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

[Jan von Löwenstein]

Hi,

getValue :: (FromJSON a) => Text -> Map Text a -> Maybe a
getValue key map = (fromResult.fromJSON) $ lookup key map

fromResult (Success a) = Just a
fromResult (Error _) = Nothing

I am not at a computer so I wouldn't be surprised if this doesn't compile.
But you should see the picture.

Best
Jan

Baa <aquagnu at gmail.com> schrieb am Di., 1. Aug. 2017, 13:48:

> Ohh, pardon! Yes, signature was
>
>   class GetValue a where
>     getValue :: Text -> Map Text Value -> Maybe a
>
> with `Maybe` sure. But I didn't understand you: what do you mean "to
> use aeson directly"? Problem is to save `FromJSON a` instances in map
> but get from the map `Text`, `Bool` and etc values. Sure, it possible
> to parameterize all related functions/types with `FromJSON a => .. a`
> but in this case I hit 2 problems:
>
> 1. rigid type error when I have `FromJSON a =>` in some signature and
>    try to use this `a` as `Text` value, for example
> 2. big refactoring (many signatures, types)
>
> So, I decided to save in the map `Value`s instead of `FromJSON a`s.
> De-facto, this map is heterogenous and keeps values of different types.
> And question for me was: to hide this fact under `FromJSON` or under
> `Value`. I'm not sure what is the best solution and I don't know how to
> solve 1.
>
>
> > 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
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> > > > > Beginners at haskell.org
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