Add example of using grammar

gg/grammar/example_test.go

@@ -0,0 +1,163 @@
+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)
+}
+
+func s(str string) fmt.Stringer {
+	return grammar.Stringer{S: str}
+}
+
+var (
+	digit = grammar.RuneFunc(
+		s("digit"), func(r rune) bool { return '0' <= r && r <= '9' },
+	)
+
+	positiveNumber = grammar.StringFromRunes(grammar.OneOrMore(digit))
+
+	negativeNumber = grammar.Reduction(
+		s("negative-number"),
+		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.Mapping(
+		s("number"),
+		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. We
+			// pass rightParen as the Stringer because we want that to still be
+			// the string identifier for this branch of the symbol, for error
+			// purposes.
+			rightParen = grammar.Rune(')')
+			listTerm   = grammar.Mapping(
+				rightParen,
+				rightParen,
+				func(grammar.Located[rune]) listState { return listState{} },
+			)
+
+			listElTail = grammar.Reduction[Element, listState, listState](
+				element, // Stringer
+				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.Reduction[
+			grammar.Located[rune], listState, Element,
+		](
+			s("list"),
+			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
+}