##[
This module implements a hookable (de)serialization for arbitrary types.
Design goal: avoid importing modules where a custom serialization is needed;
see strtabs.fromJsonHook,toJsonHook for an example.
]##
runnableExamples:
import std/[strtabs,json]
type Foo = ref object
t: bool
z1: int8
let a = (1.5'f32, (b: "b2", a: "a2"), 'x', @[Foo(t: true, z1: -3), nil], [{"name": "John"}.newStringTable])
let j = a.toJson
doAssert j.jsonTo(type(a)).toJson == j
import std/[json,tables,strutils]
#[
xxx
use toJsonHook,fromJsonHook for Table|OrderedTable
add Options support also using toJsonHook,fromJsonHook and remove `json=>options` dependency
Future directions:
add a way to customize serialization, for eg:
* allowing missing or extra fields in JsonNode
* field renaming
* allow serializing `enum` and `char` as `string` instead of `int`
(enum is more compact/efficient, and robust to enum renamings, but string
is more human readable)
* handle cyclic references, using a cache of already visited addresses
]#
import std/macros
proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
proc distinctBase(T: typedesc): typedesc {.magic: "TypeTrait".}
template distinctBase[T](a: T): untyped = distinctBase(type(a))(a)
macro getDiscriminants(a: typedesc): seq[string] =
## return the discriminant keys
# candidate for std/typetraits
var a = a.getTypeImpl
doAssert a.kind == nnkBracketExpr
let sym = a[1]
let t = sym.getTypeImpl
let t2 = t[2]
doAssert t2.kind == nnkRecList
result = newTree(nnkBracket)
for ti in t2:
if ti.kind == nnkRecCase:
let key = ti[0][0]
let typ = ti[0][1]
result.add newLit key.strVal
if result.len > 0:
result = quote do:
@`result`
else:
result = quote do:
seq[string].default
macro initCaseObject(a: typedesc, fun: untyped): untyped =
## does the minimum to construct a valid case object, only initializing
## the discriminant fields; see also `getDiscriminants`
# maybe candidate for std/typetraits
var a = a.getTypeImpl
doAssert a.kind == nnkBracketExpr
let sym = a[1]
let t = sym.getTypeImpl
var t2: NimNode
case t.kind
of nnkObjectTy: t2 = t[2]
of nnkRefTy: t2 = t[0].getTypeImpl[2]
else: doAssert false, $t.kind # xxx `nnkPtrTy` could be handled too
doAssert t2.kind == nnkRecList
result = newTree(nnkObjConstr)
result.add sym
for ti in t2:
if ti.kind == nnkRecCase:
let key = ti[0][0]
let typ = ti[0][1]
let key2 = key.strVal
let val = quote do:
`fun`(`key2`, typedesc[`typ`])
result.add newTree(nnkExprColonExpr, key, val)
proc checkJsonImpl(cond: bool, condStr: string, msg = "") =
if not cond:
# just pick 1 exception type for simplicity; other choices would be:
# JsonError, JsonParser, JsonKindError
raise newException(ValueError, msg)
template checkJson(cond: untyped, msg = "") =
checkJsonImpl(cond, astToStr(cond), msg)
template fromJsonFields(a, b, T, keys) =
checkJson b.kind == JObject, $(b.kind) # we could customize whether to allow JNull
var num = 0
for key, val in fieldPairs(a):
num.inc
when key notin keys:
if b.hasKey key:
fromJson(val, b[key])
else:
# we could customize to allow this
checkJson false, $($T, key, b)
checkJson b.len == num, $(b.len, num, $T, b) # could customize
proc fromJson*[T](a: var T, b: JsonNode) =
## inplace version of `jsonTo`
#[
adding "json path" leading to `b` can be added in future work.
]#
checkJson b != nil, $($T, b)
when compiles(fromJsonHook(a, b)): fromJsonHook(a, b)
elif T is bool: a = to(b,T)
elif T is Table | OrderedTable:
a.clear
for k,v in b:
a[k] = jsonTo(v, typeof(a[k]))
elif T is enum:
case b.kind
of JInt: a = T(b.getBiggestInt())
of JString: a = parseEnum[T](b.getStr())
else: checkJson false, $($T, " ", b)
elif T is Ordinal: a = T(to(b, int))
elif T is pointer: a = cast[pointer](to(b, int))
elif T is distinct:
when nimvm:
# bug, potentially related to https://github.com/nim-lang/Nim/issues/12282
a = T(jsonTo(b, distinctBase(T)))
else:
a.distinctBase.fromJson(b)
elif T is string|SomeNumber: a = to(b,T)
elif T is JsonNode: a = b
elif T is ref | ptr:
if b.kind == JNull: a = nil
else:
a = T()
fromJson(a[], b)
elif T is array:
checkJson a.len == b.len, $(a.len, b.len, $T)
var i = 0
for ai in mitems(a):
fromJson(ai, b[i])
i.inc
elif T is seq:
a.setLen b.len
for i, val in b.getElems:
fromJson(a[i], val)
elif T is object:
template fun(key, typ): untyped =
jsonTo(b[key], typ)
a = initCaseObject(T, fun)
const keys = getDiscriminants(T)
fromJsonFields(a, b, T, keys)
elif T is tuple:
when isNamedTuple(T):
fromJsonFields(a, b, T, seq[string].default)
else:
checkJson b.kind == JArray, $(b.kind) # we could customize whether to allow JNull
var i = 0
for val in fields(a):
fromJson(val, b[i])
i.inc
checkJson b.len == i, $(b.len, i, $T, b) # could customize
else:
# checkJson not appropriate here
static: doAssert false, "not yet implemented: " & $T
proc jsonTo*(b: JsonNode, T: typedesc): T =
## reverse of `toJson`
fromJson(result, b)
proc toJson*[T](a: T): JsonNode =
## serializes `a` to json; uses `toJsonHook(a: T)` if it's in scope to
## customize serialization, see strtabs.toJsonHook for an example.
when compiles(toJsonHook(a)): result = toJsonHook(a)
elif T is Table | OrderedTable:
result = newJObject()
for k, v in pairs(a): result[k] = toJson(v)
elif T is object | tuple:
when T is object or isNamedTuple(T):
result = newJObject()
for k, v in a.fieldPairs: result[k] = toJson(v)
else:
result = newJArray()
for v in a.fields: result.add toJson(v)
elif T is ref | ptr:
if system.`==`(a, nil): result = newJNull()
else: result = toJson(a[])
elif T is array | seq:
result = newJArray()
for ai in a: result.add toJson(ai)
elif T is pointer: result = toJson(cast[int](a))
# edge case: `a == nil` could've also led to `newJNull()`, but this results
# in simpler code for `toJson` and `fromJson`.
elif T is distinct: result = toJson(a.distinctBase)
elif T is bool: result = %(a)
elif T is Ordinal: result = %(a.ord)
else: result = %a