Merge pull request #679 from nevalang/lists-at

Add `{lists,strings}.At` flow ops to access elements of a List and runes in a string

examples/list_index/main.neva

@@ -1,10 +1,12 @@
+import { lists }
+
 const lst list<int> = [1, 1, 5, 112, 69, 420]
 
 flow Main(start) (stop) {
-	nodes { Index<list<int>>, Println }
+	nodes { lists.At<int>, Println }
 	:start -> [
-		($lst -> index:data),
-		(4 -> index:idx)
+		($lst -> at:data),
+		(4 -> at:idx)
 	]
-	[index:res, index:err] -> println -> :stop
-}
+	[at:res, at:err] -> println -> :stop
+}

internal/runtime/funcs/list_at.go

@@ -0,0 +1,77 @@
+package funcs
+
+import (
+	"context"
+	"errors"
+
+	"github.com/nevalang/neva/internal/runtime"
+)
+
+var errListIndexOutOfBounds = errors.New("list index out of bounds")
+
+type listAt struct{}
+
+func (listAt) Create(io runtime.FuncIO, _ runtime.Msg) (func(ctx 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 []runtime.Msg
+			var idx int64
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-dataIn:
+				data = msg.List()
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-idxIn:
+				idx = msg.Int()
+			}
+
+			if idx < 0 {
+				// Support negative indexing:
+				//	$l = [1, 2, 3]
+				//	$l[-1] // 3
+				idx += int64(len(data))
+			}
+
+			if idx < 0 || idx >= int64(len(data)) {
+				select {
+				case <-ctx.Done():
+					return
+				case errOut <- errorFromString(errListIndexOutOfBounds.Error()):
+					continue
+				}
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case resOut <- data[idx]:
+			}
+		}
+	}, nil
+}

internal/runtime/funcs/registry.go

@@ -63,7 +63,7 @@ func CreatorRegistry() map[string]runtime.FuncCreator {
 		"regexp_submatch": regexpSubmatch{},
 
 		// list
-		"index":      index{},
+		"list_at":    listAt{},
 		"list_len":   listlen{},
 		"list_push":  listPush{},
 		"int_sort":   listSortInt{},
@@ -73,6 +73,7 @@ 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 +1,88 @@
+package funcs
+
+import (
+	"context"
+	"errors"
+
+	"github.com/nevalang/neva/internal/runtime"
+)
+
+var errStrIndexOutOfBounds = errors.New("string 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 string
+			var idx int64
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-dataIn:
+				data = msg.Str()
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-idxIn:
+				idx = msg.Int()
+			}
+
+			if idx < 0 {
+				// Support negaitve indexing:
+				//	$s = "abc"
+				//	$s[-1] // "c"
+				idx += int64(len(data))
+			}
+
+			if idx >= 0 && idx < int64(len(data)) {
+				var res rune
+				var found bool
+				for i, r := range data {
+					if int64(i) == idx {
+						res = r
+						found = true
+						break
+					}
+				}
+				if found {
+					select {
+					case <-ctx.Done():
+						return
+					case resOut <- runtime.NewStrMsg(string(res)):
+						continue
+					}
+				}
+			}
+
+			select {
+			case <-ctx.Done():
+				return
+			case errOut <- errorFromString(errStrIndexOutOfBounds.Error()):
+			}
+		}
+	}, nil
+}

std/builtin/collections.neva

@@ -9,12 +9,6 @@ pub flow Len<T list<any> | map<any> | string>(data T) (res int)
 #extern(stream_to_list)
 pub flow List<T>(seq stream<T>) (res list<T>)
 
-// Index returns the element at the given index in the ordered collection.
-// If the index is out of bounds, it returns an error.
-// The index is zero-based.
-#extern(index)
-pub flow Index<T list<any> | string>(data T, idx int) (res T, err error)
-
 // Push creates new list with appended element.
 #extern(list_push)
 pub flow Push<T> (lst list<T>, data T) (res list<T>)

std/lists/lists.neva

@@ -1,3 +1,6 @@
+#extern(list_at)
+pub flow At<T>(data list<T>, idx int) (res T, err error)
+
 pub flow For<T>(data list<T>) (sig any) {
     nodes {
         Iter<T>