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] 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 ------------- Requires Python 3. The author is using Python 3.5.1. 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 ------- Distributed under the MIT License. See [LICENSE.txt] for details. [LICENSE.txt]: LICENSE.txt