Consider data Task = Task { title :: String, completed :: Bool, subtasks :: Maybe [Task] } Iain Barnett wrote:
Hi, I'm trying to get my head around datatypes, and wondering how I might define a simple "Task" datatype in Haskell. data Task = Task { title :: String, completed :: Bool } Ok, that's straightforward, but sometimes tasks become a list of tasks themselves data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] } But that's not really right, because obviously, some tasks don't have subtasks. So I try this: data Task = Task { title :: String, completed :: Bool } | TaskWithSubtasks { title :: String, completed :: Bool, subtasks :: [Task] } It's a bit more accurate, but it's repeating things, which is ok with a simple type. Could anyone suggest a better way to define this? If I was using C#, which I'm far more familiar with, I could overload the constructor and refer to the smaller constructor. Is there a way to do that in Haskell, or am I still thinking too OOP? Iain
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