strings: Add At command returning the n-th rune

internal/runtime/funcs/registry.go

@@ -71,6 +71,7 @@ func CreatorRegistry() map[string]runtime.FuncCreator {
 		"time_sleep": timeSleep{},
 
 		// strings
+		"string_at":   stringAt{},
 		"join":        stringJoin{},
 		"split":       stringSplit{},
 		"string_sort": listSortString{},

internal/runtime/funcs/string_at.go

@@ -0,0 +1,71 @@
+package funcs
+
+import (
+	"context"
+	"errors"
+
+	"github.com/nevalang/neva/internal/runtime"
+	"golang.org/x/exp/utf8string"
+)
+
+var errIndexOutOfBounds = errors.New("index out of bounds")
+
+type stringAt struct{}
+
+func (stringAt) Create(io runtime.FuncIO, _ runtime.Msg) (func(context.Context), error) {
+	dataIn, err := io.In.Port("data")
+	if err != nil {
+		return nil, err
+	}
+
+	idxIn, err := io.In.Port("idx")
+	if err != nil {
+		return nil, err
+	}
+
+	resOut, err := io.Out.Port("res")
+	if err != nil {
+		return nil, err
+	}
+
+	errOut, err := io.Out.Port("err")
+	if err != nil {
+		return nil, err
+	}
+
+	return func(ctx context.Context) {
+		for {
+			var data *utf8string.String
+			var idx int64
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-dataIn:
+				data = utf8string.NewString(msg.Str())
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-idxIn:
+				idx = msg.Int()
+			}
+
+			if idx < 0 || idx >= int64(data.RuneCount()) {
+				select {
+				case <-ctx.Done():
+					return
+				case errOut <- errorFromString(errIndexOutOfBounds.Error()):
+					continue
+				}
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case resOut <- runtime.NewStrMsg(string(data.At(int(idx)))):
+			}
+		}
+	}, nil
+}