Combinator with predicate

[vrom911]

Sometimes you need something to be valid only if satisfy the predicate. It would be nice to have such a function in tomland.

Recently I've done the one for Text using textBy function:

https://github.com/kowainik/summoner/blob/2ce94372c0bccaf4adf377814324f6d4fd751366/summoner-cli/src/Summoner/CustomPrelude.hs#L23-L32

However, I don't really see how this could be generalised. Do you have any idea? And do you think that this is generally useful for the library?

[chshersh]

@vrom911 That's an excellent idea! I think we should try to implement generalised combinator since it will be the most useful and composable. First idea is something like:

validate :: (a -> Bool) -> Key -> TomlCodec a

or we can use Either Text ... instead of Bool to give an ability to provide better custom errors. But the most general form probably should be somehow be a more polymorphic version of the _TextBy combinator, but it's not clear immediately how to do this, need to think more...