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

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 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.

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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:

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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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