Conditional Rules

Only validate when a flag is true

val := v.
  Is(v.String(username, "username").Not().Blank()).
  If(isAdmin, v.Is(v.String(role, "role").EqualTo("admin")))

Only continue when valid so far

val := v.Is(
  v.String(email, "email").Not().Blank(),
  v.String(country, "country").Not().Blank(),
).WhenValid(func(val *v.Validation) {
  vatRate, err := lookupVATRate(country)
  if err != nil {
    val.AddErrorMessage("vat_rate", "Error computing VAT")
    return
  }
  val.Is(v.Number(vatRate, "vat_rate").GreaterThan(0))
})