# PLATYPUS Language Specification

### 2.1 Input Elements and Tokens

<input character> ->
    ASCII characters but not SEOF

<input> ->
    <input elements> SEOF

<input elements> ->
    <input element> | <input elements> <input element>

<token> ->
    <variable identifier> | <keyword> | <floating-point literal>
    | <integer literal> | <string literal> | <separator> | <operator>

### 2.2 White Space

<white space> ->
    ASCII SP character (space)
    | ASCII HT character (horizontal tab)
    | ASCII VT character (vertical tab)
    | ASCII FF character (form feed)
    | <line terminator>

<line terminator> ->
    CR | LF | CR LF

### 2.3 Comments

<comment> ->
    !< <opt_characters in line> <line terminator>

<characters in line> ->
    <comment character> | <characters in line> <comment character>

<comment character> ->
    <input character> but not <line terminator>

### 2.4 Variable Identifiers

<variable identifier> ->
    <arithmetic variable identifier> | <string variable identifier>

<arithmetic identifier> ->
    <letter> <opt_letters or digits>

<opt_letters or digits> ->
    ε | <letters or digits>

<letters or digits> ->
    <letter or digit> | <letters or digits> <letter or digit>

<letter> -> 
    [a-z] | [A-Z]

<letter or digit> -> 
    <letter> | <digit>

<string variable identifier> ->
    <arithmetic variable identifier># 

### 2.5 Keywords

<keyword> ->
    PLATYPUS | IF | THEN | ELSE | USING | REPEAT | INPUT | OUTPUT

### 2.6 Integer Literals

<integer literal> ->
    <decimal integer literal> | <octal integer literal>

<decimal integer literal> ->
    0 | <non-zero digit> <opt_digits>

<opt_digits> ->
    ε | <digits>

<digits> ->
    <digit> | <digits> <digit>

<digit> ->
    0 | <non-zero digit>

<non-zero digit> -> one of
    1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

<octal integer literal> ->
    0 <octal digit> <octal digits>

<octal digit> ->
    0 | 1 | 2 | 3 | 4 | 5 | 6 | 7

<octal digits> ->
    <octal digit> | <octal digits> <octal digit>


### 2.7 Floating-point Literals

<floating-point literal> ->
    <decimal integer literal> . <opt_digits>

### 2.8 String Literals

<opt_string literal> ->
    ε | <string literal>

<string literal> ->
    "<opt_string characters>"

<opt_string characters> ->
    ε | <string characters>

<string characters> ->
    <input character> | <string characters> <input character>

### 2.9 Separators

<separator> ->
    ( | ) | { | } | , | ; | " |.

### 2.10 Operators

<operator> ->
    <arithmetic operator> | <string concatenation operator>
    | <relational operator> | <logical operator>
    | <assignment operator>

<arithmetic operator> ->
    + | - | * | /

<string concatenation operator> ->
    <<

<relational operator> ->
    > | < | == | <>

<logical operator> ->
    .AND. | .OR.

<assignment operator> ->
    =

## 3. The PLATYPUS Syntactic Specification
### 3.1 PLATYPUS Program

<program> ->
    PLATYPUS {<opt_statements>} SEOF

<opt_statements> ->
    ε | <statements>

<statements> ->
    <statement> | <statements> <statement>

### 3.2 Statements

<statement> ->
    <assignment statement>
    | <selection statement>
    | <iteration statement>
    | <input statement>
    | <output statement>

### 3.3 Assignment Statement

<assignment statement> ->
    <assignment expression> ;

<assignment expression> ->
    <arithmetic variable identifier> = <arithmetic expression>
    | <string variable identifier> = <string expression>

#### 3.2.2 Selection Statement (`if` statement)

<selection statement> ->
    IF (<conditional expression>) THEN <opt_statements>
    ELSE {<opt_statements>};

#### 3.2.3 Iteration Statement (the loop statement)

<iteration statement> ->
    USING (<assignment expression> , <conditional expression>, <assignment expression>) REPEAT { <opt_statements> };

#### 3.2.4 Input Statement

<input statement> ->
    INPUT (<variable list>);

<opt_variable list> ->
    ε | <variable list>

/** New Variable List version below
<variable list> ->
    <variable identifier> | <variable list>,<variable identifier>
**/

<variable list> ->
    <variable identifier> <variable list'>

<variable list'> ->
    , <variable identifier> <variable list'> | E

#### 3.2.5 Output Statement

<output statement> ->
    OUTPUT(<opt_variable list> | <opt_string literal>);

### 3.3 Expressions
#### 3.3.1 Arithmetic Expressions

<arithmetic expression> ->
    <unary arithmetic expression>
    | <additive arithmetic expression>

<unary arithmetic expression> ->
    - <primary arithmetic expression>
    | + <primary arithmetic expression>

/** New Version of Additive Arithmetic Expressions below
<additive arithmetic expression> ->
    <additive arithmetic expression> + <multiplicative arithmetic expression>
    | <additive arithmetic expression> - <multiplicative arithmetic expression>
    | <multiplicative arithmetic expression>
**/

<additive arihtmetic expression> ->
    <multiplicative arithmetic expression> <additive arithmetic expression'>

<additive arithmetic expression'> ->
    + <multiplicative arithmetic expression> <additive arithmetic expression'>
    | - <multiplicative arithmetic expression> <additive arithmetic expression'>

/** New Version of Mult. Arithmetic Expressions below
<multiplicative arithmetic expression> ->
    <multiplicative arithmetic expression> * <primary arithmetic expression>
    | <multiplicative arithmetic expression> / <primary arithmetic expression>
    | <primary arithmetic expression>
**/
<multiplicative arithmetic expression> ->
    <primary arithmetic expression> <multiplicative arithmetic expression'>

<multiplicative arithmetic expression'> ->
    * <primary arithmetic expression> <multiplicative arithmetic expression'>
    | / <primary arithmetic expression> <multiplicative arithmetic expression'>

<primary arithmetic expression> ->
    <variable identifier>
    | <floating-point literal>
    | <integer literal>
    | (<arithmetic expression>)

#### 3.3.2 String Expression
/** New version of String Expressions below
<string expression> ->
    <primary string expression>
    | <string expression> << <primary string expression>
**/
<string expression> ->
    <primary string expression> <string expression'>

<string expression'> ->
    << <primary string expression> <string expression'> | E

<primary string expression> ->
    <string variable identifier>
    | <string literal>

#### 3.3.3 Conditional Expression

<conditional expression> ->
    <logical OR expression>

/** New Logical OR and Logical AND expressions below
<logical OR expression> ->
    <logical AND expression>
    | <logical OR expression> .OR. <logical AND expression>

<logical AND expression> ->
    <relational expression>
    | <logical AND expression> .AND. <relational expression>
**/
<logical OR expression> ->
    <logical AND expression> <logical OR expression'>

<logical OR expression'> ->
    .OR. <logical AND expression> <logical OR expression'>

<logical AND expression> ->
    <relational expression> <logical AND expression'>

<logical AND expression'> ->
    .AND. <relational expression> <logical AND expression'>


#### 3.3.4 Relational Expressions

/** New version of Relational expressions below
<relational expression> ->
    <primary a_relational expression> <relational operator> <primary a_relational expression>
    | <primary s_relational expression> <relational operator> <primary s_relational expression>
**/

<relational expression> ->
    <primary a_relational expression> <primary a_relational expression'>
    | <primary s_relational expression> <primary s_relational expression'>

    
<primary a_relational expression> ->
    <floating-point literal>
    | <integer literal>
    | <variable identifier>

<primary a_relational expression'> ->
    == <primary a_relational expression>
    | <> <primary a_relational expression>
    | > <primary a_relational expression>
    | < <primary a_relational expression>

<primary s_relational expression> ->
    <string literal> | <string variable identifier>

<primary s_relational expression'> ->
    == <primary s_relational expression>
    | <> <primary s_relational expression>
    | > <primary s_relational expression>
    | < <primary s_relational expression>