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1 // A parser and evaluator for teh while language |
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2 // |
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3 //:load matcher.scala |
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4 //:load parser3.scala |
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5 |
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6 // some regular expressions |
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7 val SYM = RANGE("ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz_") |
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8 val DIGIT = RANGE("0123456789") |
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9 val ID = SEQ(SYM, STAR(ALT(SYM, DIGIT))) |
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10 val NUM = PLUS(DIGIT) |
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11 val KEYWORD = ALTS("skip", "while", "do", "if", "then", "else", "true", "false", "print") |
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12 val SEMI: Rexp = ";" |
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13 val OP: Rexp = ALTS(":=", "=", "-", "+", "*", "!", "<", ">") |
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14 val WHITESPACE = PLUS(RANGE(" \n")) |
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15 val RPAREN: Rexp = ")" |
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16 val LPAREN: Rexp = "(" |
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17 val BEGIN: Rexp = "{" |
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18 val END: Rexp = "}" |
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19 val COMMENT = SEQS("/*", NOT(SEQS(STAR(ALLC), "*/", STAR(ALLC))), "*/") |
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20 |
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21 // tokens for classifying the strings that have been recognised |
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22 abstract class Token |
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23 case object T_WHITESPACE extends Token |
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24 case object T_COMMENT extends Token |
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25 case object T_SEMI extends Token |
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26 case object T_LPAREN extends Token |
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27 case object T_RPAREN extends Token |
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28 case object T_BEGIN extends Token |
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29 case object T_END extends Token |
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30 case class T_ID(s: String) extends Token |
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31 case class T_OP(s: String) extends Token |
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32 case class T_NUM(s: String) extends Token |
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33 case class T_KWD(s: String) extends Token |
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34 |
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35 val lexing_rules: List[Rule[Token]] = |
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36 List((KEYWORD, (s) => T_KWD(s.mkString)), |
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37 (ID, (s) => T_ID(s.mkString)), |
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38 (OP, (s) => T_OP(s.mkString)), |
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39 (NUM, (s) => T_NUM(s.mkString)), |
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40 (SEMI, (s) => T_SEMI), |
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41 (LPAREN, (s) => T_LPAREN), |
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42 (RPAREN, (s) => T_RPAREN), |
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43 (BEGIN, (s) => T_BEGIN), |
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44 (END, (s) => T_END), |
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45 (WHITESPACE, (s) => T_WHITESPACE), |
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46 (COMMENT, (s) => T_COMMENT)) |
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47 |
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48 // the tokenizer |
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49 val Tok = Tokenizer(lexing_rules, List(T_WHITESPACE, T_COMMENT)) |
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50 |
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51 // the abstract syntax trees |
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52 abstract class Stmt |
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53 abstract class AExp |
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54 abstract class BExp |
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55 type Block = List[Stmt] |
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56 case object Skip extends Stmt |
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57 case class If(a: BExp, bl1: Block, bl2: Block) extends Stmt |
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58 case class While(b: BExp, bl: Block) extends Stmt |
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59 case class Assign(s: String, a: AExp) extends Stmt |
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60 case class Print(s: String) extends Stmt |
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61 |
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62 case class Var(s: String) extends AExp |
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63 case class Num(i: Int) extends AExp |
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64 case class Aop(o: String, a1: AExp, a2: AExp) extends AExp |
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65 |
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66 case object True extends BExp |
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67 case object False extends BExp |
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68 case class Bop(o: String, a1: AExp, a2: AExp) extends BExp |
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69 |
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70 // atomic parsers |
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71 case class TokParser(tok: Token) extends Parser[List[Token], Token] { |
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72 def parse(ts: List[Token]) = ts match { |
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73 case t::ts if (t == tok) => Set((t, ts)) |
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74 case _ => Set () |
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75 } |
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76 } |
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77 implicit def token2tparser(t: Token) = TokParser(t) |
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78 |
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79 case object NumParser extends Parser[List[Token], Int] { |
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80 def parse(ts: List[Token]) = ts match { |
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81 case T_NUM(s)::ts => Set((s.toInt, ts)) |
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82 case _ => Set () |
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83 } |
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84 } |
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85 |
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86 case object IdParser extends Parser[List[Token], String] { |
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87 def parse(ts: List[Token]) = ts match { |
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88 case T_ID(s)::ts => Set((s, ts)) |
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89 case _ => Set () |
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90 } |
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91 } |
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92 |
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93 |
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94 // arithmetic expressions |
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95 lazy val AExp: Parser[List[Token], AExp] = |
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96 (T ~ T_OP("+") ~ AExp) ==> { case ((x, y), z) => Aop("+", x, z): AExp } || |
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97 (T ~ T_OP("-") ~ AExp) ==> { case ((x, y), z) => Aop("-", x, z): AExp } || T |
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98 lazy val T: Parser[List[Token], AExp] = |
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99 (F ~ T_OP("*") ~ T) ==> { case ((x, y), z) => Aop("*", x, z): AExp } || F |
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100 lazy val F: Parser[List[Token], AExp] = |
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101 (T_LPAREN ~> AExp <~ T_RPAREN) || |
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102 IdParser ==> Var || |
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103 NumParser ==> Num |
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104 |
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105 // boolean expressions |
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106 lazy val BExp: Parser[List[Token], BExp] = |
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107 (AExp ~ T_OP("=") ~ AExp) ==> { case ((x, y), z) => Bop("=", x, z): BExp } || |
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108 (AExp ~ T_OP("!=") ~ AExp) ==> { case ((x, y), z) => Bop("!=", x, z): BExp } || |
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109 (AExp ~ T_OP("<") ~ AExp) ==> { case ((x, y), z) => Bop("<", x, z): BExp } || |
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110 (AExp ~ T_OP(">") ~ AExp) ==> { case ((x, y), z) => Bop(">", x, z): BExp } || |
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111 (T_KWD("true") ==> ((_) => True)) || |
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112 (T_KWD("false") ==> ((_) => False: BExp)) |
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113 |
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114 lazy val Stmt: Parser[List[Token], Stmt] = |
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115 (T_KWD("skip") ==> ((_) => Skip: Stmt)) || |
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116 (IdParser ~ T_OP(":=") ~ AExp) ==> { case ((x, y), z) => Assign(x, z): Stmt } || |
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117 (T_KWD("if") ~ BExp ~ T_KWD("then") ~ Block ~ T_KWD("else") ~ Block) ==> |
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118 { case (((((x,y),z),u),v),w) => If(y, u, w): Stmt } || |
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119 (T_KWD("while") ~ BExp ~ T_KWD("do") ~ Block) ==> { case (((x, y), z), w) => While(y, w) } || |
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120 (T_KWD("print") ~ IdParser) ==> { case (x, y) => Print(y) } |
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121 |
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122 lazy val Stmts: Parser[List[Token], Block] = |
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123 (Stmt ~ T_SEMI ~ Stmts) ==> { case ((x, y), z) => x :: z : Block } || |
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124 (Stmt ==> ((s) => List(s) : Block)) |
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125 |
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126 lazy val Block: Parser[List[Token], Block] = |
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127 (T_BEGIN ~> Stmts <~ T_END) || |
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128 (Stmt ==> ((s) => List(s))) |
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129 |
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130 // interpreter |
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131 type Env = Map[String, Int] |
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132 |
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133 def eval_bexp(b: BExp, env: Env) : Boolean = b match { |
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134 case True => true |
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135 case False => false |
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136 case Bop("=", a1, a2) => eval_aexp(a1, env) == eval_aexp(a2, env) |
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137 case Bop("!=", a1, a2) => !(eval_aexp(a1, env) == eval_aexp(a2, env)) |
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138 case Bop(">", a1, a2) => eval_aexp(a1, env) > eval_aexp(a2, env) |
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139 case Bop("<", a1, a2) => eval_aexp(a1, env) < eval_aexp(a2, env) |
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140 } |
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141 |
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142 def eval_aexp(a: AExp, env : Env) : Int = a match { |
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143 case Num(i) => i |
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144 case Var(s) => env(s) |
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145 case Aop("+", a1, a2) => eval_aexp(a1, env) + eval_aexp(a2, env) |
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146 case Aop("-", a1, a2) => eval_aexp(a1, env) - eval_aexp(a2, env) |
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147 case Aop("*", a1, a2) => eval_aexp(a1, env) * eval_aexp(a2, env) |
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148 } |
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149 |
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150 def eval_stmt(s: Stmt, env: Env) : Env = s match { |
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151 case Skip => env |
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152 case Assign(x, a) => env + (x -> eval_aexp(a, env)) |
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153 case If(b, bl1, bl2) => if (eval_bexp(b, env)) eval_bl(bl1, env) else eval_bl(bl2, env) |
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154 case While(b, bl) => |
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155 if (eval_bexp(b, env)) eval_stmt(While(b, bl), eval_bl(bl, env)) |
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156 else env |
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157 case Print(x) => { println(env(x)); env } |
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158 } |
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159 |
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160 def eval_bl(bl: Block, env: Env) : Env = bl match { |
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161 case Nil => env |
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162 case s::bl => eval_bl(bl, eval_stmt(s, env)) |
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163 } |
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164 |
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165 def eval_prog(name: String) : Env = { |
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166 val tks = Tok.fromFile(name) |
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167 val ast = Stmts.parse_all(tks).head |
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168 eval_bl(ast, Map.empty) |
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169 } |
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170 |
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171 |
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172 //examples |
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173 |
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174 eval_prog("fib.while") |