Types

Anything encoding/json decodes, gopantic decodes the same way. These are the cases worth knowing.

Type	Behaviour
Numbers, strings, booleans	Decoded, then coerced if the decoder refused
time.Time	RFC 3339, plus the other formats in the coercion guide and Unix seconds
Structs, pointers	Rules inside them run. A nil pointer has nothing to check
Slices, arrays	Rules run on each element
map[K]V	Rules run on each value. K can be a string or an integer, and interface{} for YAML, which keeps the key's type
interface{}	Holds what the decoder produced. required works, and nothing goes inside it
json.RawMessage	Keeps its bytes as they arrived
Types with UnmarshalJSON, UnmarshalYAML or UnmarshalText	Decode themselves. validate tags still apply to them

Absent, null and zero

A plain field can't tell an absent key from a zero value. A pointer can:

type Update struct {
    Name *string `json:"name"` // nil: not sent. "": sent empty
    Age  *int    `json:"age"`
}

An absent or null slice is nil, and [] is empty and not nil, as with encoding/json.

Fixed arrays

Arrays follow encoding/json: extra elements are dropped and missing ones stay zero, with no error. To require a length, use a slice and a rule:

Scores []float64 `json:"scores" validate:"len=3"`

Struct tags

json tags work as in encoding/json, including json:"-" and omitempty, embedded structs, and case-insensitive key matching. A key that matches no field is ignored.

For YAML, a field's key comes from its yaml tag, then its json tag, then its lowercased name. yaml:",inline" and merge keys behave as in yaml.v3. A field with json:"-" and no yaml tag is skipped on YAML too.

type Config struct {
    Host string `json:"host"`            // key "host" in both
    Port int    `yaml:"port" json:"p"`   // "port" in YAML, "p" in JSON
    TLS  bool                             // key "tls" in YAML, any case of "TLS" in JSON
}

Trees and cycles

A type can refer to itself, and input is a tree, so nothing loops. Validate on a value you built with a real cycle stops at the depth limit with an error. Tag a back-pointer json:"-" and Validate won't follow it:

type Node struct {
    Name     string  `json:"name" validate:"required"`
    Children []*Node `json:"children"`
    Parent   *Node   `json:"-"`
}