| 1 | // Copyright 2013 The Go Authors. All rights reserved. |
|---|---|
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | // This is an example of a goyacc program. |
| 6 | // To build it: |
| 7 | // goyacc -p "expr" expr.y (produces y.go) |
| 8 | // go build -o expr y.go |
| 9 | // expr |
| 10 | // > <type an expression> |
| 11 | |
| 12 | %{ |
| 13 | |
| 14 | package main |
| 15 | |
| 16 | import ( |
| 17 | "bufio" |
| 18 | "bytes" |
| 19 | "fmt" |
| 20 | "io" |
| 21 | "log" |
| 22 | "math/big" |
| 23 | "os" |
| 24 | "unicode/utf8" |
| 25 | ) |
| 26 | |
| 27 | %} |
| 28 | |
| 29 | %union { |
| 30 | num *big.Rat |
| 31 | } |
| 32 | |
| 33 | %type <num> expr expr1 expr2 expr3 |
| 34 | |
| 35 | %token '+' '-' '*' '/' '(' ')' |
| 36 | |
| 37 | %token <num> NUM |
| 38 | |
| 39 | %% |
| 40 | |
| 41 | top: |
| 42 | expr |
| 43 | { |
| 44 | if $1.IsInt() { |
| 45 | fmt.Println($1.Num().String()) |
| 46 | } else { |
| 47 | fmt.Println($1.String()) |
| 48 | } |
| 49 | } |
| 50 | |
| 51 | expr: |
| 52 | expr1 |
| 53 | | '+' expr |
| 54 | { |
| 55 | $$ = $2 |
| 56 | } |
| 57 | | '-' expr |
| 58 | { |
| 59 | $$ = $2.Neg($2) |
| 60 | } |
| 61 | |
| 62 | expr1: |
| 63 | expr2 |
| 64 | | expr1 '+' expr2 |
| 65 | { |
| 66 | $$ = $1.Add($1, $3) |
| 67 | } |
| 68 | | expr1 '-' expr2 |
| 69 | { |
| 70 | $$ = $1.Sub($1, $3) |
| 71 | } |
| 72 | |
| 73 | expr2: |
| 74 | expr3 |
| 75 | | expr2 '*' expr3 |
| 76 | { |
| 77 | $$ = $1.Mul($1, $3) |
| 78 | } |
| 79 | | expr2 '/' expr3 |
| 80 | { |
| 81 | $$ = $1.Quo($1, $3) |
| 82 | } |
| 83 | |
| 84 | expr3: |
| 85 | NUM |
| 86 | | '(' expr ')' |
| 87 | { |
| 88 | $$ = $2 |
| 89 | } |
| 90 | |
| 91 | |
| 92 | %% |
| 93 | |
| 94 | // The parser expects the lexer to return 0 on EOF. Give it a name |
| 95 | // for clarity. |
| 96 | const eof = 0 |
| 97 | |
| 98 | // The parser uses the type <prefix>Lex as a lexer. It must provide |
| 99 | // the methods Lex(*<prefix>SymType) int and Error(string). |
| 100 | type exprLex struct { |
| 101 | line []byte |
| 102 | peek rune |
| 103 | } |
| 104 | |
| 105 | // The parser calls this method to get each new token. This |
| 106 | // implementation returns operators and NUM. |
| 107 | func (x *exprLex) Lex(yylval *exprSymType) int { |
| 108 | for { |
| 109 | c := x.next() |
| 110 | switch c { |
| 111 | case eof: |
| 112 | return eof |
| 113 | case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9': |
| 114 | return x.num(c, yylval) |
| 115 | case '+', '-', '*', '/', '(', ')': |
| 116 | return int(c) |
| 117 | |
| 118 | // Recognize Unicode multiplication and division |
| 119 | // symbols, returning what the parser expects. |
| 120 | case '×': |
| 121 | return '*' |
| 122 | case '÷': |
| 123 | return '/' |
| 124 | |
| 125 | case ' ', '\t', '\n', '\r': |
| 126 | default: |
| 127 | log.Printf("unrecognized character %q", c) |
| 128 | } |
| 129 | } |
| 130 | } |
| 131 | |
| 132 | // Lex a number. |
| 133 | func (x *exprLex) num(c rune, yylval *exprSymType) int { |
| 134 | add := func(b *bytes.Buffer, c rune) { |
| 135 | if _, err := b.WriteRune(c); err != nil { |
| 136 | log.Fatalf("WriteRune: %s", err) |
| 137 | } |
| 138 | } |
| 139 | var b bytes.Buffer |
| 140 | add(&b, c) |
| 141 | L: for { |
| 142 | c = x.next() |
| 143 | switch c { |
| 144 | case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', 'e', 'E': |
| 145 | add(&b, c) |
| 146 | default: |
| 147 | break L |
| 148 | } |
| 149 | } |
| 150 | if c != eof { |
| 151 | x.peek = c |
| 152 | } |
| 153 | yylval.num = &big.Rat{} |
| 154 | _, ok := yylval.num.SetString(b.String()) |
| 155 | if !ok { |
| 156 | log.Printf("bad number %q", b.String()) |
| 157 | return eof |
| 158 | } |
| 159 | return NUM |
| 160 | } |
| 161 | |
| 162 | // Return the next rune for the lexer. |
| 163 | func (x *exprLex) next() rune { |
| 164 | if x.peek != eof { |
| 165 | r := x.peek |
| 166 | x.peek = eof |
| 167 | return r |
| 168 | } |
| 169 | if len(x.line) == 0 { |
| 170 | return eof |
| 171 | } |
| 172 | c, size := utf8.DecodeRune(x.line) |
| 173 | x.line = x.line[size:] |
| 174 | if c == utf8.RuneError && size == 1 { |
| 175 | log.Print("invalid utf8") |
| 176 | return x.next() |
| 177 | } |
| 178 | return c |
| 179 | } |
| 180 | |
| 181 | // The parser calls this method on a parse error. |
| 182 | func (x *exprLex) Error(s string) { |
| 183 | log.Printf("parse error: %s", s) |
| 184 | } |
| 185 | |
| 186 | func main() { |
| 187 | in := bufio.NewReader(os.Stdin) |
| 188 | for { |
| 189 | if _, err := os.Stdout.WriteString("> "); err != nil { |
| 190 | log.Fatalf("WriteString: %s", err) |
| 191 | } |
| 192 | line, err := in.ReadBytes('\n') |
| 193 | if err == io.EOF { |
| 194 | return |
| 195 | } |
| 196 | if err != nil { |
| 197 | log.Fatalf("ReadBytes: %s", err) |
| 198 | } |
| 199 | |
| 200 | exprParse(&exprLex{line: line}) |
| 201 | } |
| 202 | } |
| 203 |
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