feat: Add utilities for working with functions

- pipe
- compose
- pack
- apply

Author: nielsenko

lib/relic.dart

@@ -23,3 +23,4 @@ export 'src/middleware/middleware_logger.dart' show logRequests;
 export 'src/relic_server.dart';
 export 'src/router/lookup_result.dart';
 export 'src/router/router.dart';
+export 'src/util/functional.dart';

lib/src/util/functional.dart

@@ -0,0 +1,45 @@
+extension Pipe<T> on T {
+  R pipe<R>(final R Function(T) next) => next(this);
+}
+
+extension Compose<R, T> on R Function(T) {
+  R Function(U) compose<U>(final T Function(U) inner) =>
+      (final u) => this(inner(u));
+}
+
+// === Apply section ===
+extension Apply1<T, R> on R Function(T) {
+  R apply(final T x) => this(x);
+}
+
+extension Apply2<R, T, U> on R Function(T, U) {
+  R Function(U) apply(final T t) => (final U u) => this(t, u);
+}
+
+extension Apply3<R, T, U, V> on R Function(T, U, V) {
+  R Function(U, V) apply(final T t) => (final U u, final V v) => this(t, u, v);
+}
+
+extension Apply4<R, T, U, V, X> on R Function(T, U, V, X) {
+  R Function(U, V, X) apply(final T t) =>
+      (final U u, final V v, final X x) => this(t, u, v, x);
+}
+
+// === Pack section ===
+extension Pack1<R, T> on R Function(T) {
+  R Function((T,)) get pack => (final (T,) x) => this(x.$1);
+}
+
+extension Pack2<R, T, U> on R Function(T, U) {
+  R Function((T, U)) get pack => (final (T, U) x) => this(x.$1, x.$2);
+}
+
+extension Pack3<R, T, U, V> on R Function(T, U, V) {
+  R Function((T, U, V)) get pack =>
+      (final (T, U, V) x) => this(x.$1, x.$2, x.$3);
+}
+
+extension Pack4<R, T, U, V, X> on R Function(T, U, V, X) {
+  R Function((T, U, V, X)) get pack =>
+      (final (T, U, V, X) x) => this(x.$1, x.$2, x.$3, x.$4);
+}