Compilers 101: An introduction to programming languages and parsing

I gave this talk at Coderfaire, August 17, 2013. As usual, the slides do not make a ton of sense without the words. Sorry!

However they contain a complete recursive-descent parser for a toy grammar, in JS. I’ve never given a talk with this much code before. The slides reveal this code line by line, and that worked out very nicely.

More resources for this talk: http://gist.io/6256943

Transcript

1. 17 August 2013
   Compilers 101
   An introduction to programming languages and parsing
   Tuesday, September 3, 13
   

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2. 17 August 2013
   Compilers 101
   An introduction to programming languages and parsing
   WARNING
   CONTAINS CODE
   0
   0
   0
   1
   1
   Tuesday, September 3, 13
   

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3. gist.io/6256943
   Tuesday, September 3, 13
   

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4. Tuesday, September 3, 13
   

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5. Tuesday, September 3, 13
   

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6. green big ball
   Tuesday, September 3, 13
   

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7. green big ball
   big green ball
   Tuesday, September 3, 13
   

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8. Tuesday, September 3, 13
   

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9. A compiler is
   Tuesday, September 3, 13
   

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10. A compiler is a program that translates code
    Tuesday, September 3, 13
    

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11. A compiler is a program that translates code
    from one language to another.
    Tuesday, September 3, 13
    

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12. A compiler is a program that translates code
    from one language to another.
    ✤ to machine code (like FORTRAN, C, Go)
    Tuesday, September 3, 13
    

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13. A compiler is a program that translates code
    from one language to another.
    ✤ to machine code (like FORTRAN, C, Go)
    ✤ to bytecode (like Java, Python)
    Tuesday, September 3, 13
    

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14. A compiler is a program that translates code
    from one language to another.
    ✤ to machine code (like FORTRAN, C, Go)
    ✤ to bytecode (like Java, Python)
    ✤ to another high-level language (cfront,
    LESS & Sass, CoffeeScript, Traceur,
    emscripten)
    Tuesday, September 3, 13
    

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15. Tuesday, September 3, 13
    

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16. Language is a different kind of data.
    Tuesday, September 3, 13
    

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17. Language is a different kind of data.
    It’s not line-based.
    Tuesday, September 3, 13
    

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18. Language is a different kind of data.
    It’s not line-based.
    It’s not record-based.
    Tuesday, September 3, 13
    

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19. Language is a different kind of data.
    It’s not line-based.
    It’s not record-based.
    And you can’t fake understanding.
    Tuesday, September 3, 13
    

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20. Language is a different kind of data.
    It’s not line-based.
    It’s not record-based.
    And you can’t fake understanding.
    Language has structure.
    Tuesday, September 3, 13
    

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21. Tuesday, September 3, 13
    

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22. when flag clicked
    forever
    point towards mouse-pointer
    if
    move 3 * speed steps
    not touching mouse-pointer ? then
    Tuesday, September 3, 13
    

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23. when flag clicked
    forever
    point towards mouse-pointer
    if
    move 3 * speed steps
    not touching mouse-pointer ? then
    Tuesday, September 3, 13
    

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24. Inside a compiler
    Tuesday, September 3, 13
    

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25. Inside a compiler
    Tuesday, September 3, 13
    

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26. front end
    (parsing, analysis)
    Inside a compiler
    Tuesday, September 3, 13
    

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27. front end
    (parsing, analysis)
    Inside a compiler
    Tuesday, September 3, 13
    

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28. front end
    (parsing, analysis)
    back end
    (code generation)
    Inside a compiler
    Tuesday, September 3, 13
    

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29. front end
    (parsing, analysis)
    back end
    (code generation)
    Inside a compiler
    Tuesday, September 3, 13
    

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30. front end
    (parsing, analysis)
    back end
    (code generation)
    sleep(2*x)
    Inside a compiler
    Tuesday, September 3, 13
    

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31. front end
    (parsing, analysis)
    back end
    (code generation)
    sleep(2*x)
    pushq %rbp
    movq %rsp, %rbp
    addl %edi, %edi
    callq _sleep
    Inside a compiler
    Tuesday, September 3, 13
    

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32. front end
    (parsing, analysis)
    back end
    (code generation)
    sleep(2*x)
    call
    sleep *
    2 x
    pushq %rbp
    movq %rsp, %rbp
    addl %edi, %edi
    callq _sleep
    Inside a compiler
    Tuesday, September 3, 13
    

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33. Inside a front end
    Tuesday, September 3, 13
    

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34. Inside a front end
    tokenizer
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35. Inside a front end
    tokenizer parser
    Tuesday, September 3, 13
    

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36. Inside a front end
    tokenizer parser AST builder
    Tuesday, September 3, 13
    

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37. Inside a front end
    tokenizer parser AST builder
    12*x
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38. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    Tuesday, September 3, 13
    

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39. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    ['12', '*', 'x']
    Tuesday, September 3, 13
    

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40. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    ['12', '*', 'x']
    Tuesday, September 3, 13
    

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41. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    ['12', '*', 'x']
    out.number('12')
    Tuesday, September 3, 13
    

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42. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    ['12', '*', 'x']
    out.number('12')
    out.name('x')
    Tuesday, September 3, 13
    

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43. Inside a front end
    tokenizer parser AST builder
    12*x *
    12 x
    ['12', '*', 'x']
    out.number('12')
    out.name('x')
    out.mul(
    ,
    )
    Tuesday, September 3, 13
    

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44. IfStatement :
    if ( Expression ) Statement else Statement
    if ( Expression ) Statement
    Tuesday, September 3, 13
    

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45. Statement :
    Block
    VariableStatement
    EmptyStatement
    ExpressionStatement
    IfStatement
    IterationStatement
    ContinueStatement
    BreakStatement
    ReturnStatement
    WithStatement
    LabelledStatement
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46. Tuesday, September 3, 13
    

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47. PrimaryExpr :
    Number
    Name
    ( Expr )
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
    Tuesday, September 3, 13
    

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48. PrimaryExpr :
    Number
    Name
    ( Expr )
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
    out.number("3")
    Tuesday, September 3, 13
    

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49. PrimaryExpr :
    Number
    Name
    ( Expr )
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
    out.number("3")
    out.name("pi")
    Tuesday, September 3, 13
    

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50. PrimaryExpr :
    Number
    Name
    ( Expr )
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
    out.number("3")
    out.name("pi")
    out.mul(lhs, rhs)
    Tuesday, September 3, 13
    

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51. PrimaryExpr :
    Number
    Name
    ( Expr )
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
    out.number("3")
    out.name("pi")
    out.mul(lhs, rhs)
    out.add(lhs, rhs)
    Tuesday, September 3, 13
    

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52. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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53. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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54. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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55. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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56. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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57. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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58. function parse(code, out) {
    var tokens = tokenize(code);
    var position = 0;
    function peek() {
    return tokens[position];
    }
    function consume(token) {
    assert.strictEqual(token, tokens[position]);
    position++;
    }
    Calculator parser - page 1 of 5
    Tuesday, September 3, 13
    

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59. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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60. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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61. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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62. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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63. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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64. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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65. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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66. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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67. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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68. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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69. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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70. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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71. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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72. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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73. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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74. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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75. function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Calculator parser - page 2 of 5 PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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76. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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77. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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78. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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79. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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80. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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81. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
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82. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
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83. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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84. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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85. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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86. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
    Tuesday, September 3, 13
    

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87. function parseMulExpr() {
    var expr = parsePrimaryExpr();
    var t = peek();
    while (t === "*" || t === "/") {
    consume(t);
    var rhs = parsePrimaryExpr();
    if (t === "*")
    expr = out.mul(expr, rhs);
    else
    expr = out.div(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 3 of 5
    MulExpr :
    PrimaryExpr ( * PrimaryExpr | / PrimaryExpr )*
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88. function parseExpr() {
    var expr = parseMulExpr();
    var t = peek();
    while (t === "+" || t === "-") {
    consume(t);
    var rhs = parseMulExpr();
    if (t === "+")
    expr = out.add(expr, rhs);
    else
    expr = out.sub(expr, rhs);
    t = peek();
    }
    return expr;
    }
    Calculator parser - page 4 of 5
    Expr :
    MulExpr ( + MulExpr | - MulExpr )*
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89. Calculator parser - page 5 of 5
    var result = parseExpr();
    if (position !== tokens.length) {
    throw new SyntaxError(
    "unexpected '" + peek() + "'");
    }
    return result;
    }
    Tuesday, September 3, 13
    

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90. Calculator parser - page 5 of 5
    var result = parseExpr();
    if (position !== tokens.length) {
    throw new SyntaxError(
    "unexpected '" + peek() + "'");
    }
    return result;
    }
    Tuesday, September 3, 13
    

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91. Calculator parser - page 5 of 5
    var result = parseExpr();
    if (position !== tokens.length) {
    throw new SyntaxError(
    "unexpected '" + peek() + "'");
    }
    return result;
    }
    Tuesday, September 3, 13
    

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92. Calculator parser - page 5 of 5
    var result = parseExpr();
    if (position !== tokens.length) {
    throw new SyntaxError(
    "unexpected '" + peek() + "'");
    }
    return result;
    }
    Tuesday, September 3, 13
    

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93. Tuesday, September 3, 13
    

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94. Tuesday, September 3, 13
    

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95. Tuesday, September 3, 13
    

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96. IfStatement :
    if ( Expression ) Statement else Statement
    if ( Expression ) Statement
    Tuesday, September 3, 13
    

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97. IfStatement :
    if ( Expression ) Statement else Statement
    if ( Expression ) Statement
    Tuesday, September 3, 13
    

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98. IfStatement :
    if ( Expression ) Statement else Statement
    if ( Expression ) Statement
    PrimaryExpr :
    Number
    Name
    ( Expr )
    Tuesday, September 3, 13
    

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99. IfStatement :
    if ( Expression ) Statement else Statement
    if ( Expression ) Statement
    PrimaryExpr :
    Number
    Name
    ( Expr )
    function parsePrimaryExpr() {
    var t = peek();
    if (isNumber(t)) {
    consume(t);
    return out.number(t);
    } else if (isName(t)) {
    consume(t);
    return out.name(t);
    } else if (t === "(") {
    consume(t);
    var expr = parseExpr();
    if (peek() !== ")")
    throw new SyntaxError("expected )");
    consume(")");
    return expr;
    } else {
    throw new SyntaxError("didn't expect '" + t + "'");
    }
    }
    Tuesday, September 3, 13
    

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100. IfStatement :
     if ( Expression ) Statement else Statement
     if ( Expression ) Statement
     PrimaryExpr :
     Number
     Name
     ( Expr )
     function parsePrimaryExpr() {
     var t = peek();
     if (isNumber(t)) {
     consume(t);
     return out.number(t);
     } else if (isName(t)) {
     consume(t);
     return out.name(t);
     } else if (t === "(") {
     consume(t);
     var expr = parseExpr();
     if (peek() !== ")")
     throw new SyntaxError("expected )");
     consume(")");
     return expr;
     } else {
     throw new SyntaxError("didn't expect '" + t + "'");
     }
     }
     Tuesday, September 3, 13
     

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101. Tuesday, September 3, 13
     

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102. join me tomorrow
     for Compilers 101L
     Tuesday, September 3, 13
     

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