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# Simple interpreter
An interpreter of a simple (deficient, really) programming language.
Here you'll find a brief description of the language and the implementation.
## The language
### Data types
Integer numbers, floating-point numbers and boolean values are supported.
Literal integer numbers are represented as sequences of digits.
Negative integer number literals are not supported.
Floating-point number literals follow a bit more complicated format with
scientific notation support.
Negative floating-point numbers literals are not supported.
Boolean values are represented as either `True` or `False`.
### Variables
Variables are named memory regions.
Numbers can be stored in memory by assigning them to variables using the
assignment operator `:=`.
A variable can be used anywhere a number can be used.
x := 1;
answer_to_everything := 42;
y := x;
### Arithmetic expressions
Numbers can be computed from complex arithmetic expressions.
Addition, subtraction, multiplication, and division with grouping using
parentheses are supported.
An arithmetic expression can be used anywhere a number can be used.
microseconds := 1000000 * seconds;
### Input/output
Basic output facilities are provided by the `print` statement.
print 60 * 3.14 / 180;
print days_per_year * 24;
Only numbers can be `print`ed.
### Logical expressions
Boolean values can be computed from complex logical expressions.
The language supports the conjunction (`&&`) and disjunction (`||`) operators,
as well as comparing two Boolean values using the equality (`==`) and
inequality (`!=`) operators.
A logical expression can be used anywhere a Boolean value can be used.
if (True && False == False)
days := 366;
Please note that operators `&&` and `||` have the same precedence.
### Control flow
Control flow is supported using the conditional `if` statement.
When executed, an `if` statement executes its body if its condition evaluates
to `True`.
never_printed := 0;
if (False)
print never_printed;
### Compound statements
A compound statement (or a block) is a sequence of statements grouped together
inside a pair of curly braces (`{` and `}`).
When executed, a block executes the sequence of statements sequentially.
A block can be used anywhere a statement can be used.
if (True) {
days := 366;
hours := days * 24;
minutes := hours * 60;
seconds := minutes * 60;
}
### Language grammar
The language grammar in the [Extended Backus-Naur Form] (EBNF) is as follows:
program = { statement } ;
statement = empty_statement
| block
| assignment
| print_statement
| if_statement ;
empty_statement = ";" ;
block = "{" , { statement } , "}" ;
assignment = identifier , ":=" , arithmetic_expression , ";" ;
print_statement = "print" , arithmetic_expression , ";" ;
arithmetic_expression = arithmetic_term , { "+" , arithmetic_term
| "-" , arithmetic_term } ;
arithmetic_term = arithmetic_factor , { "*" , arithmetic_factor
| "/" , arithmetic_factor } ;
arithmetic_factor = identifier
| number
| "(" , arithmetic_expression , ")" ;
number = integer_number | floating_point_number ;
integer_number = digit , { digit } ;
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
(* Valid floating-point number literals: "1e1", "1e+1", "1e-1", ".1", "1.".
Invalid floating-point number literals: "1e", ".e1". *)
floating_point_number = integer_number , exponent
| integer_number , "." , { digit } , [ exponent ] ;
| { digit } , "." , integer_number , [ exponent ] ;
exponent = ( "e" | "E" ) , [ "+" | "-" ] , integer_number ;
identifier = ( LETTER | "_" ) , { LETTER | digit | "_" } ;
(* LETTER is a symbol Python considers a letter under the current locale
like "a", "b", "c", "X", "Y", "Z", etc. *)
if_statement = "if" , "(" , logical_expression , ")" , statement ;
logical_expression = logical_term , { "&&" , logical_term
| "||" , logical_term } ;
logical_term = logical_factor , [ "==" , logical_factor
| "!=" , logical_factor ] ;
logical_factor = "True"
| "False"
| "(" , logical_expression , ")" ;
[Extended Backus-Naur Form]: https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form
## Design
### Lexer
The *lexer* represents the contents of a source file as a sequence of *tokens*.
Token examples include identifiers (like `x` and `foo`), literal values (either
numeric like `42` and `3.14` or Boolean like `True`), parentheses (`(` and
`)`), arithmetic operation signs (`+`, `*`), etc.
The lexer is implemented in "src/lexer.py".
### Parser
The *parser* builds a program tree according to the rules described in the
language grammar.
Each tree node, when executed, processes its children accordingly.
For example, a node representing addition of two numbers must have exactly
two children.
When executed, such a node evaluates its children and adds the two values.
+
/ \
/ \
1 2
Each of the children in the example above might in turn be represented by a
complex subtree.
For example, the right-side operand of an addition might be a multiplication of
two numbers.
+
/ \
/ \
/ \
1 *
/ \
/ \
2 3
An `if` statement must also have two children (its condition and body), but
executes its body only after making sure the condition evaluates to `True`.
if
/ \
------- -------
/ \
&& :=
/ \ / \
/ \ / \
/ \ days 366
/ \
True False
The parser is implemented in "src/parser.py".
## Prerequisites
* Python 3
## Usage
To execute a script, pass the path to a file to "src/parser.py".
You can also pass the path to a script to "src/lexer.py" to examine the tokens
the script gets separated into.
## License
This project, including all of the files and their contents, is licensed under
terms of the MIT License.
See [LICENSE.txt] for details.
[LICENSE.txt]: LICENSE.txt
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