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

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 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.

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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 schrieb am Di., 1. Aug. 2017, 10:32:

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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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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