# nim-serde
Easy-to-use json serialization capabilities, and a drop-in replacement for `std/json`.
## Quick examples
Opt-in serialization by default:
```nim
import pkg/serde/json
type MyType = object
field1 {.serialize.}: bool
field2: bool
assert MyType(field1: true, field2: true).toJson == """{"field1":true}"""
```
Opt-out deserialization by default:
```nim
import pkg/serde/json
# All fields deserialized, as none are ignored
type MyType1 = object
field1: bool
field2: bool
let jsn1 = """{
"field1": true,
"field2": true
}"""
assert !MyType1.fromJson(jsn1) == MyType1(field1: true, field2: true)
# Don't deserialize ignored fields in OptOut mode
type MyType2 = object
field1 {.deserialize(ignore=true).}: bool
field2: bool
let jsn2 = """{
"field1": true,
"field2": true,
"extra": "extra fields don't error in OptOut mode"
}"""
assert !MyType2.fromJson(jsn2) == MyType2(field1: false, field2: true)
# Note, the ! operator is part of https://github.com/codex-storage/questionable, which retrieves a value if set
```
Serialize all fields of a type (OptOut mode):
```nim
import pkg/serde/json
type MyType {.serialize.} = object
field1: int
field2: int
assert MyType(field1: 1, field2: 2).toJson == """{"field1":1,"field2":2}"""
```
Alias field names in both directions!
```nim
import pkg/serde/json
type MyType {.serialize.} = object
field1 {.serialize("othername"),deserialize("takesprecedence").}: int
field2: int
assert MyType(field1: 1, field2: 2).toJson == """{"othername":1,"field2":2}"""
let jsn = """{
"othername": 1,
"field2": 2,
"takesprecedence": 3
}"""
assert !MyType.fromJson(jsn) == MyType(field1: 3, field2: 2)
```
Supports strict mode, where type fields and json fields must match
```nim
import pkg/serde/json
type MyType {.deserialize(mode=Strict).} = object
field1: int
field2: int
let jsn = """{
"field1": 1,
"field2": 2,
"extra": 3
}"""
let res = MyType.fromJson(jsn)
assert res.isFailure
assert res.error of SerdeError
assert res.error.msg == "json field(s) missing in object: {\"extra\"}"
```
## Serde modes
`nim-serde` uses three different modes to control de/serialization:
```nim
OptIn
OptOut
Strict
```
Modes can be set in the `{.serialize.}` and/or `{.deserialize.}` pragmas on type
definitions. Each mode has a different meaning depending on if the type is being
serialized or deserialized. Modes can be set by setting `mode` in the `serialize` or
`deserialize` pragma annotation, eg:
```nim
type MyType {.serialize(mode=Strict).} = object
field1: bool
field2: bool
```
### Modes reference
| | serialize | deserialize |
|:-------------------|:------------------------------------------------------------------------------------------------------------------------------------|:------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `SerdeMode.OptOut` | All object fields will be serialized, except fields marked with `{.serialize(ignore=true).}`. | All json keys will be deserialized, except fields marked with `{.deserialize(ignore=true).}`. No error if extra json fields exist. |
| `SerdeMode.OptIn` | Only fields marked with `{.serialize.}` will be serialized. Fields marked with `{.serialize(ignore=true).}` will not be serialized. | Only fields marked with `{.deserialize.}` will be deserialized. Fields marked with `{.deserialize(ignore=true).}` will not be deserialized. A `SerdeError` error is raised if the field is missing in json. |
| `SerdeMode.Strict` | All object fields will be serialized, regardless if the field is marked with `{.serialize(ignore=true).}`. | Object fields and json fields must match exactly, otherwise a `SerdeError` is raised. |
## Default modes
`nim-serde` will de/serialize types if they are not annotated with `serialize` or
`deserialize`, but will assume a default mode. By default, with no pragmas specified,
`serde` will always serialize in `OptIn` mode, meaning any fields to b Additionally, if
the types are annotated, but a mode is not specified, `serde` will assume a (possibly
different) default mode.
```nim
# Type is not annotated
# A default mode of OptIn (for serialize) and OptOut (for deserialize) is assumed.
type MyObj = object
field1: bool
field2: bool
# Type is annotated, but mode not specified
# A default mode of OptOut is assumed for both serialize and deserialize.
type MyObj {.serialize, deserialize.} = object
field1: bool
field2: bool
```
### Default mode reference
| | serialize | deserialize |
|:------------------------------|:----------|:------------|
| Default (no pragma) | `OptIn` | `OptOut` |
| Default (pragma, but no mode) | `OptOut` | `OptOut` |
## Serde field options
Type fields can be annotated with `{.serialize.}` and `{.deserialize.}` and properties
can be set on these pragmas, determining de/serialization behavior.
For example,
```nim
import pkg/serde/json
type
Person {.serialize(mode=OptOut), deserialize(mode=OptIn).} = object
id {.serialize(ignore=true), deserialize(key="personid").}: int
name: string
birthYear: int
address: string
phone: string
let person = Person(
name: "Lloyd Christmas",
birthYear: 1970,
address: "123 Sesame Street, Providence, Rhode Island 12345",
phone: "555-905-justgivemethedamnnumber!⛽️🔥")
let createRequest = """{
"name": "Lloyd Christmas",
"birthYear": 1970,
"address": "123 Sesame Street, Providence, Rhode Island 12345",
"phone": "555-905-justgivemethedamnnumber!⛽️🔥"
}"""
assert person.toJson(pretty=true) == createRequest
let createResponse = """{
"personid": 1,
"name": "Lloyd Christmas",
"birthYear": 1970,
"address": "123 Sesame Street, Providence, Rhode Island 12345",
"phone": "555-905-justgivemethedamnnumber!⛽️🔥"
}"""
assert !Person.fromJson(createResponse) == Person(id: 1)
```
### `key`
Specifying a `key`, will alias the field name. When seriazlizing, json will be written
with `key` instead of the field name. When deserializing, the json must contain `key`
for the field to be deserialized.
### `ignore`
Specifying `ignore`, will prevent de/serialization on the field.
### Serde field options reference
| | serialize | deserialize |
|:---------|:-----------------------------------------------------------------------------------------------------------|:-----------------------------------------------------------------------------------------------------------------|
| `key` | aliases the field name in json | deserializes the field if json contains `key` |
| `ignore` | **OptOut:** field not serialized**OptIn:** field not serialized**Strict:** field serialized | **OptOut:** field not deserialized**OptIn:** field not deserialized**Strict:** field deserialized |
## Deserialization
`serde` deserializes using `fromJson`, and in all instances returns `Result[T,
CatchableError]`, where `T` is the type being deserialized. For example:
```nim
type MyType = object
field1: bool
field2: bool
let jsn1 = """{
"field1": true,
"field2": true
}"""
assert !MyType.fromJson(jsn1) == MyType(field1: true, field2: true)
```
If there was an error during deserialization, the result of `fromJson` will contain it:
```nim
import pkg/serde/json
type MyType {.deserialize(mode=Strict).} = object
field1: int
field2: int
let jsn = """{
"field1": 1,
"field2": 2,
"extra": 3
}"""
let res = MyType.fromJson(jsn)
assert res.isFailure
assert res.error of SerdeError
assert res.error.msg == "json field(s) missing in object: {\"extra\"}"
```
## Custom types
If `serde` can't de/serialize a custom type, de/serialization can be supported by
overloading `%` and `fromJson`. For example:
```nim
type
Address* = distinct array[20, byte]
SerializationError* = object of CatchableError
func `%`*(address: Address): JsonNode =
%($address)
func fromJson(_: type Address, json: JsonNode): ?!Address =
expectJsonKind(Address, JString, json)
without address =? Address.init(json.getStr), error:
return failure newException(SerializationError,
"Failed to convert '" & $json & "' to Address: " & error.msg)
success address
```
## Serializing to string (`toJson`)
`toJson` is a shortcut for serializing an object into its serialized string
representation:
```nim
import pkg/serde/json
type MyType {.serialize.} = object
field1: string
field2: bool
let mt = MyType(field1: "hw", field2: true)
assert mt.toJson == """{"field1":"hw","field2":true}"""
```
This comes in handy, for example, when sending API responses:
```nim
let availability = getAvailability(...)
return RestApiResponse.response(availability.toJson,
contentType="application/json")
```
## `std/json` drop-in replacment
`nim-serde` can be used as a drop-in replacement for the [standard library's `json`
module](https://nim-lang.org/docs/json.html), with a few notable improvements.
Instead of importing `std/json` into your application, `pkg/serde/json` can be imported
instead:
```diff
- import std/json
+ import pkg/serde/json
```
As with `std/json`, `%` can be used to serialize a type into a `JsonNode`:
```nim
import pkg/serde/json
assert %"hello" == newJString("hello")
```
And `%*` can be used to serialize objects:
```nim
import pkg/serde/json
let expected = newJObject()
expected["hello"] = newJString("world")
assert %*{"hello": "world"} == expected
```
As well, serialization of types can be overridden, and serialization of custom types can
be introduced. Here, we are overriding the serialization of `int`:
```nim
import pkg/serde/json
func `%`(i: int): JsonNode =
newJInt(i + 1)
assert 1.toJson == "2"
```
## `parseJson` and exception tracking
Unfortunately, `std/json`'s `parseJson` can raise an `Exception`, so proper exception
tracking breaks, eg
```nim
## Fails to compile:
## Error: parseJson(me, false, false) can raise an unlisted exception: Exception
import std/json
{.push raises:[].}
type
MyAppError = object of CatchableError
proc parseMe(me: string): JsonNode =
try:
return me.parseJson
except CatchableError as error:
raise newException(MyAppError, error.msg)
assert """{"hello":"world"}""".parseMe == %* { "hello": "world" }
```
This is due to `std/json`'s `parseJson` incorrectly raising `Exception`. This can be
worked around by instead importing `serde` and calling its `JsonNode.parse` routine.
Note that `serde`'s `JsonNode.parse` returns a `Result[JsonNode, CatchableError]`
instead of just a plain `JsonNode` object as in `std/json`'s `parseJson`:
```nim
import pkg/serde/json
{.push raises:[].}
type
MyAppError = object of CatchableError
proc parseMe(me: string): JsonNode {.raises: [MyAppError].} =
without parsed =? JsonNode.parse(me), error:
raise newException(MyAppError, error.msg)
parsed
assert """{"hello":"world"}""".parseMe == %* { "hello": "world" }
```