Re: [Haskell-cafe] The Good, the Bad and the GUI

2014-08-12 12:46 GMT+02:00 Wojtek Narczyński :

W dniu 2014-08-12 11:30, Chris Warburton pisze:

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Wojtek Narczyński writes:

ut all of this? To regular classes, to builder classes, or to both?

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The current trend in OOP Web frameworks Model-View-Controller.

We've been doing this for 15 years, and believe me, it works only so-so.

To me the latter choice looks similar to a parsing problem. You say that

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Haskell's "great" at parsing, which I think contradicts your statement that "in Haskell it is unusual to deal with data that is incomplete or otherwise erroneous".

Grammars have been tried for GUIs, without any success. I'm not aware of any even moderate size GUI constructed this way.

Well, a formlet is essentially an applicative parser of HTTP parameters + a writer of HTML code. it works pretty well. and it can create the complete logic of a page, not only the interface if we add a monadic instance.

In Haskell we deal with incomplete data all the time using lazy

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evaluation. Erroneous data doesn't require Maybes all over our data: it just needs one big Maybe in the 'parsing' function; or, more likely, "Either [Error]" detailing the problems which were found.

Continuing my VAT Invoice example, let us say a LineItem that does not have a product description (missing value), but it does have all the numeric fields filled in. It is partly erroneous, but it can be included in calculation of the total. How would you handle it with Either [Error] Invoice style code?

-- Cheers, Wojtek Narczyński

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