On Thu, Aug 14, 2014 at 10:48:34AM +0200, Wojtek Narczyński wrote:
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 need.
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. [...]
The difficulty lies in abundance, ubiquity and complexity of the validation rules.
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.
Is there a language that gets this right? If so we can try replicating its functionality in Haskell. Tom