[Haskell-beginners] How to "cast" FromJSON instance value

[Jan von Löwenstein]

You'd still have to handle the case that the Value is not what you
expected. Otherwise you'll have a partial function (i.e. error in case of
certain inputs).

So maybe you'd best use aeson directly (which spares you the GetValue
instances as well).

getValue :: (FromJSON a) => Text -> Map Text a -> Maybe a

Best
Jan
P.S. There is no such thing as a stupid question.

Baa <aquagnu at gmail.com> schrieb am Di., 1. Aug. 2017, 13:02:

> Hello, Jan! I "remove" the question :)
> Problem was: reading of HTML form values, which can be any type and
> saving them in a map. I done it w/ `Map Text Value` (instead of previous
> `Map Text Text`). To get values from the map, I implemented class and
> several instances which returns values of different types, something
> like:
>
>   class GetValue a where
>     getValue :: Text -> Map Text Value -> a
>
> (promoting of `a` anywhere in the existing code needs big refactoring,
> so map's value is `::Value` but not `::a`). Cons is a need to implement
> N instances of `GetValue`: one per getting type.
>
> Anywhere, Jan, thanks for quick answer, I'm sorry for this stupid
> question.
>
>
> > Hi,
> >
> > You don't know anything about your a's but that they have FromJSON
> > instances. That in turn means you can get values from aeson parsing
> > functions. Think parseJSON :: Value -> Parser a
> > Could you be more specific about the problem you try to solve?
> >
> > Best
> > Jan
> >
> > Baa <aquagnu at gmail.com> schrieb am Di., 1. Aug. 2017, 10:32:
> >
> > > Hello, List!
> > >
> > > I have, for example, function with signature like
> > >
> > >   fn :: FromJSON a => Map Text a -> BlahBlah
> > >
> > > and I want to treat this `a` as String (or `Text`), to read it with
> > > `readMaybe`. Am I right that it's impossible? I get error about
> > > "...a is rigid type and could not be matched with Text...". But
> > > `Text` is `FromJSON` instance. Is it possible to do it (cast,
> > > convert, unwrap, etc)?
> > >
> > > The problem's source is that early I worked with `Text` values only
> > > (getting from HTML form) but now they become any type: int's,
> > > bool's, etc).
> > >
> > > ===
> > > Best regards,
> > >   Paul
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners at haskell.org
> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > >
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20170801/f90c4aa3/attachment.html>