Please have a look at: learning to write a compiler
Also interesting:
- how to write a programming language
- parsing where can i learn about it
- learning resources on parsers interpreters and compilers (ok you already mentioned this one.
And there are more on the topic. But I can give a short introduction:
The first step is the lexical analysis. A stream of characters is translated into a stream of tokens. Tokens can be simple like == <= + - (etc) and they can be complex like identifiers and numbers. If you like I can elaborate on this.
The next step is to translate the tokenstream into a syntaxtree or an other representation. This is called the parsing step.
Before you can create a parser, you need to write the grammar. For example we create an expression parser:
Tokens
addOp = '+' | '-';
mulOp = '*' | '/';
parLeft = '(';
parRight = ')';
number = digit, {digit};
digit = '0'..'9';
Each token can have different representations: + and = are both addOp and
23 6643 and 223322 are all numbers.
The language
exp = term | exp, addOp, term;
// an expression is a series of terms separated by addOps.
term = factor | term, mulOp, factor;
// a term is a series of factors separated by mulOps
factor = addOp, factor | parLeft, exp, parRight | number;
// a factor can be an addOp followed by another factor,
// an expression enclosed in parentheses or a number.
The lexer
We create a state engine that walks through the char stream, creating a token
s00
'+', '-' -> s01 // if a + or - is found, read it and go to state s01.
'*', '/' -> s02
'(' -> s03
')' -> s04
'0'..'9' -> s05
whitespace -> ignore and retry // if a whitespace is found ignore it
else ERROR // sorry but we don't recognize this character in this state.
s01
found TOKEN addOp // ok we have found an addOp, stop reading and return token
s02
found TOKEN mulOp
s03
found TOKEN parLeft
s04
found TOKEN parRight
s05
'0'..'9' -> s05 // as long as we find digits, keep collecting them
else found number // last digit read, we have a number
Parser
It is now time to create a simple parser/evaluator. This is complete in code. Normally they are created using tables. But we keep it simple. Read the tokens and calculate the result.
ParseExp
temp = ParseTerm // start by reading a term
while token = addOp do
// as long as we read an addop keep reading terms
if token('+') then temp = temp + ParseTerm // + so we add the term
if token('-') then temp = temp - ParseTerm // - so we subtract the term
od
return temp // we are done with the expression
ParseTerm
temp = ParseFactor
while token = mulOp do
if token('*') then temp = temp * ParseFactor
if token('/') then temp = temp / ParseFactor
od
return temp
ParseFactor
if token = addOp then
if token('-') then return - ParseFactor // yes we can have a lot of these
if token('+') then return ParseFactor
else if token = parLeft then
return ParseExpression
if not token = parRight then ERROR
else if token = number then
return EvaluateNumber // use magic to translate a string into a number
This was a simple example. In real examples you will see that error handling is a big part of the parser.
I hope this clarified a bit ;-).
