ginger/gg/grammar/example_test.go

package grammar_test

import (
	"bytes"
	"fmt"
	"strconv"
	"strings"

	"code.betamike.com/mediocregopher/ginger/gg/grammar"
	"golang.org/x/exp/slices"
)

/*
This example demonstrates how to describe the following EBNF using the grammar
package:

```
<digit>           ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
<positive-number> ::= <digit>+
<negative-number> ::= "-" <positive-number>
<number>          ::= <negative-number> | <positive-number>

<list-el-tail> ::= <element> <list-tail>
<list-tail>    ::= ")" | "," <list-el-tail>
<list-head>    ::= ")" | <list-el-tail>
<list>         ::= "(" <list-head>

<element> ::= <number> | <list>
```
*/

// Element represents an element of a list, which can be either a number or a
// sub-list.
type Element struct {
	Number int64
	List   []Element
}

func (e Element) String() string {
	if e.List != nil {
		listElStrs := make([]string, len(e.List))
		for i := range e.List {
			listElStrs[i] = e.List[i].String()
		}
		return fmt.Sprintf("(%s)", strings.Join(listElStrs, ","))
	}

	return fmt.Sprint(e.Number)
}

var (
	digit = grammar.RuneFunc(
		"digit", func(r rune) bool { return '0' <= r && r <= '9' },
	)

	positiveNumber = grammar.StringFromRunes(grammar.OneOrMore(digit))

	negativeNumber = grammar.Reduction(
		grammar.Rune('-'),
		positiveNumber,
		func(
			neg grammar.Located[rune], posNum grammar.Located[string],
		) grammar.Located[string] {
			return grammar.Locate(neg.Location, string(neg.Value)+posNum.Value)
		},
	)

	number = grammar.Named(
		"number",
		grammar.Mapping(
			grammar.FirstOf(negativeNumber, positiveNumber),
			func(str grammar.Located[string]) Element {
				i, err := strconv.ParseInt(str.Value, 10, 64)
				if err != nil {
					panic(fmt.Errorf("parsing %q as int: %w", str, err))
				}
				return Element{Number: i}
			},
		),
	)

	// Because the list/element definitions are recursive it requires using
	// SymbolPtrs, which is easier to do via a global initialization function
	// like this.
	list = func() grammar.Symbol[Element] {

		type listState []Element

		var (
			listTail = new(grammar.SymbolPtr[listState])
			list     = new(grammar.SymbolPtr[Element])
			element  = new(grammar.SymbolPtr[Element])

			// Right parenthesis indicates the end of a list, at which point we
			// can initialize the state which gets returned down the stack.
			listTerm = grammar.Mapping(
				grammar.Rune(')'),
				func(grammar.Located[rune]) listState { return listState{} },
			)

			listElTail = grammar.Reduction[Element, listState, listState](
				element,
				listTail,
				func(el Element, ls listState) listState {
					ls = append(ls, el)
					return ls
				},
			)

			listHead = grammar.FirstOf(listTerm, listElTail)
		)

		listTail.Symbol = grammar.FirstOf(
			listTerm,
			grammar.Prefixed(grammar.Rune(','), listElTail),
		)

		list.Symbol = grammar.Named(
			"list",
			grammar.Reduction[grammar.Located[rune], listState, Element](
				grammar.Rune('('),
				listHead,
				func(_ grammar.Located[rune], ls listState) Element {
					slices.Reverse(ls)
					return Element{List: []Element(ls)}
				},
			),
		)

		element.Symbol = grammar.FirstOf[Element](number, list)

		return list
	}()
)

func Example() {
	r := grammar.NewReader(bytes.NewBufferString(
		`()` + `(1,(2,-3),4)` + `(ERROR`,
	))

	l1, err := list.Decode(r)
	fmt.Println(l1, err)

	l2, err := list.Decode(r)
	fmt.Println(l2, err)

	_, err = list.Decode(r)
	fmt.Println(err)

	// Output:
	// () <nil>
	// (1,(2,-3),4) <nil>
	// 1:16: expected ')' or number or list
}