Task models are the cornerstone of user-centred design methodologies for user interface design. Therefore, they deserve attention in order to produce them effectively and efficiently, while guaranteeing the reproducibility of a task model: different persons should in principle obtain the same task model, or a similar one, for the same problem. In order to provide user interface designers with some guidance for task modelling, a list of canonical task types is proposed that offers a unified definition of frequently used tasks types in a consistent way. Each task type consists of a task action coupled with a task object, each of them being written according to design guidelines. This list provides the following benefits: tasks are modelled in a more consistent way, their definition is more communicable and shared, task models can be efficiently used for model-driven engineering of user interfaces.