fn: Added map functions

Signed-off-by: Ononiwu Maureen <59079323+Chinwendu20@users.noreply.github.com>

fn/map.go

@@ -0,0 +1,44 @@
+package fn
+
+import (
+	"fmt"
+
+	"golang.org/x/exp/maps"
+)
+
+// KeySet converts a map into a Set containing the keys of the map.
+func KeySet[K comparable, V any](m map[K]V) Set[K] {
+	return NewSet(maps.Keys(m)...)
+}
+
+// NewSubMapIntersect returns a sub-map of `m` containing only the keys found in
+// both `m` and the `keys` slice.
+func NewSubMapIntersect[K comparable, V any](m map[K]V, keys []K) map[K]V {
+	result := make(map[K]V)
+	for _, k := range keys {
+		v, ok := m[k]
+		if !ok {
+			continue
+		}
+
+		result[k] = v
+	}
+
+	return result
+}
+
+// NewSubMap creates a sub-map from a given map using specified keys. It errors
+// if any of the keys is not found in the map.
+func NewSubMap[K comparable, V any](m map[K]V, keys []K) (map[K]V, error) {
+	result := make(map[K]V, len(keys))
+	for _, k := range keys {
+		v, ok := m[k]
+		if !ok {
+			return nil, fmt.Errorf("NewSubMap: missing key %v", k)
+		}
+
+		result[k] = v
+	}
+
+	return result, nil
+}