Fail language

Language based on lox and tweaked from book “Crafting Interpreters” by @munificent.

Properties

• regular grammar
• recursive descent parser

Keywords

Constructs

• if, else
• for, while, do-while (break, continue)
• fun
• class (this, super)

Data types

• var
• Boolean: true, false, and, or
• none

Misc

• print

Grammar

Statements:

program      → declaration* EOF ;
declaration  → classDecl
             | funDecl
             | varDecl
             | statement ;
classDecl    → "class" IDENTIFIER ( "<" IDENTIFIER )?
               "{" function* "}" ;
funDecl      → "fun" function ;
varDecl      → "var" IDENTIFIER ( "=" expression )? ";" ;
statement    → exprStmt
             | forStmt
             | ifStmt
             | printStmt
             | returnStmt
             | whileStmt
             | block;
exprStmt     → expression ";" ;
forStmt      → "for" "(" ( varDecl | exprStmt | ";" )
                         expression? ";"
                         expression? ")" statement ;
ifStmt       → "if" "(" expression ")" statement ( "else" statement )? ;
printStmt    → "print" expression ";" ;
doWhileStmt  → "do" statement "while" "(" expression ")" ";" ;
whileStmt    → "while" "(" expression ")" statement ;
breakStmt    → "break" ";" ;
continueStmt → "continue" ";" ;
block        → "{" declaration* "}" ;

Expressions:

expression  → comma ;
comma       → assignment ( "," assignment )*
assignment  → ( call "." )? ( ( "=" | "+=" | "-=" | "*=" | "/=" | "**=" ) assignment )?
            | ternary ;
ternary     → logic_or ( "?" expression ":" ternary )?
logic_or    → logic_and ( "or" logic_and )*
logic_and   → equality ( "and" equality )*
equality    → comparison ( ( "!=" | "==" ) comparison )*
comparison  → term ( ( ">" | ">=" | "<" | "<=" ) term )*
term        → factor ( ( "-" | "+" ) factor )*
factor      → unary ( ( "/" | "*" ) unary )*
unary       → ( "!" | "-" | "++" | "--" ) unary
            | exponent ;
exponent    → (prefix "**" unary)
            | prefix ;
prefix      → ("++" | "--") primary
            | postfix ;
postfix     → primary ( "++" | "--" )* 
            | call ; 
call        → primary ( "(" arguments? ")" | "." IDENTIFIER )* ;
primary     → "true" | "false" | "none" | "this"
            | NUMBER | STRING | IDENTIFIER | "(" expression ")"
            | lambda
            | "super" "." IDENTIFIER
            // Error productions...
            | ( "!=" | "==" ) equality
            | ( ">" | ">=" | "<" | "<=" ) comparison
            | ( "+" ) term
            | ( "/" | "*" ) factor
            | ("**") exponent;

Other:

arguments    → expression ( "," expression )* ;
parameters   → IDENTIFIER ( "," IDENTIFIER )* ;
function     → IDENTIFIER functionBody;
method       → function;
static method→ "class" method;
constructor  → "init" functionBody; # returns this
functionBody → "(" parameters? ")" block ;
lambda       → "fun" functionBody ;

Notes

Unary ‘+’ operator is not supported.

Currently continue statement does not work as expected for for loops, incrementors must be manually incremented.

Rules

Operator precedence (highest → lowest)

Name          Operators                   Associates
Call          a()                      Left
Postfix       a++ a--                  Left
Prefix        ++a --a                  Right
Exponent      **                       Right
Unary          ! -                      Right
Factor          / *                      Left
Term          - +                      Left
Comparison    > >= < <=                   Left
Equality      == !=                    Left
Logical And   and                      Left
Logical Or    or                       Left
Ternary       ?:                       Right
Assignment    =, +=, -=, /=, *=, **=   Right
Comma         ,                        Left

Truthyness

Fail follows Ruby’s simple rule: false and none are falsey and everything else is truthy.

Escape sequences

\" – double quote
\\ – single backslash
\b – backspace
\r – carriage return
\n – newline
\t – tab

Data types

Variables can change it’s data type at runtime as in Python. Data types are implied from expressions.

Built-in data types

• Boolean: true or false
• string
• number (double precision)
• none - a null pointer

  User defined data types

  Those can be defined via class keyword.

Standard library

Global functions

• clock() - Gets the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC. Then it changes milliseconds to seconds as a floating point value.
• len(x) - This function changes input to a string and gets its length.
• str(x) - Changes input to its string representation.

Added features

Additional features mostly based on tasks from book:

• multiline comments
• postfix and prefix increment/decrement operators
• ternary operator
• exponent operator
• prevent access to unassigned variables (no implicit initialization to none)
• accept escape sequences
• comma operator
• operator overload for string multiplication (“abc” * 2 → “abcabc”)
• break and continue
• do-while statement
• shorthand assignment operators +=, -=, =, /=, *=
• prevention of assignment inside if, loop and ternary condition expressions
• lambdas
• warnings, if local variable is unused
• static methods, getters and setters