json - Override how Data.Aeson handles only one field of my record -

json - Override how Data.Aeson handles only one field of my record -

i making rest api university courses:

data course of study = course of study { id :: maybe text, name :: text, deleted :: bool } deriving(show, generic) instance fromjson course of study instance tojson course of study

i allow deleted optional in serialized json structure, not in application. want set deleted false if isn't specified when parsing.

i write manual instance fromjson, don't want have write out fields. want declare how deleted handled , allow automatic instance handle else.

how this?

to knowledge there not way customize generic instance, construction type bit differently:

data course of study = course of study { courseid :: maybe text -- don't utilize `id`, it's function , name :: text } deriving (show, generic) info deletable = deletable { deleted :: bool , item :: } deriving (show) instance fromjson course of study instance tojson course of study instance fromjson => fromjson (deletable a) parsejson (object v) = <- parsejson (object v) d <- v .:? "deleted" .!= false homecoming $ deletable d parsejson _ = mzero

now can do

> allow nodeleted = "{\"name\":\"math\",\"courseid\":\"12345\"}" :: text > allow withdeleted = "{\"name\":\"math\",\"courseid\":\"12345\",\"deleted\":true}" :: text > decode nodeleted :: maybe (deletable course) (deletable {deleted = false, item = course of study {courseid = "12345", name = "math"}}) > decode nodeleted :: maybe course of study (course {courseid = "12345", name = "math"}) > decode withdeleted :: maybe (deletable course) (deletable {deleted = true, item = course of study {courseid = "12345", name = "math"}}) > decode withdeleted :: maybe course of study (course {courseid = "12345", name = "math"})

and can optionally tag course of study deletable when need it, , fromjson instances take care of everything.

json haskell aeson