currently, you have to include type signatures
for <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all</I
></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
></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="sec-precedences.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="happy.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="sec-monads.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Using Precedences</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="sec-using.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Monadic Parsers</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -