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{{languageReferenceNavbar|Lexical Elements}}

The Visual Prolog Compiler is applied to a source file.  This source file may include other source files, which are (conceptually) inserted into the original source file to constitute one '''''compilation unit'''''. The compilation of a compilation unit is done in two conceptual steps:
*first the input is transformed into a sequence of tokens; 
*and then these tokens are syntactically analyzed and transformed into executable code.

The lexical analysis of the program will break the compilation unit <vpbnf><CompilationUnit></vpbnf> into a list of input elements <vpbnf><InputElement></vpbnf>

<vipbnf><CompilationUnit>:
   <InputElement>-list</vipbnf>

<vipbnf><InputElement>:
   <Comment>
   <WhiteSpace>
   <Token></vipbnf>

Only tokens are significant to the subsequent syntax analysis.

=== Comments ===

A Visual Prolog comment is written in one of the following ways:
*The <vpbnf>/*</vpbnf> (slash, asterisk) characters, followed by any sequence of characters (including new lines), terminated by the <vpbnf>*/</vpbnf> (asterisk, slash) characters. These comments can be multi-lined. They can also be nested.
*The <vpbnf>%</vpbnf> (percent sign) character, followed by any sequence of characters. Comments that begin with character <vpbnf>%</vpbnf> (percent sign) continue until the end of the line. Therefore, they are commonly called ''single-line comments''.

Notice the following comment example:

<vipbnf>/* Begin of Comment1
   % Nested Comment2 */ This mark does not close a multi-line comment because it is inside a single-line comment
   This is the real termination of Comment1 */</vipbnf>

=== Whitespace ===

<vipbnf><WhiteSpace>:
   <Blank>
   <Tab>
   <NewLine></vipbnf>

Here {{lang2|Lexical_Elements|Character_Literals|blank}} is a blank character, {{lang2|Lexical_Elements|Character_Literals|tab}} is a tabulation character, and {{lang2|Lexical_Elements|Character_Literals|newLine}} is a new line.

=== Tokens ===

<vipbnf><Token>:
   <Identifier>
   <Keyword>
   <Punctuator>
   <Operator>
   <Literal></vipbnf>
   
==== Identifiers ====

<vipbnf><Identifier>:
   <LowercaseIdentifier>
   <UppercaseIdentifier>
   <AnonymousIdentifier>
   <Ellipsis></vipbnf>

A <vpbnf><LowercaseIdentifier></vpbnf> is a sequence of letters, digits, and underscores that starts with a small letter. An <vpbnf><UppercaseIdentifier></vpbnf> is a sequence of letters, digits, and underscores that starts either with a capital letter or with an underscore.

<vipbnf><AnonymousIdentifier> :    _</vipbnf>

<vipbnf><Ellipsis> :
   ...</vipbnf>

==== Keywords ====

Keywords are divided into major and minor keywords, this division is only cosmetic however, there is no formal difference between major and minor keywords.  In the sequel we will however use different coloring for them.

<vipbnf><Keyword> :
   <MajorKeyword>
   <MinorKeyword></vipbnf>

<vipbnf><MajorKeyword> : one of
   class clauses constants constructors
   delegate domains
   end
   facts
   goal guards
   implement inherits interface
   monitor
   namespace
   open
   predicates
   properties
   resolve
   supports</vipbnf>

<vipbnf><MinorKeyword> : one of
   align and anyflow as 
   bitsize
   catch
   determ digits div do
   else elseif  erroneous externally
   failure finally foreach from
   if
   language
   mod multi
   nondeterm
   or
   procedure
   quot
   rem
   single
   then to try</vipbnf>

All keywords except <vp>as</vp> and <vp>language</vp> are reserved words.

Notice that <vp>div</vp> and <vp>mod</vp> are also reserved words, but these are categorized as operators.

<vp>guards</vp> and <vp>monitor</vp> are not used in the language, but are reserved for future usage.

==== Punctuation Marks ====

Punctuation marks in Visual Prolog have syntactic and semantic meaning to the compiler, but do not specify by themselves an operation that yields a value. Some punctuation marks, either alone or in combinations, can also be Visual Prolog operators.

Punctuation marks are:

<vipbnf><PunctuationMarks>: one of
   ;    !    ,    .    #    [    ]    |    (    )   :-    :    ::</vipbnf>

==== Operators ====

{{lang2|Terms|Operators|Operators}} specify an evaluation to be performed on involved operands.
<vipbnf><Operators>: one of
      +    -    /    *    ^    =  div mod quot rem
   <    >    <>    ><    <=    >=    :=</vipbnf>

All operators are {{lang2|Terms|Operators|binary}}, but <vp>-</vp> and <vp>+</vp> also exist as {{lang2|Terms|Operators|unary}} operators.

<vp>div</vp> and <vp>mod</vp> are reserved words.

===  Literals ===

Literals fall into following categories: integer, character, floating-point, string, binary and list.

<vipbnf><Literal>:
   <IntegerLiteral>
   <RealLiteral>
   <CharacterLiteral>
   <StringLiteral>
   <BinaryLiteral>
   <ListLiteral>
   <CompoundDomainLiteral></vipbnf>

==== Integral Literals ====

<vipbnf><IntegerLiteral>:
   <UnaryPlus>-opt    <DecimalDigit>-list
   <UnaryMinus>-opt   <DecimalDigit>-list
   <UnaryPlus>-opt    <OctalPrefix> <OctalDigit>-list
   <UnaryMinus>-opt   <OctalPrefix> <OctalDigit>-list
   <UnaryPlus>-opt    <HexadecimalPrefix> <HexadecimalDigit>-list
   <UnaryMinus>-opt   <HexadecimalPrefix> <HexadecimalDigit>-list

<UnaryPlus>:
   +

<UnaryMinus>:
   -

<OctalPrefix>:
   0o

<OctalDigit>: one of
   0 1 2 3 4 5 6 7

<DecimalDigit>: one of
   0 1 2 3 4 5 6 7 8 9

<HexadecimalPrefix>:
   0x

<HexadecimalDigit>: one of
   0 1 2 3 4 5 6 7 8 9 A a B b C c D d E e F f</vipbnf>

An integral literal can belong to {{lang2|Built-in_entities|integer|integer}} or {{lang2|Built-in_entities|unsigned|unsigned}} domains and it should not exceed maximum and minimum {{lang2|Built-in_entities|integer|integer}} or {{lang2|Built-in_entities|unsigned|unsigned}} values.

==== Real Literal ====

<vipbnf><RealLiteral>:
   <UnaryMinus>-opt <DecimalDigit>-list <FractionOfFloat>-opt <Exponent>-opt

<FractionOfFloat>:
   . <DecimalDigit>-list

<Exponent>:
   <ExponentSymbol> <ExponentSign>-opt <DecimalDigit>-list

<ExponentSymbol>: one of
   e E

<ExponentSign>: one of
   - +</vipbnf>

A floating literal should not exceed maximum and minimum real values. 

Either fractional part or exponent part is required for <vpbnf><RealLiteral></vpbnf>, otherwise it is interpreted as an integer literal.

==== Character Literals ====

<vipbnf><CharacterLiteral>:
   ' <CharacterValue> '</vipbnf>

<vpbnf><CharacterValue></vpbnf> can be any printable character or an escape sequence:

*<vpbnf>\\</vpbnf> representing <vp>\</vp> 
*<vp>\t</vp> representing Tab character
*<vp>\n</vp> representing New Line character
*<vp>\r</vp> representing carriage return
*<vp>\'</vp> representing single quoter
*<vp>\"</vp> representing double quoter
*<vp>\u</vp><vpbnf><XXXX></vpbnf>, here <vpbnf><XXXX></vpbnf> should be exactly four <vpbnf><HexadecimalDigit></vpbnf>'s representing the Unicode character corresponding to the digits.

==== String Literals ====

<vipbnf><StringLiteral>:
   <StringLiteralPart>-list</vipbnf>

<vipbnf><StringLiteralPart>:
   @" anyCharacter-list-opt "
   " characterValue-list-opt "</vipbnf>

A string literal consists of one or more <vpbnf><StringLiteralPart></vpbnf>'s, which are concatenated. 

<vpbnf><StringLiteralPart></vpbnf>'s starting with <vp>@</vp> does not use escape sequences, except for the:
* <vp>""</vp> representing double quote.

Whereas <vpbnf><StringLiteralPart></vpbnf>'s without <vp>@</vp> uses the following escape sequences:
*<vp>\\</vp> representing <vp>\</vp> .
*<vp>\t</vp> representing Tab character.
*<vp>\n</vp> representing New Line character.
*<vp>\r</vp> representing carriage return.
*<vp>\</vp><vp>"</vp> representing double quoter.
*<vp>\u</vp><vpbnf><XXXX></vpbnf>, here <vpbnf><XXXX></vpbnf> should be exactly four <vpbnf><HexadecimalDigit></vpbnf>'s representing the Unicode character corresponding to the digits.

==== Binary Literals ====

<vipbnf><BinaryLiteral>:
   $[ <ElementValue>-comma-sep-list-opt ]
<ElementValue>:
   <IntegerLiteral></vipbnf>

<vpbnf><ElementValue></vpbnf> should be any integral arithmetic expression (for example, constant), which should be calculated while compilation-time and be in the range from <vp>0</vp> till <vp>255.</vp>

==== List Literals ====

All elements in a list literal must belong to the same domain (or to compatible domains). This domain can be any {{lang2|Built-in_entities|Domains|built-in}} or {{lang|Domains|user defined domain}}, for example, it can be {{lang2|Lexical_Elements|Integral_Literals|integral}}, {{lang2|Lexical_Elements|Character_Literals|character}}, {{lang2|Lexical_Elements|Binary_Literals|binary}}, {{lang2|Lexical_Elements|Compound_Domain_Literals|compound domain}}, etc.

<vipbnf><ListLiteral>:
   [ <SimpleLiteral>-comma-sep-list-opt ]
   [ <SimpleLiteral>-comma-sep-list | <ListLiteral> ]</vipbnf>

Here <vpbnf><SimpleLiteral></vpbnf> can be:

<vipbnf><SimpleLiteral>:
   <IntegerLiteral>
   <RealLiteral>
   <CharacterLiteral>
   <StringLiteral>
   <BinaryLiteral>
   <ListLiteral>
   <CompoundDomainLiteral></vipbnf>

Examples of list literals are:

<vip>
[]                    % empty list
[1,2,3]               % list of integers
["abc", "defg"]       % list of strings
[1,"abc"]             % this is INVALID list</vip>

==== Compound Domain Literals ====

Terms of {{lang2|Domains|Compound_Domains|user defined compound}} domains can be treated as literals if all their arguments are literals.
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