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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<!-- $Id: revsynt.html,v 6.3 2012-01-09 14:22:20 deraugla Exp $ -->
<!-- Copyright (c) INRIA 2007-2012 -->
<title>revised syntax</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<link rel="stylesheet" type="text/css" href="styles/base.css"
title="Normal" />
</head>
<body>
<div id="menu">
</div>
<div id="content">
<h1 class="top">The revised syntax</h1>
<p>The revised syntax is an alternative syntax of OCaml. It is close
to the normal syntax. We present here only the differences between
the two syntaxes.</p>
<p>Notice that there is a simple way to know how the normal syntax is
written in revised syntax: write the code in a file "foo.ml" in
normal syntax and type, in a shell:</p>
<pre>
camlp5o pr_r.cmo pr_rp.cmo foo.ml
</pre>
<p>And, conversely, how a file "bar.ml" written in revised syntax is
displayed in normal syntax:</p>
<pre>
camlp5r pr_o.cmo pr_op.cmo bar.ml
</pre>
<p>Even simpler, without creating a file:</p>
<pre>
camlp5o pr_r.cmo pr_op.cmo -impl -
... type in normal syntax ...
... type control-D ...
camlp5r pr_o.cmo pr_rp.cmo -impl -
... type in revised syntax ...
... type control-D ...
</pre>
<div id="tableofcontents">
</div>
<h2>Lexing</h2>
<ul>
<li>The character quote (<tt>'</tt>) can be written without backslash:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>'\''</tt></td>
<td><tt>'''</tt></td>
</tr>
</table>
</li>
</ul>
<h2>Modules, Structure and Signature items</h2>
<ul>
<li>Structure and signature items always end with a single semicolon
which is required.</li>
<li>In structures, the declaration of a value is introduced by the
keyword "value", instead of "let":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>let x = 42;;</tt></td>
<td><tt>value x = 42;</tt></td>
</tr>
<tr>
<td><tt>let x = 42 in x + 7;;</tt></td>
<td><tt>let x = 42 in x + 7;</tt></td>
</tr>
</table>
</li>
<li>In signatures, the declaration of a value is also introduced by
the keyword "value", instead of "val":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>val x : int;;</tt></td>
<td><tt>value x : int;</tt></td>
</tr>
</table>
</li>
<li>In signatures, abstract module types are represented by a quote
and an (any) identifier:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>module type MT;;</tt></td>
<td><tt>module type MT = 'a;</tt></td>
</tr>
</table>
</li>
<li>Functor application uses currying. Parentheses are not
required for the parameters:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = Set.Make(M).t;;</tt></td>
<td><tt>type t = (Set.Make M).t;</tt></td>
</tr>
<tr>
<td><tt>module M = Mod.Make (M1) (M2);;</tt></td>
<td><tt>module M = Mod.Make M1 M2;</tt></td>
</tr>
</table>
</li>
<li>It is possible to group several declarations together either in
an interface or in an implementation by enclosing them between
"declare" and "end" (this is useful when using syntax extensions
to generate several declarations from one). Example in an
interface:
<pre>
declare
type foo = [ Foo of int | Bar ];
value f : foo -> int;
end;
</pre>
</li>
</ul>
<h2>Expressions and Patterns</h2>
<h3>Imperative constructions</h3>
<ul>
<li>The sequence is introduced by the keyword "do" followed by "{"
and terminated by "}"; it is possible to put a semicolon after the
last expression:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>e1; e2; e3; e4</tt></td>
<td><tt>do { e1; e2; e3; e4 }</tt></td>
</tr>
</table>
</li>
<li>The "do" after the "while" loop and the "for" loop are followed
by a "{" and the loop end with "}"; it is possible to put a
semicolon after the last expression:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>while e1 do</tt></td>
<td><tt>while e1 do {</tt></td>
</tr>
<tr>
<td><tt> e1; e2; e3</tt></td>
<td><tt> e1; e2; e3</tt></td>
</tr>
<tr>
<td><tt>done</tt></td>
<td><tt>}</tt></td>
</tr>
<tr>
<td><tt></tt></td>
<td><tt></tt></td>
</tr>
<tr>
<td><tt>for i = e1 to e2 do </tt></td>
<td><tt>for i = e1 to e2 do {</tt></td>
</tr>
<tr>
<td><tt> e1; e2; e3</tt></td>
<td><tt> e1; e2; e3</tt></td>
</tr>
<tr>
<td><tt>done</tt></td>
<td><tt>}</tt></td>
</tr>
</table>
</li>
</ul>
<h3>Tuples and Lists</h3>
<ul>
<li>Parentheses are required in tuples:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>1, "hello", World</tt></td>
<td><tt>(1, "hello", World)</tt></td>
</tr>
</table>
</li>
<li>Lists are always enclosed with brackets. A list is a left
bracket, followed by a list of elements separated with semicolons,
optionally followed by colon-colon and an element, and ended by a
right bracket. Warning: the colon-colon is not an infix but is
just part of the syntactic construction.
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>x :: y</tt></td>
<td><tt>[x :: y]</tt></td>
</tr>
<tr>
<td><tt>[x; y; z]</tt></td>
<td><tt>[x; y; z]</tt></td>
</tr>
<tr>
<td><tt>x :: y :: z :: t</tt></td>
<td><tt>[x; y; z :: t]</tt></td>
</tr>
</table>
</li>
</ul>
<h3>Records</h3>
<ul>
<li>In record update, parentheses are required around the initial
expression:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>{e with field = a}</tt></td>
<td><tt>{(e) with field = a}</tt></td>
</tr>
</table>
</li>
<li>It is allowable to use function binding syntax in record field
definitions:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>{field = fun a -> e}</tt></td>
<td><tt>{field a = e}</tt></td>
</tr>
</table>
</li>
</ul>
<h3>Irrefutable patterns</h3>
<p>An <em>irrefutable pattern</em> is a pattern which is
syntactically visible and never fails. They are used in some
syntactic constructions. It is either:</p>
<ul>
<li>A variable,</li>
<li>The wildcard "_",</li>
<li>The constructor "()",</li>
<li>A tuple with irrefutable patterns,</li>
<li>A record with irrefutable patterns,</li>
<li>A type constraint with an irrefutable pattern.</li>
</ul>
<p>Notice that this definition is only syntactic: a constructor
belonging to a type having only one constructor is not considered as
an irrefutable pattern (except "()").</p>
<h3>Constructions with matching</h3>
<ul>
<li>The keyword "function" no longer exists. Only "fun" is used.</li>
<li>The pattern matching, in constructions with "fun", "match" and
"try" is closed with brackets: an open bracket "[" before the
first case, and a close bracket "]" after the last case:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>match e with</tt></td>
<td><tt>match e with</tt></td>
</tr>
<tr>
<td><tt> p1 -> e1</tt></td>
<td><tt>[ p1 -> e1</tt></td>
</tr>
<tr>
<td><tt>| p2 -> e2</tt></td>
<td><tt>| p2 -> e2 ]</tt></td>
</tr>
</table>
If there is only one case and if the pattern is irrefutable,
the brackets are not required. These examples work identically
in OCaml and in revised syntax:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>fun x -> x</tt></td>
<td><tt>fun x -> x</tt></td>
</tr>
<tr>
<td><tt>fun {foo=(y, _)} -> y</tt></td>
<td><tt>fun {foo=(y, _)} -> y</tt></td>
</tr>
</table>
</li>
<li>It is possible to write the empty function which always raises the
exception "Match_failure" when a parameter is applied. It is also possible to write and
empty "match", raising "Match_failure" after having evaluated its
expression and the empty "try", equivalent to its expression
without try:
<pre>
fun []
match e with []
try e with []
</pre>
</li>
<li>The patterns after "let" and "value" must be irrefutable. The
following OCaml expression:
<pre>
let f (x::y) = ...
</pre>
must be written:
<pre>
let f = fun [ [x::y] -> ...
</pre>
</li>
<li>It is possible to use a construction "where", equivalent to "let",
but usable only when where is only one binding. The expression:
<pre>
e1 where p = e
</pre>
is equivalent to:
<pre>
let p = e in e1
</pre>
</li>
</ul>
<h3>Mutables and Assignment</h3>
<ul>
<li>The statement "<tt><-</tt>" is written "<tt>:=</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>x.f <- y</tt></td>
<td><tt>x.f := y</tt></td>
</tr>
</table>
</li>
<li>The "ref" type is declared as a record type with one field named
"val", instead of "contents". The operator "!" does not exist any
more, and references are assigned like the other mutables:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>x := !x + y</tt></td>
<td><tt>x.val := x.val + y</tt></td>
</tr>
</table>
</li>
</ul>
<h3>Miscellaneous</h3>
<ul>
<li>The "<tt>else</tt>" is required in the "<tt>if</tt>"
statement:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>if a then b</tt></td>
<td><tt>if a then b else ()</tt></td>
</tr>
</table>
</li>
<li>The boolean operations "<tt>or</tt>" and "<tt>and</tt>" can only
be written with "<tt>||</tt>" and "<tt>&&</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>a or b & c</tt></td>
<td><tt>a || b && c</tt></td>
</tr>
<tr>
<td><tt>a || b && c</tt></td>
<td><tt>a || b && c</tt></td>
</tr>
</table>
</li>
<li>No more "<tt>begin end</tt>" construction. One must use
parentheses.</li>
<li>The operators as values are written with an backslash:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>(+)</tt></td>
<td><tt>\+</tt></td>
</tr>
<tr>
<td><tt>(mod)</tt></td>
<td><tt>\mod</tt></td>
</tr>
</table>
</li>
<li>Nested "as" patterns require parenthesis:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>function Some a as b, c -></tt></td>
<td><tt>fun [ ((Some a as b), c) -></tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
</table>
But they are not required before the right arrow:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>function Some a as b -></tt></td>
<td><tt>fun [ Some a as b -></tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
</table>
</li>
<li>The operators with special characters are not automatically
infix. To define infixes, use syntax extensions.
</li>
</ul>
<h2>Types and Constructors</h2>
<ul>
<li>The type constructors are before their type parameters, which
are curryfied:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>int list</tt></td>
<td><tt>list int</tt></td>
</tr>
<tr>
<td><tt>('a, bool) Hashtbl.t</tt></td>
<td><tt>Hashtbl.t 'a bool</tt></td>
</tr>
<tr>
<td><tt>type 'a foo = 'a list list</tt></td>
<td><tt>type foo 'a = list (list 'a)</tt></td>
</tr>
</table>
</li>
<li>The abstract types are represented by an unbound type variable:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type 'a foo;;</tt></td>
<td><tt>type foo 'a = 'b;</tt></td>
</tr>
<tr>
<td><tt>type bar;;</tt></td>
<td><tt>type bar = 'a;</tt></td>
</tr>
</table>
</li>
<li>Parentheses are required in tuples of types:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>int * bool</tt></td>
<td><tt>(int * bool)</tt></td>
</tr>
</table>
</li>
<li>In declarations of a concrete type, brackets must enclose the
constructor declarations:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = A of i | B;;</tt></td>
<td><tt>type t = [ A of i | B ];</tt></td>
</tr>
</table>
</li>
<li>It is possible to make the empty type, without constructor:
<pre>
type foo = [];
</pre>
</li>
<li>There is a syntax difference between data constructors with
several parameters and data constructors with one parameter of type
tuple:<br/><br/>
The declaration of a data constructor with several parameters
is done by separating the types with "and". In expressions and
patterns, these constructor parameters must be curryfied:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = C of t1 * t2;;</tt></td>
<td><tt>type t = [ C of t1 and t2 ];</tt></td>
</tr>
<tr>
<td><tt>C (x, y);;</tt></td>
<td><tt>C x y;</tt></td>
</tr>
</table>
The declaration of a data constructor with one parameter of type
tuple is done by using a tuple type. In expressions and patterns,
the parameter must not to be curryfied, since it is alone. In that
case the syntax of constructor parameters is the same between the
two syntaxes:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = D of (t1 * t2);;</tt></td>
<td><tt>type t = [ D of (t1 * t2) ];</tt></td>
</tr>
<tr>
<td><tt>D (x, y);;</tt></td>
<td><tt>D (x, y);</tt></td>
</tr>
</table>
</li>
<li>The bool constructors start with an uppercase letter. The identifiers
"true" and "false" are not keywords:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>true && false</tt></td>
<td><tt>True && False</tt></td>
</tr>
</table>
</li>
<li>In record types, the keyword "mutable" must appear after the
colon:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = {mutable x : t1};;</tt></td>
<td><tt>type t = {x : mutable t1};</tt></td>
</tr>
</table>
</li>
<li>Manifest types are with "==":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type 'a t = 'a option =</tt></td>
<td><tt>type t 'a = option 'a ==</tt></td>
</tr>
<tr>
<td><tt> None</tt></td>
<td><tt> [ None</tt></td>
</tr>
<tr>
<td><tt> | Some of 'a</tt></td>
<td><tt> | Some of 'a ]</tt></td>
</tr>
</table>
</li>
<li>Polymorphic types start with "<tt>!</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t =</tt></td>
<td><tt>type t =</tt></td>
</tr>
<tr>
<td><tt> { f : 'a . 'a list }</tt></td>
<td><tt> { f : ! 'a . list 'a }</tt></td>
</tr>
</table>
</li>
</ul>
<h2>Streams and Parsers</h2>
<ul>
<li>The streams and the stream patterns are bracketed with "<tt>[:</tt>"
and "<tt>:]</tt>" instead of "<tt>[<</tt>" and "<tt>>]</tt>".
</li>
<li>The stream component "terminal" is written with a back-quote
instead of a quote:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>[< '1; '2; s; '3 >]</tt></td>
<td><tt>[: `1; `2; s; `3 :]</tt></td>
</tr>
</table>
</li>
<li>The cases of parsers are bracketed with "<tt>[</tt>" and
"<tt>]</tt>", as with "fun", "match" and "try". If there is one
case, the brackets are not required:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>parser</tt></td>
<td><tt>parser</tt></td>
</tr>
<tr>
<td><tt> [< 'Foo >] -> e</tt></td>
<td><tt>[ [: `Foo :] -> e</tt></td>
</tr>
<tr>
<td><tt>| [< p = f >] -> f;;</tt></td>
<td><tt>| [: p = f :] -> f ];</tt></td>
</tr>
<tr>
<td><tt>parser [< 'x >] -> x;;</tt></td>
<td><tt>parser [: `x :] -> x;</tt></td>
</tr>
</table>
</li>
<li>It is possible to write the empty parser raising the exception
"Stream.Failure" whatever parameter is applied, and the empty
stream matching always raising "Stream.Failure":
<pre>
parser []
match e with parser []
</pre>
</li>
<li>In normal syntax, the error indicator starts with a double question
mark, in revised syntax with a simple question mark:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>parser</tt></td>
<td><tt>parser</tt></td>
</tr>
<tr>
<td><tt> [< '1; '2 ?? "error" >] -></tt></td>
<td><tt> [: `1; `2 ? "error" :] -></tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
</table>
</li>
<li>In normal syntax, the component optimization starts with
"<tt>?!</tt>", in revised syntax with "<tt>!</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>parser</tt></td>
<td><tt>parser</tt></td>
</tr>
<tr>
<td><tt> [< '1; '2 ?! >] -></tt></td>
<td><tt> [: `1; `2 ! :] -></tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
</table>
</li>
</ul>
<h2>Classes and Objects</h2>
<ul>
<li>Object items end with a single semicolon which is required.</li>
<li>Class type parameters follow the class identifier:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>class ['a, 'b] point = ...</tt></td>
<td><tt>class point ['a, 'b] = ...</tt></td>
</tr>
<tr>
<td><tt>class c = [int] color;;</tt></td>
<td><tt>class c = color [int];</tt></td>
</tr>
</table>
</li>
<li>In the type of class with parameters, the type of the parameters
are between brackets. Example in signature:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>class c : int -> point;;</tt></td>
<td><tt>class c : [int] -> point;</tt></td>
</tr>
</table>
</li>
<li>The keywords "virtual" and "private" must be in this order:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>method virtual private m :</tt></td>
<td><tt>method virtual private m :</tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
<tr>
<td><tt>method private virtual m :</tt></td>
<td><tt>method virtual private m :</tt></td>
</tr>
<tr>
<td><tt> ...</tt></td>
<td><tt> ...</tt></td>
</tr>
</table>
</li>
<li>Object variables are introduced with "value" instead of "val":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>object val x = 3 end</tt></td>
<td><tt>object value x = 3; end</tt></td>
</tr>
</table>
</li>
<li>Type constraints in objects are introduced with "type" instead of
"constraint":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>object constraint 'a = int end</tt></td>
<td><tt>object type 'a = int; end</tt></td>
</tr>
</table>
</li>
</ul>
<h2>Labels and Variants</h2>
<ul>
<li>Labels in types must start with "<tt>~</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>val x : num:int -> bool;;</tt></td>
<td><tt>value x : ~num:int -> bool;</tt></td>
</tr>
</table>
</li>
<li>Types whose number of variants are fixed start with "<tt>[ =</tt>":
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = [`On | `Off];;</tt></td>
<td><tt>type t = [ = `On | `Off];</tt></td>
</tr>
</table>
</li>
<li>The "<tt>[</tt>" and the "<tt><</tt>" in variant types must
not be sticked:
<table width="75%" class="centered" cellspacing="0" cellpadding="0">
<tr>
<th align="left" class="half">OCaml</th>
<th align="left" class="half">Revised</th>
</tr>
<tr>
<td><tt>type t = [< `Foo | `Bar ];;</tt></td>
<td><tt>type t = [ < `Foo | `Bar ];</tt></td>
</tr>
</table>
</li>
</ul>
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