Instantly share code, notes, and snippets.

Embed
What would you like to do?
Parser example (GoWayFest 2018)
package main
import (
"fmt"
"os"
"regexp"
"strings"
)
type token = string
type node interface{}
type binaryNode struct {
op token
left node
right node
}
var (
check = regexp.MustCompile("[a-z0-9]+")
stack = make([]node, 0)
tokens []token
lookahead token
)
func main() {
tokens = lex(os.Args[1])
lookahead = next()
expr()
fmt.Println(pop())
}
func expr() {
term()
exprR()
}
func exprR() {
if lookahead == "+" {
match("+")
term()
emit("+")
exprR()
} else if lookahead == "-" {
match("-")
term()
emit("-")
exprR()
}
}
func term() {
factor()
termR()
}
func termR() {
if lookahead == "*" {
match("*")
factor()
emit("*")
exprR()
} else if lookahead == "/" {
match("/")
factor()
emit("/")
exprR()
}
}
func factor() {
if lookahead == "(" {
match("(")
expr()
match(")")
} else {
atom()
}
}
func atom() {
if check.MatchString(lookahead) {
stack = append(stack, lookahead)
lookahead = next()
} else {
panic("expected atom")
}
}
func pop() node {
top := stack[len(stack)-1]
stack = stack[:len(stack)-1]
return top
}
func emit(op token) {
right := pop()
left := pop()
node := binaryNode{
op,
left,
right,
}
stack = append(stack, node)
}
func next() token {
if len(tokens) > 0 {
x := tokens[0]
tokens = tokens[1:]
return x
}
return ""
}
func match(terminal token) {
if lookahead == terminal {
lookahead = next()
} else {
panic("expected token " + terminal)
}
}
func lex(input string) []token {
return strings.Split(input, "")
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment