Aeson: derive some (but not all) fields of a struct
I have a large struct which I need to be an instance of FromJSON so that I can parse my json data into it.
I would like to derive automatically, but a single field needs "special care" in that it is an object in json and I want it to be an array of the values in my struct. How can I do this without writing a huge FromJson implementation repeating all the fields?
Example json:
{"myobject": {"one": 1, "two": 2}, ...many_more_fields...}
Example struct:
data MyStruct = MyStruct {
myobject :: [Int],
...many_more_fields,...
} deriving (Generic)
How do I do this elegantly?
json haskell aeson
asked Nov 23 '18 at 15:01
Marius Melzer
466
add a comment |
I have a large struct which I need to be an instance of FromJSON so that I can parse my json data into it.
I would like to derive automatically, but a single field needs "special care" in that it is an object in json and I want it to be an array of the values in my struct. How can I do this without writing a huge FromJson implementation repeating all the fields?
Example json:
{"myobject": {"one": 1, "two": 2}, ...many_more_fields...}
Example struct:
data MyStruct = MyStruct {
myobject :: [Int],
...many_more_fields,...
} deriving (Generic)
How do I do this elegantly?
json haskell aeson
asked Nov 23 '18 at 15:01
Marius Melzer
466
related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42
add a comment |
I have a large struct which I need to be an instance of FromJSON so that I can parse my json data into it.
I would like to derive automatically, but a single field needs "special care" in that it is an object in json and I want it to be an array of the values in my struct. How can I do this without writing a huge FromJson implementation repeating all the fields?
Example json:
{"myobject": {"one": 1, "two": 2}, ...many_more_fields...}
Example struct:
data MyStruct = MyStruct {
myobject :: [Int],
...many_more_fields,...
} deriving (Generic)
How do I do this elegantly?
json haskell aeson
asked Nov 23 '18 at 15:01
Marius Melzer
466
I have a large struct which I need to be an instance of FromJSON so that I can parse my json data into it.
I would like to derive automatically, but a single field needs "special care" in that it is an object in json and I want it to be an array of the values in my struct. How can I do this without writing a huge FromJson implementation repeating all the fields?
Example json:
{"myobject": {"one": 1, "two": 2}, ...many_more_fields...}
Example struct:
data MyStruct = MyStruct {
myobject :: [Int],
...many_more_fields,...
} deriving (Generic)
How do I do this elegantly?
json haskell aeson
json haskell aeson
asked Nov 23 '18 at 15:01
Marius Melzer
466
asked Nov 23 '18 at 15:01
Marius Melzer
466
asked Nov 23 '18 at 15:01
Marius Melzer
466
asked Nov 23 '18 at 15:01
Marius Melzer
466
asked Nov 23 '18 at 15:01
Marius Melzer
466
466
related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42
add a comment |
related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42
related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42
related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42
add a comment |
2 Answers
2
active
oldest
votes
You should create a newtype for your special field:
newtype MySpecialType = MySpecialType [Int]
instance FromJSON MySpecialType where ....
data MyStruct = MyStruct {
myobject:: MySpecialType,
...
}
Now the instance for MyStruct becomes entirely regular and can be handed off to Template Haskell in the normal way.
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
add a comment |
To avoid carrying the newtype from Paul Johnson's very good answer all across the codebase, you can also generalize your type as follows, making the type of myobject a parameter:
data MyStruct_ intList = MyStruct {
myobject :: intlist,
...
} deriving (Functor, Generic)
type MyStruct = MyStruct [Int]
instance FromJSON MyStruct where
parseJSON = (fmap . fmap) ((MySpecialType i) -> i)
. genericParseJSON defaultOptions
genericParseJSON above gets instantiated with MyStruct MySpecialType, and then the field gets unwrapped via fmap (noting MyStruct_ is a Functor)
I also just wrote a blogpost about "type surgery", applied to this kind of problem so that you can keep the original type unmodified.
The generic-data-surgery library can derive a generic type with the same Generic structure as MyStruct_ MySpecialType above, to be used by aeson's genericParseJSON. The surgery modifyRField then applies the function (MySpecialType i) -> i to the myobject field, finally yielding MyStruct.
import Generic.Data.Surgery (fromOR, toOR', modifyRField)
-- The original type
data MyStruct = MyStruct {
myobject :: [Int],
...
} deriving (Generic)
instance FromJSON MyStruct where
parseJSON = fmap (fromOR . modifyRField @"myobject" ((MySpecialType i) -> i) . toOR')
. genericParseJSON defaultOptions
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
You should create a newtype for your special field:
newtype MySpecialType = MySpecialType [Int]
instance FromJSON MySpecialType where ....
data MyStruct = MyStruct {
myobject:: MySpecialType,
...
}
Now the instance for MyStruct becomes entirely regular and can be handed off to Template Haskell in the normal way.
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
add a comment |
You should create a newtype for your special field:
newtype MySpecialType = MySpecialType [Int]
instance FromJSON MySpecialType where ....
data MyStruct = MyStruct {
myobject:: MySpecialType,
...
}
Now the instance for MyStruct becomes entirely regular and can be handed off to Template Haskell in the normal way.
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
add a comment |
You should create a newtype for your special field:
newtype MySpecialType = MySpecialType [Int]
instance FromJSON MySpecialType where ....
data MyStruct = MyStruct {
myobject:: MySpecialType,
...
}
Now the instance for MyStruct becomes entirely regular and can be handed off to Template Haskell in the normal way.
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
You should create a newtype for your special field:
newtype MySpecialType = MySpecialType [Int]
instance FromJSON MySpecialType where ....
data MyStruct = MyStruct {
myobject:: MySpecialType,
...
}
Now the instance for MyStruct becomes entirely regular and can be handed off to Template Haskell in the normal way.
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
answered Nov 23 '18 at 15:13
Paul Johnson
13.5k23449
13.5k23449
add a comment |
add a comment |
To avoid carrying the newtype from Paul Johnson's very good answer all across the codebase, you can also generalize your type as follows, making the type of myobject a parameter:
data MyStruct_ intList = MyStruct {
myobject :: intlist,
...
} deriving (Functor, Generic)
type MyStruct = MyStruct [Int]
instance FromJSON MyStruct where
parseJSON = (fmap . fmap) ((MySpecialType i) -> i)
. genericParseJSON defaultOptions
genericParseJSON above gets instantiated with MyStruct MySpecialType, and then the field gets unwrapped via fmap (noting MyStruct_ is a Functor)
I also just wrote a blogpost about "type surgery", applied to this kind of problem so that you can keep the original type unmodified.
The generic-data-surgery library can derive a generic type with the same Generic structure as MyStruct_ MySpecialType above, to be used by aeson's genericParseJSON. The surgery modifyRField then applies the function (MySpecialType i) -> i to the myobject field, finally yielding MyStruct.
import Generic.Data.Surgery (fromOR, toOR', modifyRField)
-- The original type
data MyStruct = MyStruct {
myobject :: [Int],
...
} deriving (Generic)
instance FromJSON MyStruct where
parseJSON = fmap (fromOR . modifyRField @"myobject" ((MySpecialType i) -> i) . toOR')
. genericParseJSON defaultOptions
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
add a comment |
To avoid carrying the newtype from Paul Johnson's very good answer all across the codebase, you can also generalize your type as follows, making the type of myobject a parameter:
data MyStruct_ intList = MyStruct {
myobject :: intlist,
...
} deriving (Functor, Generic)
type MyStruct = MyStruct [Int]
instance FromJSON MyStruct where
parseJSON = (fmap . fmap) ((MySpecialType i) -> i)
. genericParseJSON defaultOptions
genericParseJSON above gets instantiated with MyStruct MySpecialType, and then the field gets unwrapped via fmap (noting MyStruct_ is a Functor)
I also just wrote a blogpost about "type surgery", applied to this kind of problem so that you can keep the original type unmodified.
The generic-data-surgery library can derive a generic type with the same Generic structure as MyStruct_ MySpecialType above, to be used by aeson's genericParseJSON. The surgery modifyRField then applies the function (MySpecialType i) -> i to the myobject field, finally yielding MyStruct.
import Generic.Data.Surgery (fromOR, toOR', modifyRField)
-- The original type
data MyStruct = MyStruct {
myobject :: [Int],
...
} deriving (Generic)
instance FromJSON MyStruct where
parseJSON = fmap (fromOR . modifyRField @"myobject" ((MySpecialType i) -> i) . toOR')
. genericParseJSON defaultOptions
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
add a comment |
To avoid carrying the newtype from Paul Johnson's very good answer all across the codebase, you can also generalize your type as follows, making the type of myobject a parameter:
data MyStruct_ intList = MyStruct {
myobject :: intlist,
...
} deriving (Functor, Generic)
type MyStruct = MyStruct [Int]
instance FromJSON MyStruct where
parseJSON = (fmap . fmap) ((MySpecialType i) -> i)
. genericParseJSON defaultOptions
genericParseJSON above gets instantiated with MyStruct MySpecialType, and then the field gets unwrapped via fmap (noting MyStruct_ is a Functor)
I also just wrote a blogpost about "type surgery", applied to this kind of problem so that you can keep the original type unmodified.
The generic-data-surgery library can derive a generic type with the same Generic structure as MyStruct_ MySpecialType above, to be used by aeson's genericParseJSON. The surgery modifyRField then applies the function (MySpecialType i) -> i to the myobject field, finally yielding MyStruct.
import Generic.Data.Surgery (fromOR, toOR', modifyRField)
-- The original type
data MyStruct = MyStruct {
myobject :: [Int],
...
} deriving (Generic)
instance FromJSON MyStruct where
parseJSON = fmap (fromOR . modifyRField @"myobject" ((MySpecialType i) -> i) . toOR')
. genericParseJSON defaultOptions
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
To avoid carrying the newtype from Paul Johnson's very good answer all across the codebase, you can also generalize your type as follows, making the type of myobject a parameter:
data MyStruct_ intList = MyStruct {
myobject :: intlist,
...
} deriving (Functor, Generic)
type MyStruct = MyStruct [Int]
instance FromJSON MyStruct where
parseJSON = (fmap . fmap) ((MySpecialType i) -> i)
. genericParseJSON defaultOptions
genericParseJSON above gets instantiated with MyStruct MySpecialType, and then the field gets unwrapped via fmap (noting MyStruct_ is a Functor)
I also just wrote a blogpost about "type surgery", applied to this kind of problem so that you can keep the original type unmodified.
The generic-data-surgery library can derive a generic type with the same Generic structure as MyStruct_ MySpecialType above, to be used by aeson's genericParseJSON. The surgery modifyRField then applies the function (MySpecialType i) -> i to the myobject field, finally yielding MyStruct.
import Generic.Data.Surgery (fromOR, toOR', modifyRField)
-- The original type
data MyStruct = MyStruct {
myobject :: [Int],
...
} deriving (Generic)
instance FromJSON MyStruct where
parseJSON = fmap (fromOR . modifyRField @"myobject" ((MySpecialType i) -> i) . toOR')
. genericParseJSON defaultOptions
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
answered Nov 26 '18 at 14:44
Li-yao Xia
11.5k1327
11.5k1327
add a comment |
add a comment |
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related: stackoverflow.com/questions/53352563/…
– assembly.jc
Nov 23 '18 at 15:42