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<h2 class="section" id="sec62">6.1  Lexical conventions</h2>
<h4 class="subsubsection" id="sec63">Blanks</h4>
<p>The following characters are considered as blanks: space,
horizontal tabulation, carriage return, line feed and form feed. Blanks are
ignored, but they separate adjacent identifiers, literals and
keywords that would otherwise be confused as one single identifier,
literal or keyword.</p><h4 class="subsubsection" id="sec64">Comments</h4>
<p>Comments are introduced by the two characters <span class="c008">(*</span>, with no
intervening blanks, and terminated by the characters <span class="c008">*)</span>, with
no intervening blanks. Comments are treated as blank characters.
Comments do not occur inside string or character literals. Nested
comments are handled correctly.</p><h4 class="subsubsection" id="sec65">Identifiers</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="ident"><span class="c014">ident</span></a></td><td class="c022">::=</td><td class="c024"> ( <a class="syntax" href="#letter"><span class="c014">letter</span></a> ∣  <span class="c008">_</span> ) { <a class="syntax" href="#letter"><span class="c014">letter</span></a> ∣  <span class="c008">0</span> … <span class="c008">9</span> ∣  <span class="c008">_</span> ∣  <span class="c008">'</span> }  </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="capitalized-ident"><span class="c014">capitalized-ident</span></a></td><td class="c022">::=</td><td class="c024"> (<span class="c008">A</span> … <span class="c008">Z</span>) { <a class="syntax" href="#letter"><span class="c014">letter</span></a> ∣  <span class="c008">0</span> … <span class="c008">9</span> ∣  <span class="c008">_</span> ∣  <span class="c008">'</span> }  </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="lowercase-ident"><span class="c014">lowercase-ident</span></a></td><td class="c022">::=</td><td class="c024">
(<span class="c008">a</span> … <span class="c008">z</span> ∣  <span class="c008">_</span>) { <a class="syntax" href="#letter"><span class="c014">letter</span></a> ∣  <span class="c008">0</span> … <span class="c008">9</span> ∣  <span class="c008">_</span> ∣  <span class="c008">'</span> }  </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="letter"><span class="c014">letter</span></a></td><td class="c022">::=</td><td class="c024"> <span class="c008">A</span> … <span class="c008">Z</span> ∣  <span class="c008">a</span> … <span class="c008">z</span>
</td></tr>
</table></td></tr>
</table><p>Identifiers are sequences of letters, digits, <span class="c007">_</span> (the underscore
character), and <span class="c007">'</span> (the single quote), starting with a
letter or an underscore.
Letters contain at least the 52 lowercase and uppercase
letters from the ASCII set. The current implementation
also recognizes as letters some characters from the ISO
8859-1 set (characters 192–214 and 216–222 as uppercase letters;
characters 223–246 and 248–255 as lowercase letters). This
feature is deprecated and should be avoided for future compatibility.</p><p>All characters in an identifier are
meaningful. The current implementation accepts identifiers up to
16000000 characters in length.</p><p>In many places, OCaml makes a distinction between capitalized
identifiers and identifiers that begin with a lowercase letter. The
underscore character is considered a lowercase letter for this
purpose.</p><h4 class="subsubsection" id="sec66">Integer literals</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="integer-literal"><span class="c014">integer-literal</span></a></td><td class="c022">::=</td><td class="c024">
[<span class="c008">-</span>] (<span class="c008">0</span>…<span class="c008">9</span>) { <span class="c008">0</span>…<span class="c008">9</span> ∣  <span class="c008">_</span> }
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> [<span class="c008">-</span>] (<span class="c008">0x</span>∣ <span class="c008">0X</span>) (<span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">A</span>…<span class="c008">F</span>∣ <span class="c008">a</span>…<span class="c008">f</span>)
{ <span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">A</span>…<span class="c008">F</span>∣ <span class="c008">a</span>…<span class="c008">f</span>∣ <span class="c008">_</span> }
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> [<span class="c008">-</span>] (<span class="c008">0o</span>∣ <span class="c008">0O</span>) (<span class="c008">0</span>…<span class="c008">7</span>) { <span class="c008">0</span>…<span class="c008">7</span>∣ <span class="c008">_</span> }
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> [<span class="c008">-</span>] (<span class="c008">0b</span>∣ <span class="c008">0B</span>) (<span class="c008">0</span>…<span class="c008">1</span>) { <span class="c008">0</span>…<span class="c008">1</span>∣ <span class="c008">_</span> }
</td></tr>
</table></td></tr>
</table><p>An integer literal is a sequence of one or more digits, optionally
preceded by a minus sign. By default, integer literals are in decimal
(radix 10). The following prefixes select a different radix:
</p><div class="center"><table class="c001 cellpadding1" border=1><tr><td class="c021"><span class="c019">Prefix</span></td><td class="c021"><span class="c019">Radix</span> </td></tr>
<tr><td class="c023">
<span class="c007">0x</span>, <span class="c007">0X</span></td><td class="c023">hexadecimal (radix 16) </td></tr>
<tr><td class="c023"><span class="c007">0o</span>, <span class="c007">0O</span></td><td class="c023">octal (radix 8) </td></tr>
<tr><td class="c023"><span class="c007">0b</span>, <span class="c007">0B</span></td><td class="c023">binary (radix 2) </td></tr>
</table></div><p>
(The initial <span class="c008">0</span> is the digit zero; the <span class="c008">O</span> for octal is the letter O.)
The interpretation of integer literals that fall outside the range of
representable integer values is undefined.</p><p>For convenience and readability, underscore characters (<span class="c008">_</span>) are accepted
(and ignored) within integer literals.</p><h4 class="subsubsection" id="sec67">Floating-point literals</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="float-literal"><span class="c014">float-literal</span></a></td><td class="c022">::=</td><td class="c024">
[<span class="c008">-</span>] (<span class="c008">0</span>…<span class="c008">9</span>) { <span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">_</span> } [<span class="c008">.</span> { <span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">_</span> }]
[(<span class="c008">e</span>∣ <span class="c008">E</span>) [<span class="c008">+</span>∣ <span class="c008">-</span>] (<span class="c008">0</span>…<span class="c008">9</span>) { <span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">_</span> }]
</td></tr>
</table></td></tr>
</table><p>Floating-point decimals consist in an integer part, a decimal part and
an exponent part. The integer part is a sequence of one or more
digits, optionally preceded by a minus sign. The decimal part is a
decimal point followed by zero, one or more digits.
The exponent part is the character <span class="c008">e</span> or <span class="c008">E</span> followed by an
optional <span class="c008">+</span> or <span class="c008">-</span> sign, followed by one or more digits.
The decimal part or the exponent part can be omitted but not both, to
avoid ambiguity with integer literals.
The interpretation of floating-point literals that fall outside the
range of representable floating-point values is undefined.</p><p>For convenience and readability, underscore characters (<span class="c008">_</span>) are accepted
(and ignored) within floating-point literals.</p><h4 class="subsubsection" id="sec68">Character literals</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="char-literal"><span class="c014">char-literal</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">'</span> <span class="c014">regular-char</span> <span class="c008">'</span>
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <span class="c008">'</span> <a class="syntax" href="#escape-sequence"><span class="c014">escape-sequence</span></a> <span class="c008">'</span>
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="escape-sequence"><span class="c014">escape-sequence</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">\</span> ( <span class="c008">\</span> ∣  <span class="c008">"</span> ∣  <span class="c008">'</span> ∣  <span class="c008">n</span> ∣  <span class="c008">t</span> ∣  <span class="c008">b</span> ∣  <span class="c008">r</span> ∣  <span class="c014">space</span> )
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <span class="c008">\</span> (<span class="c008">0</span>…<span class="c008">9</span>) (<span class="c008">0</span>…<span class="c008">9</span>) (<span class="c008">0</span>…<span class="c008">9</span>)
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <span class="c008">\x</span> (<span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">A</span>…<span class="c008">F</span>∣ <span class="c008">a</span>…<span class="c008">f</span>)
(<span class="c008">0</span>…<span class="c008">9</span>∣ <span class="c008">A</span>…<span class="c008">F</span>∣ <span class="c008">a</span>…<span class="c008">f</span>)
</td></tr>
</table></td></tr>
</table><p>Character literals are delimited by <span class="c008">'</span> (single quote) characters.
The two single quotes enclose either one character different from
<span class="c008">'</span> and <span class="c008">\</span>, or one of the escape sequences below:
</p><div class="center"><table class="c001 cellpadding1" border=1><tr><td class="c021"><span class="c019">Sequence</span></td><td class="c021"><span class="c019">Character denoted</span> </td></tr>
<tr><td class="c023">
<span class="c007">\\</span></td><td class="c023">backslash (<span class="c007">\</span>) </td></tr>
<tr><td class="c023"><span class="c007">\"</span></td><td class="c023">double quote (<span class="c007">"</span>) </td></tr>
<tr><td class="c023"><span class="c007">\'</span></td><td class="c023">single quote (<span class="c007">'</span>) </td></tr>
<tr><td class="c023"><span class="c007">\n</span></td><td class="c023">linefeed (LF) </td></tr>
<tr><td class="c023"><span class="c007">\r</span></td><td class="c023">carriage return (CR) </td></tr>
<tr><td class="c023"><span class="c007">\t</span></td><td class="c023">horizontal tabulation (TAB) </td></tr>
<tr><td class="c023"><span class="c007">\b</span></td><td class="c023">backspace (BS) </td></tr>
<tr><td class="c023"><span class="c007">\</span><span class="c013">space</span></td><td class="c023">space (SPC) </td></tr>
<tr><td class="c023"><span class="c007">\</span><span class="c013">ddd</span></td><td class="c023">the character with ASCII code <span class="c013">ddd</span> in decimal </td></tr>
<tr><td class="c023"><span class="c007">\x</span><span class="c013">hh</span></td><td class="c023">the character with ASCII code <span class="c013">hh</span> in hexadecimal </td></tr>
</table></div><h4 class="subsubsection" id="sec69">String literals</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="string-literal"><span class="c014">string-literal</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">"</span> { <a class="syntax" href="#string-character"><span class="c014">string-character</span></a> } <span class="c008">"</span>
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="string-character"><span class="c014">string-character</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c014">regular-string-char</span>
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <a class="syntax" href="#escape-sequence"><span class="c014">escape-sequence</span></a>
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <span class="c008">\</span> <span class="c014">newline</span>  { <span class="c014">space</span> ∣  <span class="c014">tab</span> }
</td></tr>
</table></td></tr>
</table><p>String literals are delimited by <span class="c008">"</span> (double quote) characters.
The two double quotes enclose a sequence of either characters
different from <span class="c008">"</span> and <span class="c008">\</span>, or escape sequences from the
table given above for character literals.</p><p>To allow splitting long string literals across lines, the sequence
<span class="c007">\</span><span class="c013">newline</span> <span class="c013">spaces-or-tabs</span> (a backslash at the end of a line
followed by any number of spaces and horizontal tabulations at the
beginning of the next line) is ignored inside string literals.</p><p>The current implementation places practically no restrictions on the
length of string literals.</p><h4 class="subsubsection" id="sec70">Naming labels</h4>
<p>To avoid ambiguities, naming labels in expressions cannot just be defined
syntactically as the sequence of the three tokens <span class="c007">~</span>, <a class="syntax" href="#ident"><span class="c014">ident</span></a> and
<span class="c007">:</span>, and have to be defined at the lexical level.</p><table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="label-name"><span class="c014">label-name</span></a></td><td class="c022">::=</td><td class="c024"> <a class="syntax" href="#lowercase-ident"><span class="c014">lowercase-ident</span></a>
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="label"><span class="c014">label</span></a></td><td class="c022">::=</td><td class="c024"> <span class="c008">~</span> <a class="syntax" href="#label-name"><span class="c014">label-name</span></a> <span class="c008">:</span>
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="optlabel"><span class="c014">optlabel</span></a></td><td class="c022">::=</td><td class="c024"> <span class="c008">?</span> <a class="syntax" href="#label-name"><span class="c014">label-name</span></a> <span class="c008">:</span>
</td></tr>
</table></td></tr>
</table><p>Naming labels come in two flavours: <a class="syntax" href="#label"><span class="c014">label</span></a> for normal arguments and
<a class="syntax" href="#optlabel"><span class="c014">optlabel</span></a> for optional ones. They are simply distinguished by their
first character, either <span class="c007">~</span> or <span class="c007">?</span>.</p><p>Despite <a class="syntax" href="#label"><span class="c014">label</span></a> and <a class="syntax" href="#optlabel"><span class="c014">optlabel</span></a> being lexical entities in expressions,
their expansions <span class="c008">~</span> <a class="syntax" href="#label-name"><span class="c014">label-name</span></a> <span class="c008">:</span> and <span class="c008">?</span> <a class="syntax" href="#label-name"><span class="c014">label-name</span></a> <span class="c008">:</span> will be
used in grammars, for the sake of readability. Note also that inside
type expressions, this expansion can be taken literally, <em>i.e.</em>
there are really 3 tokens, with optional blanks between them.</p><h4 class="subsubsection" id="sec71">Prefix and infix symbols</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="infix-symbol"><span class="c014">infix-symbol</span></a></td><td class="c022">::=</td><td class="c024">
(<span class="c008">=</span> ∣  <span class="c008"><</span> ∣  <span class="c008">></span> ∣  <span class="c008">@</span> ∣  <span class="c008">^</span> ∣  <span class="c008">|</span> ∣  <span class="c008">&</span> ∣ 
<span class="c008">+</span> ∣  <span class="c008">-</span> ∣  <span class="c008">*</span> ∣  <span class="c008">/</span> ∣  <span class="c008">$</span> ∣  <span class="c008">%</span>) { <a class="syntax" href="#operator-char"><span class="c014">operator-char</span></a> }
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="prefix-symbol"><span class="c014">prefix-symbol</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">!</span> { <a class="syntax" href="#operator-char"><span class="c014">operator-char</span></a> }
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> (<span class="c008">?</span> ∣  <span class="c008">~</span>) { <a class="syntax" href="#operator-char"><span class="c014">operator-char</span></a> }<sup>+</sup>
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="operator-char"><span class="c014">operator-char</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">!</span> ∣  <span class="c008">$</span> ∣  <span class="c008">%</span> ∣  <span class="c008">&</span> ∣  <span class="c008">*</span> ∣  <span class="c008">+</span> ∣  <span class="c008">-</span> ∣  <span class="c008">.</span> ∣ 
<span class="c008">/</span> ∣  <span class="c008">:</span> ∣  <span class="c008"><</span> ∣  <span class="c008">=</span> ∣  <span class="c008">></span> ∣  <span class="c008">?</span> ∣  <span class="c008">@</span> ∣ 
<span class="c008">^</span> ∣  <span class="c008">|</span> ∣  <span class="c008">~</span>
</td></tr>
</table></td></tr>
</table><p>Sequences of “operator characters”, such as <span class="c007"><=></span> or <span class="c007">!!</span>,
are read as a single token from the <a class="syntax" href="#infix-symbol"><span class="c014">infix-symbol</span></a> or <a class="syntax" href="#prefix-symbol"><span class="c014">prefix-symbol</span></a>
class. These symbols are parsed as prefix and infix operators inside
expressions, but otherwise behave like normal identifiers.
</p><h4 class="subsubsection" id="sec72">Keywords</h4>
<p>The identifiers below are reserved as keywords, and cannot be employed
otherwise:
</p><pre> and as assert asr begin class
constraint do done downto else end
exception external false for fun function
functor if in include inherit initializer
land lazy let lor lsl lsr
lxor match method mod module mutable
new object of open or private
rec sig struct then to true
try type val virtual when while
with
</pre><p> <br>
The following character sequences are also keywords:
</p><pre>
<span class="c007"> != # & && ' ( ) * + , -</span>
<span class="c007"> -. -> . .. : :: := :> ; ;; <</span>
<span class="c007"> <- = > >] >} ? [ [< [> [| ]</span>
<span class="c007"> _ ` { {< | |] || } ~</span>
</pre><p>
Note that the following identifiers are keywords of the Camlp4
extensions and should be avoided for compatibility reasons.
</p><pre> parser value $ $$ $: <: << >> ??
</pre><h4 class="subsubsection" id="sec73">Ambiguities</h4>
<p>Lexical ambiguities are resolved according to the “longest match”
rule: when a character sequence can be decomposed into two tokens in
several different ways, the decomposition retained is the one with the
longest first token.</p><h4 class="subsubsection" id="sec74">Line number directives</h4>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="linenum-directive"><span class="c014">linenum-directive</span></a></td><td class="c022">::=</td><td class="c024">
<span class="c008">#</span> {<span class="c008">0</span> … <span class="c008">9</span>}<sup>+</sup>
 </td></tr>
<tr><td class="c025"> </td><td class="c022">∣</td><td class="c024"> <span class="c008">#</span> {<span class="c008">0</span> … <span class="c008">9</span>}<sup>+</sup> <span class="c008">"</span> { <a class="syntax" href="#string-character"><span class="c014">string-character</span></a> } <span class="c008">"</span>
</td></tr>
</table></td></tr>
</table><p>Preprocessors that generate OCaml source code can insert line number
directives in their output so that error messages produced by the
compiler contain line numbers and file names referring to the source
file before preprocessing, instead of after preprocessing.
A line number directive is composed of a <span class="c008">#</span> (sharp sign), followed by
a positive integer (the source line number), optionally followed by a
character string (the source file name).
Line number directives are treated as blanks during lexical
analysis.</p>
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