haskell - Parse Array in nested JSON with Aeson -


i'm trying write fromjson function aeson.

the json:

{   "total": 1,   "movies": [     {       "id": "771315522",       "title": "harry potter , philosophers stone (wizard's collection)",       "posters": {         "thumbnail": "http://content7.flixster.com/movie/11/16/66/11166609_mob.jpg",         "profile": "http://content7.flixster.com/movie/11/16/66/11166609_pro.jpg",         "detailed": "http://content7.flixster.com/movie/11/16/66/11166609_det.jpg",         "original": "http://content7.flixster.com/movie/11/16/66/11166609_ori.jpg"       }     }   ] }

the adt: data movie = movie {id::string, title::string}

my attempt:

instance fromjson movie     parsejson (object o) =        movies <- parsejson =<< (o .: "movies") :: parser array        v <- head $ decode movies        return $ movie <$>            (v .: "movies" >>= (.: "id") ) <*>            (v .: "movies" >>= (.: "title") )     parsejson _ = mzero

this gives couldn't match expected type 'parser t0' actual type 'maybe a0' in first argument of 'head'.

as can see, i'm trying pick first of movies in array, wouldn't mind getting list of movies either (in case there several in array).

if want parse single movie json array of movies, can this:

instance fromjson movie     parsejson (object o) =         movievalue <- head <$> o .: "movies"         movie <$> movievalue .: "id" <*> movievalue .: "title"     parsejson _ = mzero

but safer route parse [movie] via newtype wrapper:

main = print $ movielist <$> decode "{\"total\":1,\"movies\":[ {\"id\":\"771315522\",\"title\":\"harry potter , philosophers stone (wizard's collection)\",\"posters\":{\"thumbnail\":\"http://content7.flixster.com/movie/11/16/66/11166609_mob.jpg\",\"profile\":\"http://content7.flixster.com/movie/11/16/66/11166609_pro.jpg\",\"detailed\":\"http://content7.flixster.com/movie/11/16/66/11166609_det.jpg\",\"original\":\"http://content7.flixster.com/movie/11/16/66/11166609_ori.jpg\"}}]}"  newtype movielist = movielist {movielist :: [movie]}  instance fromjson movielist     parsejson (object o) = movielist <$> o .: "movies"     parsejson _ = mzero  data movie = movie {id :: string, title :: string}  instance fromjson movie     parsejson (object o) = movie <$> o .: "id" <*> o .: "title"     parsejson _ = mzero

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