hydra

1. Introduction

This document contains the complete technical specification of the Hydra programming language.

The Hydra of Lerna is a many-headed serpent or monster in Greek mythology. As soon as one head was cut off, two more heads would emerge from the fresh wound.
— Merriam-Webster Dictionary

This language specification was developed for the 2020 spring semester TC3048 Compiler Design course at the Tecnológico de Monterrey, Campus Estado de Mexico.

2. Lexicon

In the following sections, a letter is any character from the English alphabet from A to Z (both lowercase and uppercase). A digit is any character from 0 to 9.

2.1. Tokens

There are five kinds of tokens: identifiers, keywords, literals, operators, and other separators. Spaces, tabulators, newlines, and comments (collectively, “white space”) are used as delimiters between tokens, but are otherwise ignored.

If the input stream has been separated into tokens up to a given character, the next token is the longest string of characters that could constitute a token.

2.2. Comments

Comments can be either single or multi-line. Single line comments start with two slashes (//) and conclude at the end of the line. Multi-line comments start with a slash and an asterisk (/*) and end with an asterisk and a slash (*/). Comments cannot be placed inside string literals. Multi-line comments cannot nest.

2.3. Identifiers

An identifier is composed of a letter and a sequence of zero or more letters, digits and the underscore character (_). Uppercase and lowercase letters are considered different. Identifiers can be of any length.

An identifier token appears as an ‹id› terminal symbol in the language grammar.

2.4. Keywords

The following nine identifiers are reserved for use as keywords and may not be used for any other purpose:

break

return

elif

true

else

var

false

while

if

2.5. Literals

At a lexical level there are four kinds of literals: booleans, integers, characters, and strings.

2.5.1. Booleans

A boolean literal is either the keyword true or the keyword false. These are equivalent to 1 and 0, respectively.

A boolean literal token appears as a ‹lit-bool› terminal symbol in the language grammar.

2.5.2. Integers

An integer literal is a sequence of one or more digits from 0 to 9. It can optionally start with a minus (−) sign. Only decimal (base 10) numbers are supported. Valid range: −2,147,483,648 to 2,147,483,647 (\(-2^{31}\) to \(2^{31} - 1\)).

An integer literal token appears as a ‹lit-int› terminal symbol in the language grammar.

2.5.3. Characters

A character literal is a Unicode character enclosed in single quotes, as in 'x'. The compiler translates the specified character into its corresponding Unicode integer code point.

Character literals cannot contain the quote character (') or a newline character; in order to represent them, and certain other characters, the following escape sequences may be used:

Name Escape Sequence Code Point

Newline

\(\backslash \texttt{n}\)

10

Carriage Return

\(\backslash \texttt{r}\)

13

Tab

\(\backslash \texttt{t}\)

9

Backslash

\(\backslash \backslash\)

92

Single Quote

\(\backslash \texttt{'}\)

39

Double Quote

\(\backslash \texttt{"}\)

34

Unicode Character

\(\backslash \texttt{u} hhhhhh\)

\(hhhhhh\)

The escape sequence \(\backslash \texttt{u} hhhhhh\) consists of the backslash, followed by the lower case letter “u”, followed by six hexadecimal digits (digits “0” to “9” and the upper or lower case letters “a” to “f”), which are taken to specify the code point in base 16 of the desired character.

A character literal token appears as a ‹lit-char› terminal symbol in the language grammar.

2.5.4. Strings

A string literal is a sequence of zero or more Unicode characters delimited by double quotes, for example: "this is a string". String literals do not contain newline or double-quote characters; in order to represent them, the same escape sequences as for character literals are available.

A string literal is stored in memory as an array (accessible through a 32-bit handle) containing zero or more int32 values. Each value is the code point of the character in the corresponding position of the given string. In other words, a string is stored using the UTF-32 encoding.

A string literal token appears as a ‹lit-str› terminal symbol in the language grammar.

3. Syntax

The following BNF context free grammar defines the syntax of the Hydra programming language. The red elements represent explicit terminal symbols (tokens).

‹program›

\(\rightarrow\)

‹def-list›

‹def-list›

\(\rightarrow\)

‹def-list› ‹def›

‹def-list›

\(\rightarrow\)

ε

‹def›

\(\rightarrow\)

‹var-def›

‹def›

\(\rightarrow\)

‹fun-def›

‹var-def›

\(\rightarrow\)

var ‹var-list› ;

‹var-list›

\(\rightarrow\)

‹id-list›

‹id-list›

\(\rightarrow\)

‹id› ‹id-list-cont›

‹id-list-cont›

\(\rightarrow\)

, ‹id› ‹id-list-cont›

‹id-list-cont›

\(\rightarrow\)

ε

‹fun-def›

\(\rightarrow\)

‹id› ( ‹param-list› ) { ‹var-def-list› ‹stmt-list› }

‹param-list›

\(\rightarrow\)

‹id-list›

‹param-list›

\(\rightarrow\)

ε

‹var-def-list›

\(\rightarrow\)

‹var-def-list› ‹var-def›

‹var-def-list›

\(\rightarrow\)

ε

‹stmt-list›

\(\rightarrow\)

‹stmt-list› ‹stmt›

‹stmt-list›

\(\rightarrow\)

ε

‹stmt›

\(\rightarrow\)

‹stmt-assign›

‹stmt›

\(\rightarrow\)

‹stmt-incr›

‹stmt›

\(\rightarrow\)

‹stmt-decr›

‹stmt›

\(\rightarrow\)

‹stmt-fun-call›

‹stmt›

\(\rightarrow\)

‹stmt-if›

‹stmt›

\(\rightarrow\)

‹stmt-while›

‹stmt›

\(\rightarrow\)

‹stmt-break›

‹stmt›

\(\rightarrow\)

‹stmt-return›

‹stmt›

\(\rightarrow\)

‹stmt-empty›

‹stmt-assign›

\(\rightarrow\)

‹id› = ‹expr› ;

‹stmt-incr›

\(\rightarrow\)

‹id› ++ ;

‹stmt-decr›

\(\rightarrow\)

‹id› −− ;

‹stmt-fun-call›

\(\rightarrow\)

‹fun-call› ;

‹fun-call›

\(\rightarrow\)

‹id› ( ‹expr-list› )

‹expr-list›

\(\rightarrow\)

‹expr› ‹expr-list-cont›

‹expr-list›

\(\rightarrow\)

ε

‹expr-list-cont›

\(\rightarrow\)

, ‹expr› ‹expr-list-cont›

‹expr-list-cont›

\(\rightarrow\)

ε

‹stmt-if›

\(\rightarrow\)

if ( ‹expr› ) { ‹stmt-list› } ‹else-if-list› ‹else›

‹else-if-list›

\(\rightarrow\)

‹else-if-list› elif ( ‹expr› ) { ‹stmt-list› }

‹else-if-list›

\(\rightarrow\)

ε

‹else›

\(\rightarrow\)

else { ‹stmt-list› }

‹else›

\(\rightarrow\)

ε

‹stmt-while›

\(\rightarrow\)

while ( ‹expr› ) { ‹stmt-list› }

‹stmt-break›

\(\rightarrow\)

break ;

‹stmt-return›

\(\rightarrow\)

return ‹expr› ;

‹stmt-empty›

\(\rightarrow\)

;

‹expr›

\(\rightarrow\)

‹expr-or›

‹expr-or›

\(\rightarrow\)

‹expr-or› || ‹expr-and›

‹expr-or›

\(\rightarrow\)

‹expr-and›

‹expr-and›

\(\rightarrow\)

‹expr-and› && ‹expr-comp›

‹expr-and›

\(\rightarrow\)

‹expr-comp›

‹expr-comp›

\(\rightarrow\)

‹expr-comp› ‹op-comp› ‹expr-rel›

‹expr-comp›

\(\rightarrow\)

‹expr-rel›

‹op-comp›

\(\rightarrow\)

==

‹op-comp›

\(\rightarrow\)

!=

‹expr-rel›

\(\rightarrow\)

‹expr-rel› ‹op-rel› ‹expr-add›

‹expr-rel›

\(\rightarrow\)

‹expr-add›

‹op-rel›

\(\rightarrow\)

<

‹op-rel›

\(\rightarrow\)

<=

‹op-rel›

\(\rightarrow\)

>

‹op-rel›

\(\rightarrow\)

>=

‹expr-add›

\(\rightarrow\)

‹expr-add› ‹op-add› ‹expr-mul›

‹expr-add›

\(\rightarrow\)

‹expr-mul›

‹op-add›

\(\rightarrow\)

+

‹op-add›

\(\rightarrow\)

−

‹expr-mul›

\(\rightarrow\)

‹expr-mul› ‹op-mul› ‹expr-unary›

‹expr-mul›

\(\rightarrow\)

‹expr-unary›

‹op-mul›

\(\rightarrow\)

*

‹op-mul›

\(\rightarrow\)

/

‹op-mul›

\(\rightarrow\)

%

‹expr-unary›

\(\rightarrow\)

‹op-unary› ‹expr-unary›

‹expr-unary›

\(\rightarrow\)

‹expr-primary›

‹op-unary›

\(\rightarrow\)

+

‹op-unary›

\(\rightarrow\)

−

‹op-unary›

\(\rightarrow\)

!

‹expr-primary›

\(\rightarrow\)

‹id›

‹expr-primary›

\(\rightarrow\)

‹fun-call›

‹expr-primary›

\(\rightarrow\)

‹array›

‹expr-primary›

\(\rightarrow\)

‹lit›

‹expr-primary›

\(\rightarrow\)

( ‹expr› )

‹array›

\(\rightarrow\)

[ ‹expr-list› ]

‹lit›

\(\rightarrow\)

‹lit-bool›

‹lit›

\(\rightarrow\)

‹lit-int›

‹lit›

\(\rightarrow\)

‹lit-char›

‹lit›

\(\rightarrow\)

‹lit-str›

4. Semantics

4.1. Compile Time Validations

  1. The language only supports a 32-bit signed two’s complement integer (int32) data type. This is the data type for every variable, parameter and function return value. This means that a Hydra compiler doesn’t need to verify type consistency.

  2. Every program starts its execution in a function called main. It is an error if the program does not contain a function with this name.

  3. Any variable defined outside a function is a global variable. The scope of a global variable is the body of all the functions in the program, even those defined before the variable itself.

  4. Function names and global variables exist in different namespaces. This means that you can have a global variable with the same name as a function and vice versa.

  5. It’s an error to define two global variables with the same name.

  6. It’s an error to define two functions with the same name.

  7. A function definition is visible from the body of all the functions in a program, even from itself. Thus, functions can call themselves recursively directly or indirectly.

  8. In every function call the number of arguments must match the number of parameters contained in the corresponding function definition.

  9. The following names are part of the initial namespace for functions and constitute Hydra’s API (the number after the slash symbol (/) is the arity of the given function):

    • printi/1

    • printc/1

    • prints/1

    • println/0

    • readi/0

    • reads/0

    • new/1

    • size/1

    • add/2

    • get/2

    • set/3

  10. Each function has its own independent namespace for its local names. This means that parameter and local variable names have to be unique inside the body of each function. It’s valid to have a parameter or local variable name with the same name as a global variable. In that case the local name shadows the global variable.

  11. It’s an error to refer to a variable, parameter or function not in scope in the current namespace.

  12. The break statement can only be used inside the body of a while statement.

  13. Values of integer literals should be between -2147483648 and 2147483647 (\(-2^{31}\) and \(2^{31} - 1\), respectively).

4.2. Runtime Behavior

  1. A function returns zero by default, except if it executes an explicit return statement with some other value.

  2. The value returned by the main function must be the exit code returned by the program to the operating system. This can be accomplished by using the exit(code) function from node’s process module.

  3. For the conditional and loop (if and while) statements the number 0 is the only value considered false, everything else is considered true.

  4. All the Hydra statements (assignment, increment, decrement, function call, conditional, while, break, and return) behave like their C# counterparts.

  5. The Hydra syntax supports string and array literals. Both of these are represented in memory as arrays accessible through API managed 32-bit handles.

  6. The following are the supported operators. Precedence and associativity are established in the language grammar.

    Table 1. Arithmetic operators
    Operator Syntax Description

    Unary minus

    − x

    Returns x negated (with its sign changed).

    Unary plus

    + x

    Returns x.

    Multiplication

    x * y

    Returns x times y.

    Division

    x / y

    Returns x divided by y truncating the result towards zero. An exception is thrown if y is zero.

    Remainder

    x % y

    Returns the remainder of dividing x by y. An exception is thrown if y is zero.

    Addition

    x + y

    Returns x plus y.

    Subtraction

    x − y

    Return x minus y.
    Table 2. Logical operators
    Operator Syntax Description

    Not

    ! x

    Evaluates x and returns 1 if its result is equal to 0. Otherwise returns 0.

    And

    x && y

    Returns 1 if both x and y evaluate to non-zero values. Otherwise, returns 0. The operation is short circuited, this means that if x evaluates to 0 then y is not evaluated.

    Or

    x || y

    Returns 1 if either x or y evaluate to a non-zero value. Otherwise, returns 0. The operation is short circuited, this means that if x evaluates to a non-zero value then y is not evaluated.
    Table 3. Comparison and relational operators
    Operator Syntax Description

    Equal to

    x == y

    Returns 1 if x is equal to y, otherwise returns 0.

    Not equal to

    x != y

    Returns 1 if x is not equal to y, otherwise returns 0.

    Greater than

    x > y

    Returns 1 if x is greater than y, otherwise returns 0.

    Less than

    x < y

    Returns 1 if x is less than y, otherwise returns 0.

    Greater than or equal to

    x >= y

    Returns 1 if x is greater than or equal to y, otherwise returns 0.

    Less than or equal to

    x <= y

    Returns 1 if x is less than or equal to y, otherwise returns 0.
    Table 4. Function call
    Operator Syntax Description

    Function call

    f (arg1, arg2, …​, argn)

    Invoke f with the given arguments and return its result. All arguments are fully evaluated before the call.

5. API

This section documents all the functions from the Hydra application programming interface (API).

Table 5. Input/Output Operations
Signature Description

printi(i)

Prints i to stdout as a decimal integer. Does not print a new line at the end. Returns 0.

printc(c)

Prints a character to stdout, where c is its Unicode code point. Does not print a new line at the end. Returns 0.

prints(s)

Prints s to stdout as a string. s must be a handle to an array containing zero or more Unicode code points. Does not print a new line at the end. Returns 0.

println()

Prints a newline character to stdout. Returns 0.

readi()

Reads from stdin an optionally signed decimal integer and returns its value. Does not return until a valid integer has been read.

reads()

Reads from stdin a string (until the end of line) and returns a handle to a newly created array containing the Unicode code points of all the characters read.
Table 6. Array List Operations
Signature Description

new(n)

Creates a new array object with n elements and returns its handle. All the elements of the array are set to zero. Throws an exception if n is less than zero.

size(h)

Returns the size (number of elements) of the array referenced by handle h. Throws an exception if h is not a valid handle.

add(h, x)

Adds x at the end of the array referenced by handle h. Returns 0. Throws an exception if h is not a valid handle.

get(h, i)

Returns the value at index i from the array referenced by handle h. Throws an exception if i is out of bounds or if h is not a valid handle.

set(h, i, x)

Sets to x the element at index i of the array referenced by handle h. Returns 0. Throws an exception if i is out of bounds or if h is not a valid handle.

6. Code Examples

Source File Description

001_ultimate.hydra

Prints the answer to the ultimate question of life, the universe, and everything.

002_binary.hydra

Converts decimal numbers into binary.

003_palindrome.hydra

Determines if a string is a palindrome.

004_factorial.hydra

Computes factorials using iteration and recursion.

005_arrays.hydra

Implementation of typical array operations.

006_next_day.hydra

Given the date of a certain day, determines the date of the day after.

007_literals.hydra

Verifies that the implementation of literal values meet the specified requirements.