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|
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This is an example of a goyacc program.
// To build it:
// go tool yacc -p "expr" expr.y (produces y.go)
// go build -o expr y.go
// expr
// > <type an expression>
%{
// This tag will be copied to the generated file to prevent that file
// confusing a future build.
// +build ignore
package main
import (
"bufio"
"bytes"
"fmt"
"io"
"log"
"math/big"
"os"
"unicode/utf8"
)
%}
%union {
num *big.Rat
}
%type <num> expr expr1 expr2 expr3
%token <num> NUM
%%
top:
expr
{
if $1.IsInt() {
fmt.Println($1.Num().String())
} else {
fmt.Println($1.String())
}
}
expr:
expr1
| '+' expr
{
$$ = $2
}
| '-' expr
{
$$.Neg($2)
}
expr1:
expr2
| expr1 '+' expr2
{
$$.Add($1, $3)
}
| expr1 '-' expr2
{
$$.Sub($1, $3)
}
expr2:
expr3
| expr2 '*' expr3
{
$$.Mul($1, $3)
}
| expr2 '/' expr3
{
$$.Quo($1, $3)
}
expr3:
NUM
| '(' expr ')'
{
$$ = $2
}
%%
// The parser expects the lexer to return 0 on EOF. Give it a name
// for clarity.
const eof = 0
// The parser uses the type <prefix>Lex as a lexer. It must provide
// the methods Lex(*<prefix>SymType) int and Error(string).
type exprLex struct {
line []byte
peek rune
}
// The parser calls this method to get each new token. This
// implementation returns operators and NUM.
func (x *exprLex) Lex(yylval *exprSymType) int {
for {
c := x.next()
switch c {
case eof:
return eof
case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
return x.num(c, yylval)
case '+', '-', '*', '/', '(', ')':
return int(c)
// Recognize Unicode multiplication and division
// symbols, returning what the parser expects.
case '×':
return '*'
case '÷':
return '/'
case ' ', '\t', '\n', '\r':
default:
log.Printf("unrecognized character %q", c)
}
}
}
// Lex a number.
func (x *exprLex) num(c rune, yylval *exprSymType) int {
add := func(b *bytes.Buffer, c rune) {
if _, err := b.WriteRune(c); err != nil {
log.Fatalf("WriteRune: %s", err)
}
}
var b bytes.Buffer
add(&b, c)
L: for {
c = x.next()
switch c {
case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', 'e', 'E':
add(&b, c)
default:
break L
}
}
if c != eof {
x.peek = c
}
yylval.num = &big.Rat{}
_, ok := yylval.num.SetString(b.String())
if !ok {
log.Printf("bad number %q", b.String())
return eof
}
return NUM
}
// Return the next rune for the lexer.
func (x *exprLex) next() rune {
if x.peek != eof {
r := x.peek
x.peek = eof
return r
}
if len(x.line) == 0 {
return eof
}
c, size := utf8.DecodeRune(x.line)
x.line = x.line[size:]
if c == utf8.RuneError && size == 1 {
log.Print("invalid utf8")
return x.next()
}
return c
}
// The parser calls this method on a parse error.
func (x *exprLex) Error(s string) {
log.Printf("parse error: %s", s)
}
func main() {
in := bufio.NewReader(os.Stdin)
for {
if _, err := os.Stdout.WriteString("> "); err != nil {
log.Fatalf("WriteString: %s", err)
}
line, err := in.ReadBytes('\n')
if err == io.EOF {
return
}
if err != nil {
log.Fatalf("ReadBytes: %s", err)
}
exprParse(&exprLex{line: line})
}
}
|
