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

[Baa]

Jan, this is the exactly what I'm looked for! Thank you again! :)

  getValue :: FromJSON a => T.Text -> M.Map T.Text a -> Maybe a
  getValue k m = do
    j <- fromJSON <$> M.lookup k m
    case j of Success a -> return a
              _         -> Nothing


> 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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