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 Language extensions
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<TR><TD><A NAME="htoc90"><B><FONT SIZE=6>Chapter&nbsp;7</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=6>Language extensions</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE> <A NAME="c:extensions"></A>
<BR>
This chapter describes syntactic extensions and convenience features
that are implemented in Objective Caml, but not described in the
Objective Caml reference manual.<BR>
<BR>
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<TR><TD><A NAME="htoc91"><B><FONT SIZE=5>7.1</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Streams and stream parsers</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="s:streams"></A><BR>
Streams and stream parsers are no longer part of the Objective Caml
language, but available through a CamlP4 syntax extension. See the
CamlP4 reference manual for more information. Objective Caml programs
that use streams and stream parsers can be compiled with the
<TT>-pp camlp4o</TT> option to <TT>ocamlc</TT> and <TT>ocamlopt</TT>. For interactive use,
run <TT>ocaml</TT> and issue the <CODE>#load "camlp4o.cma";;</CODE> command.<BR>
<BR>
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<TR><TD><A NAME="htoc92"><B><FONT SIZE=5>7.2</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Range patterns</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE><BR>
In patterns, Objective Caml recognizes the form
<TT><FONT COLOR=blue>'</FONT></TT> <TT><I><FONT COLOR=maroon>c</FONT></I></TT> <TT><FONT COLOR=blue>'</FONT></TT> <TT><FONT COLOR=blue>..</FONT></TT> <TT><FONT COLOR=blue>'</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>d</FONT></I></TT> <TT><FONT COLOR=blue>'</FONT></TT>
(two character literals separated by&nbsp;<TT>..</TT>) as shorthand for the pattern
<DIV ALIGN=center>
<TT><FONT COLOR=blue>'</FONT></TT> <TT><I><FONT COLOR=maroon>c</FONT></I></TT> <TT><FONT COLOR=blue>'</FONT></TT> <TT><FONT COLOR=blue>|</FONT></TT> <TT><FONT COLOR=blue>'</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>c</FONT></I></TT><SUB><FONT SIZE=2>1</FONT></SUB> <TT><FONT COLOR=blue>'</FONT></TT> <TT><FONT COLOR=blue>|</FONT></TT> <TT><FONT COLOR=blue>'</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>c</FONT></I></TT><SUB><FONT SIZE=2>2</FONT></SUB> <TT><FONT COLOR=blue>'</FONT></TT> <TT><FONT COLOR=blue>|</FONT></TT> ...
 <TT><FONT COLOR=blue>|</FONT></TT> <TT><FONT COLOR=blue>'</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>c</FONT></I></TT><SUB><FONT SIZE=2><I>n</I></FONT></SUB> <TT><FONT COLOR=blue>'</FONT></TT> <TT><FONT COLOR=blue>|</FONT></TT> <TT><FONT COLOR=blue>'</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>d</FONT></I></TT> <TT><FONT COLOR=blue>'</FONT></TT>
</DIV>
where <I>c</I><SUB><FONT SIZE=2>1</FONT></SUB>, <I>c</I><SUB><FONT SIZE=2>2</FONT></SUB>, ..., <I>c</I><SUB><FONT SIZE=2><I>n</I></FONT></SUB> are the characters
that occur between <I>c</I> and <I>d</I> in the ASCII character set. For
instance, the pattern <TT>'0'..'9'</TT> matches all characters that are digits.<BR>
<BR>
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<TR><TD><A NAME="htoc93"><B><FONT SIZE=5>7.3</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Assertion checking</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="@manual.kwd168"></A><BR>
Objective Caml supports the <TT>assert</TT> construct to check debugging assertions.
The expression <TT><FONT COLOR=blue>assert</FONT></TT> <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> evaluates the expression <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> and
returns <TT>()</TT> if <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> evaluates to <TT>true</TT>. Otherwise, the exception
<TT>Assert_failure</TT> is raised with the source file name and the
location of <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> as arguments. Assertion
checking can be turned off with the <TT>-noassert</TT> compiler option.<BR>
<BR>
As a special case, <TT>assert false</TT> is reduced to
<TT>raise (Assert_failure ...)</TT>, which is polymorphic (and
is not turned off by the <TT>-noassert</TT> option).
<A NAME="@manual3"></A><BR>
<BR>
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<TR><TD BGCOLOR="#66ff66"><DIV ALIGN=center><TABLE>
<TR><TD><A NAME="htoc94"><B><FONT SIZE=5>7.4</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Deferred computations</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="@manual.kwd169"></A><BR>
The expression <TT><FONT COLOR=blue>lazy</FONT></TT> <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> returns a value <I>v</I> of type <TT>Lazy.t</TT> that
encapsulates the computation of <TT><I><FONT COLOR=maroon>expr</FONT></I></TT>. The argument <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> is not
evaluated at this point in the program. Instead, its evaluation will
be performed the first time <TT>Lazy.force</TT> is applied to the value
<I>v</I>, returning the actual value of <TT><I><FONT COLOR=maroon>expr</FONT></I></TT>. Subsequent applications
of <TT>Lazy.force</TT> to <I>v</I> do not evaluate <TT><I><FONT COLOR=maroon>expr</FONT></I></TT> again.
For more information, see the description of module <TT>Lazy</TT> in the
standard library
(<a HREF=libref/Lazy.html>Module <tt>Lazy</tt></a>
 ).
<A NAME="@manual4"></A><A NAME="@manual5"></A><BR>
<BR>
<TABLE CELLPADDING=0 CELLSPACING=0 WIDTH="100%">
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<TR><TD><A NAME="htoc95"><B><FONT SIZE=5>7.5</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Local modules</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE>
<A NAME="@manual.kwd170"></A>
<A NAME="@manual.kwd171"></A><BR>
The expression
<TT><FONT COLOR=blue>let</FONT></TT> <TT><FONT COLOR=blue>module</FONT></TT> <TT><I><FONT COLOR=maroon>module-name</FONT></I></TT> <TT><FONT COLOR=blue>=</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>module-expr</FONT></I></TT> <TT><FONT COLOR=blue>in</FONT></TT> &nbsp;<TT><I><FONT COLOR=maroon>expr</FONT></I></TT>
locally binds the module expression <TT><I><FONT COLOR=maroon>module-expr</FONT></I></TT> to the identifier
<TT><I><FONT COLOR=maroon>module-name</FONT></I></TT> during the evaluation of the expression <TT><I><FONT COLOR=maroon>expr</FONT></I></TT>.
It then returns the value of <TT><I><FONT COLOR=maroon>expr</FONT></I></TT>. For example:
<PRE>
        let remove_duplicates comparison_fun string_list =
          let module StringSet =
            Set.Make(struct type t = string
                            let compare = comparison_fun end) in
          StringSet.elements
            (List.fold_right StringSet.add string_list StringSet.empty)
</PRE>
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<TR><TD><A NAME="htoc96"><B><FONT SIZE=5>7.6</FONT></B></A></TD>
<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=5>Grouping in integer and floating-point literals</FONT></B></TD>
</TR></TABLE></DIV></TD>
</TR></TABLE><BR>
In integer and floating-point literals, the character <TT>_</TT> (underscore)
can be used to separate groups of digits, as in <TT>1_000_000</TT>,
<TT>0x45_FF</TT>, or <TT>1_234.567_89</TT>. The underscore characters are simply
ignored when reading the literal.
<BR>
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