Absent values

A value is absent when there is nothing to check:

• a nil pointer field: *string, *int, *uuid.UUID;
• a nil pointer to a slice: *[]string;
• a field of a nested struct stored by a nil pointer: City of a nil *Address;
• a nil element of a slice of pointers: []*string{nil}.

Every rule skips absent values, only Required fails on them. So an optional field needs no special code: the rules check it only when it is set.

package main

import (
    "context"
    "fmt"

    "github.com/insei/valigo"
    "github.com/insei/valigo/rules/num"
    "github.com/insei/valigo/rules/str"
    "github.com/insei/valigo/shared"
)

type Address struct {
    City string
}

type Order struct {
    Comment  *string
    Discount *int
    Coupons  *[]string
    Delivery *Address
}

func main() {
    v := valigo.New()
    valigo.Configure[Order](v, func(b valigo.Configurator[Order], o *Order) {
        b.Field(&o.Comment).As(str.Of).MaxLen(200)
        b.Field(&o.Discount).As(num.Ptr).AnyOfInterval(1, 50)
        b.Field(&o.Coupons).As(shared.SlicePtr).As(str.Slice).MaxLen(3)
        b.Field(&o.Delivery.City).As(str.Of).Required()
    })

    // nothing is set: only the Required rule fails
    fmt.Println(v.Validate(context.Background(), &Order{}))

    discount := 70
    fmt.Println(v.Validate(context.Background(), &Order{Discount: &discount, Delivery: &Address{City: "Paris"}}))
}

Output:

[Should be fulfilled (Delivery.City: <nil>)]
[Only interval[1 - 50] is allowed (Discount: 70)]

Required

What Required checks depends on the type:

Conversion	Fails on
str.Of	absent value, empty string
num.Of	zero
num.Ptr	absent value; zero is a valid value of a pointer
uuid.Of	absent value, uuid.Nil
Slice, str.Slice, uuid.Slice	absent slice, empty slice

Nil slices of value fields ([]string(nil)) are present and empty: length rules check them, Each has nothing to check.

Conditions

A field When doesn't call its condition for an absent value and skips its rules, including Required: the condition is about the value, and there is no value. Put Required before When, or use When of the struct, if the field is required under a condition. See Conditions.