Add example from the readme

example_readme_test.go

@@ -0,0 +1,108 @@
+package prattle_test
+
+import (
+	"fmt"
+	"strconv"
+	"unicode"
+
+	"github.com/askeladdk/prattle"
+)
+
+type driver struct {
+	stack []int
+}
+
+func (d *driver) push(i int) {
+	d.stack = append(d.stack, i)
+}
+
+func (d *driver) pop() (i int) {
+	n := len(d.stack)
+	i, d.stack = d.stack[n-1], d.stack[:n-1]
+	return
+}
+
+func (d *driver) number(p *prattle.Parser, t prattle.Token) error {
+	n, _ := strconv.Atoi(t.Text)
+	d.push(n)
+	return nil
+}
+
+func (d *driver) add(p *prattle.Parser, t prattle.Token) error {
+	// First parse the right hand operator.
+	if err := p.ParseExpression(d.Precedence(t.Kind)); err != nil {
+		return err
+	}
+
+	right := d.pop()
+	left := d.pop()
+	acc := left + right
+	fmt.Printf("%d + %d = %d\n", left, right, acc)
+	d.push(acc)
+	return nil
+}
+
+func (d *driver) Prefix(k int) prattle.ParseFunc {
+	if k == 2 {
+		return d.number
+	}
+	return nil
+}
+
+func (d *driver) Infix(k int) prattle.ParseFunc {
+	if k == 1 {
+		return d.add
+	}
+	return nil
+}
+
+func (d *driver) Statement(k int) prattle.ParseFunc {
+	return nil
+}
+
+func (d *driver) Precedence(k int) int {
+	return k
+}
+
+func (d *driver) ParseError(t prattle.Token) error {
+	return fmt.Errorf("%s", t)
+}
+
+// This example is described in the readme.
+func Example_readme() {
+	scanner := prattle.Scanner{
+		Scan: func(s *prattle.Scanner) int {
+			// Skip whitespaces.
+			s.ExpectAny(unicode.IsSpace)
+			s.Skip()
+			switch {
+			case s.Done(): // Stop if the entire input has been consumed.
+				return 0
+			case s.Expect('+'):
+				return 1
+			case s.ExpectOne(unicode.IsDigit): // Read a number consisting of one or more digits.
+				s.ExpectAny(unicode.IsDigit)
+				return 2
+			}
+
+			s.Advance()
+			return -1
+		},
+	}
+
+	parser := prattle.Parser{
+		Driver: &driver{},
+	}
+
+	source := "1 + 23 + 456 + 7890"
+	scanner.InitWithString(source)
+	parser.Init(&scanner)
+	if err := parser.ParseExpression(0); err != nil {
+		fmt.Println(err)
+	}
+
+	// Output:
+	// 1 + 23 = 24
+	// 24 + 456 = 480
+	// 480 + 7890 = 8370
+}