[Haskell-cafe] The Good, the Bad and the GUI
wojtek at power.com.pl
Thu Aug 14 08:48:34 UTC 2014
On 14.08.2014 01:21, Tom Ellis wrote:
> Perhaps I don't grasp exactly what you're getting at, but this seems easy.
> Please let me know where my proposed solution fails to provide what you
> I do see that you originally said "In Haskell you'd need two data types: the
> usual proper Haskell data type, and another which wraps every field in
> Maybe, facilitates editing, validation, etc.". You don't actually *need*
> the version without the Maybe, but you can provide it if you want some
> additional type safety.
Yes! This is what I mean.
But there is much more to validation, than just missing values. For text
fields you have lengths, regexps. For numeric fields you have ranges,
steps. Then come rules that are inter-fleld: if this field is "Yes",
that field must be filled, if the user chooses "No" a field should be
hidden, if the user changes his mind, the value entered should be
preserved (not validation, but related). Then come rules that are
inter-record: if this field is "ExportInvoice", all VAT fields in the
LineItem must be zero.
The difficulty lies in abundance, ubiquity and complexity of the
Empty fields are just an innocent example. The code you provided solves
it well, but this is just a tip of an iceberg.
And we still haven't even touched the subject of assisting the user to
fix the inconsistencies.
Therefore, quite frankly, I was hoping for radical new ideas how to
tackle this, when I was starting this thread. DSLs, logic, Attribute
Grammars, something like that.
More information about the Haskell-Cafe