Re: [Haskell-beginners] Phantom types and export lists

Karl Voelker wrote:

On Sat, Oct 20, 2012 at 12:41 AM, Emmanuel Surleau wrote:

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Is this the recommended way of doing something like this?

It looks to me like using a "smart constructor" would be sufficient in this case. That is, you have a function:

makeTask :: String -> Either String Task

This function performs your validation. Then you make sure not to export the data constructor for Task - that way, the only way to make a Task outside of this module is to call the validating constructor function.

l see. However, with phantom types, you get more flexibility. For instance, you can generate a blank Task as a template. I think I want to retain the ability to work with non-well-formed data. But I'll keep smart constructors in mind.

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When I'm exporting "Task", I seem to be exporting the data type, but is there a way to export the Task constructor itself (not that I would want that)?

You can say Foo(..) to export all the data constructors of type Foo. Or, you can list out the data constructors you want to export inside the parens. If you want to be explicit about the fact that you are not exporting any data constructors, you should probably write Foo().

Note that the same syntax applies to classes and their methods.

Thanks. This means I can export Sanitise (..) instead of Sanitise and sanitise.

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Of course, it's quite possible that I'm reinventing the wheel and that I've missed a perfectly useable data validation module on hackage.

I don't know of any. But I'm also not convinced that such a thing is necessary - smart constructors are usually all you need. Perhaps in a context with more assumptions (i.e., the web), there would be some useful work that a validation library could do.

I guess you are right. Not many use cases for a completely generic validation library. Thanks, Emm -- Sent from my Android phone with K-9 Mail. Please excuse my brevity.