Thank you, David. 1st way seems to be most close to my task. But from your answers I see that actually no way to "run" Aeson Parser monad except with ByteString argument (even not a direct "run"), right? I check Aeson sources and IMHO no exporting functions to do it... Anyway, thank you again!

There are three ways to go about this.

Method one, we could turn all the Values into Maybe MyTypes. Method two, we could filter out any Value which cannot be turned into an MyType. Method three, we could fail to parse entirely if any Value cannot be turned into a MyType succesfully.

Here is some code that demonstrates all three approaches.

import Data.Aeson import Data.Aeson.Types (Parser) import Control.Monad import qualified Data.Text as T import Data.Maybe (catMaybes)

data Resp = Resp { rHeadings :: [T.Text], rows1 :: [Maybe MyType], rows2 :: [MyType], rows3 :: [MyType] }

data MyType = MyType

instance FromJSON Resp where parseJSON = withObject "Resp" $ \v -> do hds <- v .: "headings"

-- we could catch all of mytypes as maybes r1 <- fmap f <$> v .: "rows" :: Parser [Maybe MyType]

-- we could throw away values which can't be turned into MyType r2 <- catMaybes . fmap f <$> v .: "rows" :: Parser [MyType]

-- we could fail the parse entirely if any value fails. r3 <- v .: "rows" >>= traverse f' pure $ Resp hds r1 r2 r3

f :: Value -> Maybe MyType f v = undefined

f' :: Value -> Parser MyType f' (String t) = pure MyType f' (Bool t) = pure MyType f' (Number t) = pure MyType f' (Null) = pure MyType -- Anything that is not one of the above will cause a failure of the entire parse. f' _ = mzero

On Tue, Aug 29, 2017 at 11:58 AM, Baa wrote:

...

Hello List again! :)

If I have a `FromJSON` instance, with `parseJSON` which translates `Value` to MyType, how can I run this parser like I "run" state/writer/reader monads? I mean not decoding of a bytestring representation:

decode::ByteString -> Maybe MyType

but

run::Value -> Maybe MyType

The reason is that I have already collected Value's , and I have parsers, so what is the problem, it would seem, to run parser on the Value - to convert `Value` into `Maybe MyType`... Is it possible?

=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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