Writing a parser: introduction to ADL

This post will introduce ADL (Acronymic Demonstrational Language), a language I designed to demonstrate how to write a parser.

Statements

ADL has 5 different types of statements:

• if-statement
• while-loop
• for-loop
• assignment
• function call

An if-statement looks like this:

if expression then
    statements
end if

An if-statement can also have an else-clause:

if expression then
    statements
else
    statements
end if

A while-loop looks like this:

while expression do
    statements
end while

A for-loop looks like this:

for variable := expression to expression do
    statements
end for

A for-loop can also have a by-clause that defines the number to add to the variable on each iteration:

for variable := expression to expression by expression do
    statements
end for

An assignment looks like this:

variable := expression

A function call looks like this:

function()

It can also have one or more arguments separated by commas:

function(expression, expression, expression)

Expressions

Expressions in ADL consist of variables, constants, function calls, unary expressions, binary expressions and groups.

Variable names start with a letter, followed by zero or more letters or numbers.

ADL has 2 types of constants: strings and integer numbers. A string is a sequence of characters, enclosed between 2 quotes. An integer is a sequence of 1 or more digits.

Unary expressions are expressions with an operator that has only 1 operand:

• positive operator: +expression
• negative operator: -expression
• logical NOT-operator: not expression

Binary expressions are expressions with an operator that has 2 operands:

• logical AND-operator: expression and expression
• logical OR-operator: expression or expression
• comparison: expression comparator expression (comparator is one of the following: ==, <>, <, >, <=, >=)
• addition: expression + expression
• subtraction: expression - expression
• multiplication: expression * expression
• division: expression / expression

A group is an expression between parentheses: ( expression )

Precedence

When an expression is built from multiple operators, the expression is evaluated in the following order:

• Grouping: ( )
• Unary expression: + - not
• Multiplicative expression: * /
• Additive expression: + -
• Comparison: == <> < > <= >=
• Logical OR: or
• Logical AND: and

Technorati tags: Parser, ADL, Acronymic Demonstrational Language, Programming Language.