instance_eval.rb

class SimpleBuilder
  def set(key, value)
    @stuff[key] = value
  end

  def self.build(&block)
    new.tap { |builder| builder.instance_eval(&block) }
  end
end

SimpleBuilder.build do
  set :foo, "bar"
end

no_instance_eval.m

@implementation SimpleBuilder

- (void)set:(NSString *)key value:(NSString *)value
{
  [stuff setObject:value forKey:key];
}

+ (id)build:(void (^)(SimpleBuilder *builder))block
{
  SimpleBuilder *builder = [self new];
  block(builder);
  return builder;
}

@end

[SimpleBuilder build:^(SimpleBuilder *builder) {
  [builder set:@"foo" value:@"bar"];
}];

// would like to do

[SimpleBuilder build:^{
  [self set:@"foo" value:@"bar"];
}];