[Haskell-cafe] n00b question: defining datatype
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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Re: [Haskell-cafe] n00b question: defining datatype
Iain Barnett wrote:
data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] }
This one looks the best to me. Remember, you can just use an empty list
if the task has no subtasks.
- Jake
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Re: [Haskell-cafe] n00b question: defining datatype
On Thu, Jul 23, 2009 at 8:43 PM, Iain Barnett wrote:
> data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] }
> But that's not really right, because obviously, some tasks don't have
> subtasks.
The empty list is a list.
--Max
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Re: [Haskell-cafe] n00b question: defining datatype
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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>
>
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Re: [Haskell-cafe] n00b question: defining datatype
Hello Iain,
Thursday, July 23, 2009, 10:43:02 PM, you wrote:
> data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] }
> But that's not really right, because obviously, some tasks don't have
> subtasks.
don't see a problem - subtasks list may be empty
--
Best regards,
Bulatmailto:bulat.zigans...@gmail.com
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Re: [Haskell-cafe] n00b question: defining datatype
> data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] }
> But that's not really right, because obviously, some tasks don't have
As Jake said - that's fine and you can still pattern match on the null
([]) taskes when looking for tasks without subtaskes.
> 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.
So do you want:
data Job = Job {title :: String, completed :: Bool }
data Task = Task Job | MasterTask String [Job]
Tom
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Re: [Haskell-cafe] n00b question: defining datatype
Actually, how about this? import Data.Tree newtype Task = Task (Tree (String, Bool)) Now you already have that tree structure you wanted. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] n00b question: defining datatype
Ok, thanks to everyone, that's certainly answered my question and given me some more avenues to pursue. I can see now that because I can pattern match against the empty list it's not really a problem to have it there. I didn't realise I could use Maybe in the constructor because it's a monad, but that's good because I was wondering about the best way to make a nullable value. That Data.Tree module looks interesting too! It does seem to be a naturally recursive type, but I'm still trying to become easy with that sort of thought :) Thanks for all the help, it's nice to get a bit of feedback when still getting used to things. Iain ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] n00b question: defining datatype
Kim-Ee Yeoh wrote:
Consider
data Task = Task { title :: String, completed :: Bool, subtasks :: Maybe
[Task] }
Note that unless you have some meaning in mind for the difference
between a subtask value of Nothing vs. (Just []), the Maybe is redundant.
Anton
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Re: [Haskell-cafe] n00b question: defining datatype
On Thu, Jul 23, 2009 at 08:17:34PM +0100, Iain Barnett wrote: > [..] > against the empty list it's not really a problem to have it there. I didn't > realise I could use Maybe in the constructor because it's a monad, but > that's good because I was wondering about the best way to make a nullable > value. Actually, this has nothing to do with Maybe being a monad. The reason you can do this is because "Maybe" itself is not a type, but a (unary) type constructor(It has kind * -> *), so you need to apply it to another type. Doing something like "test :: Maybe" would be an error. > That Data.Tree module looks interesting too! It does seem to be a > naturally recursive type, but I'm still trying to become easy with that sort > of thought :) > [..] A list is also recursively defined, so it is not really more difficult to use a Tree instead. E.g. one could define a list type like this: data List a = Nil | Cons a (List a) - Daniel pgpx25fhhGIwn.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
