Conditional Validation in Go

All conditional methods return the same *Validation, so they can be used in a fluent chain.

If(condition, validation)

If() merges the supplied validation result only when the condition is true.

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

Go evaluates function arguments before calling If(). The supplied validation is therefore built in either case; only the merge is conditional. Use When() when creating the validation is expensive or has side effects.

Do(function)

Do() always calls the function with the current validation session.

val := v.
  Is(v.String(username, "username").Not().Blank()).
  Do(func(val *v.Validation) {
    if isAdmin {
      val.Is(v.String(role, "role").EqualTo("admin"))
    }
  })

When(condition, function)

When() calls the function only when the condition is true.

val := v.
  Is(v.String(username, "username").Not().Blank()).
  When(isAdmin, func(val *v.Validation) {
    val.Is(v.String(role, "role").EqualTo("admin"))
  })

Validity-gated merges

These methods inspect results already recorded in the current session and merge another validation when the predicate passes:

val.IfValid(other)
val.IfPathValid("email", other)
val.IfAllValid([]string{"email", "password"}, other)
val.IfAnyValid([]string{"email", "phone"}, other)

Like If(), these methods receive an already-created validation. In particular, IfAllValid and IfAnyValid take a []string, not variadic path arguments.

An empty path slice makes IfAllValid use the overall Valid() result. An empty path slice makes IfAnyValid a no-op.

Validity-gated callbacks

The corresponding When methods execute a callback only when the predicate passes:

val.WhenValid(func(val *v.Validation) { /* ... */ })
val.WhenPathValid("email", func(val *v.Validation) { /* ... */ })
val.WhenAllValid([]string{"email", "password"}, func(val *v.Validation) { /* ... */ })
val.WhenAnyValid([]string{"email", "phone"}, func(val *v.Validation) { /* ... */ })

Use these callback forms to defer expensive work. Their empty-slice behavior matches the corresponding IfAllValid and IfAnyValid methods.