start the dart fuctions chapter (#115)

Author: robsonoduarte

flutter_course/dart_functions/lib/functions.dart

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+main() {
+  // Functions
+  // https://dart.dev/language/functions
+
+  // using the dynamic as parameters :eyes:
+  union(a, b) {
+    print(a + b);
+  }
+
+  union(2.8, 47);
+
+  // Named parameters
+  // https://dart.dev/language/functions#named-parameters
+  String named({String name = 'Robson', int age = 47}) {
+    return '$name has age';
+  }
+
+  print(named()); // using default parameters
+  print(named(age: 48, name: 'Ana Mara')); // using named parameters
+
+  // Optional positional parameters
+  // https://dart.dev/language/functions#optional-positional-parameters
+  String say(String from, String msg, [String? device]) {
+    var result = '$from says $msg';
+    if (device != null) {
+      result = '$result with a $device';
+    }
+    return result;
+  }
+
+  print(say("Robson", "Hello Dart and Flutter"));
+  print(say("Robson", "Hello Dart and Flutter", 'Android'));
+
+  // Functions as first-class objects
+  // https://dart.dev/language/functions#functions-as-first-class-objects
+
+  // function as var
+  final sum = (int a, int b) => a + b;
+  print(sum);
+  print(sum(10, 30));
+
+  // function as param
+  var result = [1, 2, 3].fold(1, sum);
+  print(result);
+}

flutter_course/dart_functions/pubspec.lock

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+# Generated by pub
+# See https://dart.dev/tools/pub/glossary#lockfile
+packages: {}
+sdks:
+  dart: ">=3.1.3 <4.0.0"