Mapping To Object Variants

Mapping to object variants

Just like how you can use object variants as sources to create objects/tuples, you can also create them from objects! However, mapping any input to object variants requires telling mapster which possible kind of an object variant you want to instantiate. Mapster can not infer this.

As such, you should provide a parameter with the desired kind, annotate your proc with mapVariant and pass the pragma the name of the parameter that you intend to use:

import mapster

type A = object
  str: string
  num: int

type Kind = enum
  ka, kb

type B = object
  case kind: Kind
  of ka: str: string
  of kb: num: int

let a = A(str: "str", num: 5)

proc mapToB(x: A, bKind: Kind): B {.mapVariant: "bKind".} = discard

let myBa: B = mapToB(a, Kind.ka)
doAssert myBa.kind == Kind.ka
doAssert myBa.str == "str"

let myBb: B = mapToB(a, Kind.kb)
doAssert myBb.kind == Kind.kb
doAssert myBb.num == 5

In regards to mapping with custom logic, mapping with non-field parameters, mapping with multiple parameters and mapping with object variant sources it acts the same as the map pragma. Excluding/ignoring parameters via a mapVariantExcept pragma is not yet supported.