509 lines
15 KiB
Markdown
509 lines
15 KiB
Markdown
|
|
# 3 Parser
|
|
|
|
In this chaper I'll show how I would make a parser.
|
|
|
|
A parser, in addition to our lexer, transforms the input program as text, meaning an unstructured sequence of characters, into a structered representation. Structured meaning the representation tells us about the different constructs such as if statements and expressions.
|
|
|
|
## 3.1 Abstract Syntax Tree AST
|
|
|
|
The result of parsing is a tree structure representing the input program.
|
|
|
|
This structure is a recursive acyclic structure storing the different parts of the program.
|
|
|
|
This is how I would define an AST data type.
|
|
|
|
```ts
|
|
type Stmt = {
|
|
kind: StmtKind,
|
|
pos: Pos,
|
|
};
|
|
|
|
type StmtKind =
|
|
| { type: "error" }
|
|
// ...
|
|
| { type: "let", ident: string, value: Expr }
|
|
// ...
|
|
;
|
|
|
|
type Expr = {
|
|
kind: ExprKind,
|
|
pos: Pos,
|
|
};
|
|
|
|
type ExprKind =
|
|
| { type: "error" }
|
|
// ...
|
|
| { type: "int", value: number }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
Both `Stmt` (statement) and `Expr` (expression) are polymorphic types, meaning an expression, for example, can be either an addition operation containing 2 inner expressions or an integer expression containing the integer value, etc. This can also be implemented with classes and sub classes.
|
|
|
|
For both `Stmt` and `Expr` there's an error-kind. This makes the parser simpler, as we won't need to manage parsing failures differently than successful parslings.
|
|
|
|
## 3.2 Consumer of lexer
|
|
|
|
To start, we'll implement a `Parser` class, which for now is simply a consumer of a token iterater, meaning the lexer. In simple terms, whereas the lexer is a transformation from text to tokens, the parser is a transformation from token to an AST, except that the parser is not an iterator.
|
|
|
|
```ts
|
|
class Parser {
|
|
private currentToken: Token | null;
|
|
|
|
public constructor(private lexer: Lexer) {
|
|
this.currentToken = lexer.next();
|
|
}
|
|
// ...
|
|
private step() { this.currentToken = this.lexer.next() }
|
|
private done(): bool { return this.currentToken == null; }
|
|
private current(): Token { return this.currentToken!; }
|
|
// ...
|
|
}
|
|
```
|
|
|
|
This implementation should look familiar compared to the lexer. We use the `currentToken` as a 'buffer', and then just use the `.next()` on the `lexer`.
|
|
|
|
Just as the lexer, we'll have a `.pos()` method, returning the current position.
|
|
|
|
For convenience, although there are other ways of doing it, we'll implement another public method on `Lexer`, which will return the lexer's current position.
|
|
|
|
```ts
|
|
class Lexer {
|
|
// ...
|
|
public currentPos(): Pos { return this.pos(); }
|
|
// ...
|
|
}
|
|
```
|
|
The reason, is that when the lexer has reached the end of the file, the `.next()` method will return `null` instead of a token with a position, meaning we won't get the position after the last token.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
private pos(): Pos {
|
|
if (this.done())
|
|
return this.lexer.currentPos();
|
|
return this.current().pos;
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
The parser does not need to keep track of `index`, `line` and `col` as those are stored in the tokens. The token's position is prefered to the lexer's.
|
|
|
|
Also like the lexer, we'll have a `.test()` method in the parser, which will test for token type rather than strings or regex.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
private test(type: string): bool {
|
|
return !this.done() && this.current().type === type;
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
When testing, we first check that we have not reach the end. Either we have to do that here, or the caller will have to write something like `!this.done() && this.test(...)`, and it's easy to do it here.
|
|
|
|
We'll also want a method for reporting errors.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
private report(msg: string, pos = this.pos()) {
|
|
console.log(`Parser: ${msg} at ${pos.line}:${pos.col}`);
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
## 3.3 Operands
|
|
|
|
Operands are the individual parts of an operation. For example, in the math expression `a + b`, (would be `+ a b` in the input language), `a` and `b` are the *operands*, while `+` is the *operator*. In the expression `a + b * c`, the operands are `a`, `b` and `c`. But in the expression `a * (b + c)`, the operands of the multiply operation are `a` and `(b + c)`. `(b + c)` is an operands, because it is enclosed on both sides. This is how we'll define operands.
|
|
|
|
We'll make a public method in `Parser` called `parseOperand`.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parseOperand(): Expr {
|
|
const pos = this.pos();
|
|
// ...
|
|
this.report("expected expr", pos);
|
|
this.step();
|
|
return { kind: { type: "error" }, pos };
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
### 3.3.1 Identifiers and literals
|
|
|
|
Identifiers and literals (integers, strings) are single token constructs, meaning the parsing consists of translating a token into an ast-node with the value.
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "ident", value: string }
|
|
| { type: "int", value: number }
|
|
| { type: "string", value: string }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parseOperand(): Expr {
|
|
// ...
|
|
if (this.test("ident")) {
|
|
const value = this.current().identValue;
|
|
this.step();
|
|
return { kind: { type: "ident", value }, pos };
|
|
}
|
|
if (this.test("int")) {
|
|
const value = this.current().intValue;
|
|
this.step();
|
|
return { kind: { type: "int", value }, pos };
|
|
}
|
|
if (this.test("string")) {
|
|
const value = this.current().stringValue;
|
|
this.step();
|
|
return { kind: { type: "string", value }, pos };
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
### 3.3.2 Group expressions
|
|
|
|
A group expression is an expression enclosed in parenthesis, eg `(1 + 2)`. Because the expression is enclosed, meaning starts with a `(`-token and ends with a `)`-token, we will treat is like an operand.
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "group", expr: Expr }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
If we find a `(`-token in `.parseOperand()`, we know that we should parse a group expression. We do this by ignoring the `(`-token, parsing an expression using `.parseExpr()` and checking that we find a `)`-token afterwards.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parseOperand(): Expr {
|
|
// ...
|
|
if (this.test("(")) {
|
|
this.step();
|
|
const expr = this.parseExpr();
|
|
if (!this.test(")")) {
|
|
this.report("expected ')'");
|
|
return { kind: { type: "error" }, pos };
|
|
}
|
|
this.step();
|
|
return { kind: { type: "group", expr }, pos };
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
If we do not find the closing `)`-token, we report an error and return an error expression.
|
|
|
|
### 3.3.3 Block, if and loop operands
|
|
|
|
We want to be able to use blocks, if and loop constructs as expressions.
|
|
|
|
Example:
|
|
```rs
|
|
let temperature_feeling = if > temperature 20 { "hot" } else { "cold" };
|
|
```
|
|
|
|
Each construct will have their own `.parse...()`-method, so we'll just look for the first `{`-, `if`-, or `loop`-token and call the relevant method.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parseOperand(): Expr {
|
|
// ...
|
|
if (this.test("{"))
|
|
return this.parseBlock();
|
|
if (this.test("if"))
|
|
return this.parseIf();
|
|
if (this.test("loop"))
|
|
return this.parseLoop();
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
## 3.4 Postfix operators
|
|
|
|
Postfix operations are expressions were the operators come after the subject expression. This includes field expressions (eg. `subject.field`), index expressions (eg. `subject[index]`) and call expressions (eg. `subject(...args)`).
|
|
|
|
A notable detail, is that postfix operations are chainable, eg. `subject[index].field` is valid, likewise with `subject.method(arg)` and `matrix[y][x]`.
|
|
|
|
We'll make a method `.parsePostfix()` to parse postfix operators.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePostfix(): Expr {
|
|
let subject = this.parseOperand();
|
|
while (true) {
|
|
const pos = this.pos();
|
|
// ...
|
|
break;
|
|
}
|
|
return subject;
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
We start by parsing an operand. Then we enter a loop, which runs until we no longer find any relevant operator tokens. When we parse a postfix expression, the `subject` will be replaced with the new parsed expression.
|
|
|
|
Notice we don't define `pos` at the start, but after we've parsed the subject. That's because we want `pos` to the reflect the start of the postfix operator, not the start of the subject.
|
|
|
|
### 3.4.1 Field expression
|
|
|
|
A field expression is for accessing fields on an object, and consists of a `.`-token and an identifier, eg. `.field`.
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "field", subject: Expr, value: string }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePostfix(): Expr {
|
|
// ...
|
|
while (true) {
|
|
// ...
|
|
if (this.test(".")) {
|
|
this.step();
|
|
if (!this.test("ident")) {
|
|
this.report("expected ident");
|
|
return { kind: { type: "error" }, pos };
|
|
}
|
|
const value = this.current().identValue;
|
|
this.step();
|
|
subject = { kind: { type: "field", subject, value }, pos };
|
|
continue;
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
If we find a `.`-token, we step over it, and make sure that we've hit an identifier. We save the identifier value and step over the identifier. Then we replace `subject` with a new field expression containing the previous `subject` value. Then we continue to look for the next postfix operator.
|
|
|
|
### 3.4.2 Index expression
|
|
|
|
An index operation consists of the subject and an index. The index is an expression, and it is contained in `[`- and `]`-tokens, eg. `subject[value]`.
|
|
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "index", subject: Expr, value: Expr }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePostfix(): Expr {
|
|
// ...
|
|
while (true) {
|
|
// ...
|
|
if (this.test("[")) {
|
|
this.step();
|
|
const value = this.parseExpr();
|
|
if (!this.test("]") {
|
|
this.report("expected ']'");
|
|
return { kind: { type: "error" }, pos };
|
|
}
|
|
this.step();
|
|
subject = { kind: { type: "index", subject, value }, pos };
|
|
continue;
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
If we find a `[`-token, we parse the index part exactly the same way, we parse a group expression.
|
|
|
|
### 3.4.3 Call expression
|
|
|
|
A call expression is like an index expression, except that it uses `(` and `)` instead of `[` and `]` and that there can be 0 or more expressions (arguments or args) inside the `(` and `)`. The arguments are seperated by `,`.
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "call", subject: Expr, args: Expr[] }
|
|
// ...
|
|
;
|
|
```
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePostfix(): Expr {
|
|
// ...
|
|
while (true) {
|
|
// ...
|
|
if (this.test("(")) {
|
|
this.step();
|
|
let args: Expr[] = [];
|
|
if (!this.test(")") {
|
|
args.push(this.parseExpr());
|
|
while (this.test(",")) {
|
|
this.step();
|
|
if (this.test(")"))
|
|
break;
|
|
args.push(this.parseExpr());
|
|
}
|
|
}
|
|
const value = this.parseExpr();
|
|
if (!this.test(")") {
|
|
this.report("expected ')'");
|
|
return { kind: { type: "error" }, pos };
|
|
}
|
|
this.step();
|
|
subject = { kind: { type: "call", subject, args }, pos };
|
|
continue;
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
Similarly to index epxressions, if we find a `(`-token, we step over it, parse the arguments, check for a `)` and replace `subject` with a call expression containing the previous `subject`.
|
|
|
|
When parsing the arguments, we start by testing if we've reached a `)` to check if there are any arguments. If not, we parse the first argument.
|
|
The consecutive arguments are all preceded by a `,`-token. There we test or `,`, to check if we should keep parsing arguments. After checking for a seperating `,`, we check if we've reached a `)` and break if so. This is to allow for trailing comma.
|
|
|
|
```ts
|
|
func(
|
|
a,
|
|
b, // trailing comma
|
|
)
|
|
```
|
|
|
|
## 3.5 Prefix expressions
|
|
|
|
Contrasting postfix expressions, prefix expression are operations where the operator comes first, then the operands are listed. In some languages, operations such as negation (eg. `-value`) and not-operations (eg. `!value`) are prefix operations. In the language we're making, all binary and unary arithmetic operations are prefix. This includes both expressions with a single operand, such as not (eg. `not value`), but also expressions with 2 operands, such ass addition (eg. `+ a b`) and equation (eg. `== a b`).
|
|
|
|
This is because infix operators (eg. `a + b`) makes parsing more complicated, as it requires reasoning about operator precedence, eg. why `2 + 3 * 4 != (2 + 3) * 4`.
|
|
|
|
Operations with 1 operand are called unary expression. Operations with 2 are called binary expressions.
|
|
|
|
```ts
|
|
type ExprKind =
|
|
// ...
|
|
| { type: "unary", unaryType: UnaryType, subject: Expr }
|
|
| { type: "binary", binaryType: BinaryType, left: Expr, right: Expr }
|
|
// ...
|
|
;
|
|
|
|
type UnaryType = "not" /*...*/;
|
|
type BinaryType = "+" | "*" | "==" /*...*/;
|
|
```
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePrefix(): Expr {
|
|
const pos = this.pos();
|
|
// ...
|
|
return this.parsePostfix();
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
We again get the position immediately, because the operation, eg. `+ a b`, starts at the first `+`-token.
|
|
|
|
If we don't find any operators, we proceed to try to parse a postfix expression.
|
|
|
|
### 3.5.1 Unary expressions
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePrefix(): Expr {
|
|
// ...
|
|
if (this.test("not")) {
|
|
this.step();
|
|
const subject = this.parsePrefix();
|
|
return { kind: { type: "unary", unaryType: "not", subject }, pos };
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
If we find a `not`-token, we ignore it, parse a prefix expression recursively, and return a unary expression with the `subject` and unary type.
|
|
|
|
### 3.5.2 Binary expressions
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parsePrefix(): Expr {
|
|
// ...
|
|
if (this.test("+")) {
|
|
this.step();
|
|
const left = this.parsePrefix();
|
|
const right = this.parsePrefix();
|
|
return { kind: { type: "binary", binaryType: "+", left, right }, pos };
|
|
}
|
|
// ...
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
Just as with unary, if we find a `+`-token, we ignore it and parse prefix expression recursively. Then we parse the second operand, by parsing another prefix expressions. And then we return a binary expression with the `left` and `right` operands and the binary type.
|
|
|
|
## 3.6 Expressions
|
|
|
|
Lastly for expressions, we'll make a method `.parseExpr()` for parsing an expression.
|
|
|
|
```ts
|
|
class Parser {
|
|
// ...
|
|
public parseExpr(): Expr {
|
|
return this.parsePrefix();
|
|
}
|
|
// ...
|
|
}
|
|
```
|
|
|
|
The method just proceeds to try and parse a prefix expression.
|
|
|
|
## 3.7 If
|
|
|