cmd/yacc: replace units example with simpler expr example

The units example is nice but is covered by the Lucent
license, which may be a concern for some people making a
commercial source code distribution of Go.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13283045

src/cmd/yacc/Makefile

@@ -2,9 +2,9 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-units: yacc.go units.y
-	go run yacc.go -p units_ units.y
-	go build -o units y.go
+expr: yacc.go expr.y
+	go run yacc.go -p expr expr.y
+	go build -o expr y.go
 
 clean:
-	rm -f y.go y.output units
+	rm -f y.go y.output expr

src/cmd/yacc/doc.go

@@ -20,10 +20,8 @@ written in C and documented at
 Adepts of the original yacc will have no trouble adapting to this
 form of the tool.
 
-The file units.y in this directory is a yacc grammar for a version of
-the Unix tool units, also written in Go and largely transliterated
-from the Plan 9 C version. It needs the flag "-p units_" (see
-below).
+The file expr.y in this directory is a yacc grammar for a very simple
+expression parser.  It needs the flag "-p expr" (see below).
 
 The generated parser is reentrant. Parse expects to be given an
 argument that conforms to the following interface:

src/cmd/yacc/expr.y

+// 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':
+		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})
+	}
+}