chapter 3, operands, postfix expressions
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Simon From Jakobsen
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@ -417,7 +417,7 @@ while (token !== null) {
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## 2.10 Exercises
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1. Implement the operators: `-`, `*`, `/`, `(`, `)`, `[`, `]`, `!=`, `<`, `>`, `<=` and `>=`.
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1. Implement the operators: `-`, `*`, `/`, `(`, `)`, `.`, `,`, `,`, `[`, `]`, `!=`, `<`, `>`, `<=` and `>=`.
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2. Implement the keywords: `true`, `false`, `null`, `or`, `and`, `not`, `loop`, `break`, `let`, `fn` and `return`.
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3. \* Implement single line comments using `//` and multiline comments using `\*` and `*\` (\*\* extra points if multiline comments can be nested, eg. `/* ... /* ... */ ... */`).
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4. \* Reimplement integers such that integers are either `0` or start with `[1-9]`.
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@ -1,11 +1,11 @@
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# Parser
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# 3 Parser
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In this chaper I'll show how I would make a parser.
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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.
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## Abstract Syntax Tree AST
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## 3.1 Abstract Syntax Tree AST
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The result of parsing is a tree structure representing the input program.
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@ -43,7 +43,7 @@ Both `Stmt` (statement) and `Expr` (expression) are polymorphic types, meaning a
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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.
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## Consumer of lexer
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## 3.2 Consumer of lexer
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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.
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@ -110,14 +110,14 @@ We'll also want a method for reporting errors.
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```ts
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class Parser {
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// ...
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private report(pos: Pos, msg: string) {
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private report(msg: string, pos = this.pos()) {
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console.log(`Parser: ${msg} at ${pos.line}:${pos.col}`);
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}
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// ...
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}
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```
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## Operands
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## 3.3 Operands
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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.
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@ -128,12 +128,8 @@ class Parser {
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// ...
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public parseOperand(): Expr {
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const pos = this.pos();
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if (this.test("int")) {
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const value = this.current().intValue;
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this.step();
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return { kind: { type: "int", value }, pos };
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}
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this.report(pos "expected expr");
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// ...
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this.report("expected expr", pos);
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this.step();
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return { kind: { type: "error" }, pos };
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}
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@ -141,14 +137,16 @@ class Parser {
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}
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```
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### Integer
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### 3.3.1 Identifiers and literals
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Parsing an integer is a 1:1 translation between the integer token and an integer expression.
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Identifiers and literals (integers, strings) are single token constructs, meaning the parsing consists of translating a token into an ast-node with the value.
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```ts
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type ExprKind =
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// ...
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| { type: "ident", value: string }
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| { type: "int", value: number }
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| { type: "string", value: string }
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// ...
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;
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```
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@ -158,14 +156,266 @@ class Parser {
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// ...
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public parseOperand(): Expr {
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// ...
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if (this.test("ident")) {
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const value = this.current().identValue;
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this.step();
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return { kind: { type: "ident", value }, pos };
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}
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if (this.test("int")) {
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const value = this.current().intValue;
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this.step();
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return { kind: { type: "int", value }, pos };
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}
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if (this.test("string")) {
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const value = this.current().stringValue;
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this.step();
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return { kind: { type: "string", value }, pos };
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}
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// ...
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}
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// ...
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}
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```
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### 3.3.2 Group expressions
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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.
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```ts
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type ExprKind =
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// ...
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| { type: "group", expr: Expr }
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// ...
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;
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```
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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.
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```ts
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class Parser {
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// ...
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public parseOperand(): Expr {
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// ...
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if (this.test("(")) {
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this.step();
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const expr = this.parseExpr();
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if (!this.test(")")) {
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this.report("expected ')'");
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return { kind: { type: "error" }, pos };
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}
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this.step();
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return { kind: { type: "group", expr }, pos };
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}
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// ...
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}
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// ...
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}
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```
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If we do not find the closing `)`-token, we report an error and return an error expression.
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### 3.3.3 Block, if and loop operands
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We want to be able to use blocks, if and loop constructs as expressions.
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Example:
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```rs
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let temperature_feeling = if > temperature 20 { "hot" } else { "cold" };
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```
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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.
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```ts
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class Parser {
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// ...
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public parseOperand(): Expr {
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// ...
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if (this.test("{"))
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return this.parseBlock();
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if (this.test("if"))
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return this.parseIf();
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if (this.test("loop"))
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return this.parseLoop();
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// ...
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}
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// ...
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}
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```
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## 3.4 Postfix operators
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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)`).
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A notable detail, is that postfix operations are chainable, eg. `subject[index].field` is valid, likewise with `subject.method(arg)` and `matrix[y][x]`.
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We'll make a method `.parsePostfix()` to parse postfix operators.
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```ts
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class Parser {
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// ...
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public parsePostfix(): Expr {
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let subject = this.parseOperand();
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while (true) {
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const pos = this.pos();
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// ...
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break;
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}
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return subject;
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}
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// ...
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}
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```
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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.
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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.
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### 3.4.1 Field expression
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A field expression is for accessing fields on an object, and consists of a `.`-token and an identifier, eg. `.field`.
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```ts
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type ExprKind =
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// ...
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| { type: "field", subject: Expr, value: string }
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// ...
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;
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```
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```ts
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class Parser {
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// ...
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public parsePostfix(): Expr {
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// ...
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while (true) {
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// ...
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if (this.test(".")) {
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this.step();
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if (!this.test("ident")) {
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this.report("expected ident");
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return { kind: { type: "error" }, pos };
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}
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const value = this.current().identValue;
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this.step();
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subject = { kind: { type: "field", subject, value }, pos };
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continue;
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}
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// ...
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}
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// ...
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}
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// ...
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}
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```
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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.
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### 3.4.2 Index expression
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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]`.
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```ts
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type ExprKind =
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// ...
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| { type: "index", subject: Expr, value: Expr }
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// ...
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;
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```
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```ts
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class Parser {
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// ...
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public parsePostfix(): Expr {
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// ...
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while (true) {
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// ...
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if (this.test("[")) {
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this.step();
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const value = this.parseExpr();
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if (!this.test("]") {
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this.report("expected ']'");
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return { kind: { type: "error" }, pos };
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}
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this.step();
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subject = { kind: { type: "index", subject, value }, pos };
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continue;
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}
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// ...
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}
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// ...
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}
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// ...
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}
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```
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If we find a `[`-token, we parse the index part exactly the same way, we parse a group expression.
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### 3.4.3 Call expression
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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 `,`.
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```ts
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type ExprKind =
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// ...
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| { type: "call", subject: Expr, args: Expr[] }
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// ...
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;
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```
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```ts
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class Parser {
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// ...
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public parsePostfix(): Expr {
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// ...
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while (true) {
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// ...
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if (this.test("(")) {
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this.step();
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let args: Expr[] = [];
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if (!this.test(")") {
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args.push(this.parseExpr());
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while (this.test(",")) {
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this.step();
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if (this.test(")"))
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break;
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args.push(this.parseExpr());
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}
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}
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const value = this.parseExpr();
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if (!this.test(")") {
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this.report("expected ')'");
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return { kind: { type: "error" }, pos };
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}
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this.step();
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subject = { kind: { type: "call", subject, args }, pos };
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continue;
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}
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// ...
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}
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// ...
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}
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// ...
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}
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```
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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`.
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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.
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The consecutive arguments are all preceded by a `,`-token. There we test or `,`, to check if we should keep parsing arguments.
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After checking for a seperating `,`, we check if we've reached a `)` and break if so. This is to allow for trailing comma, eg.
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```ts
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func(
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a,
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b, // trailing comma
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)
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```
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### 3.5 Prefix expressions
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