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GALA-E0057 — Validate method with the wrong signature

What it means. A codec would decode a struct that has an unexported field, and the struct’s Validate method does not have the one signature decoding calls, func (x T) Validate() Try[T]. A pointer receiver, a parameter, a type parameter, or a result other than Try[T] (bool, error, Option[T]) all fire it, wherever the struct sits in the codec: the root, a nested field, or inside an Option, Array, List or HashMap.

Code that triggers it

package main

import (
    "martianoff/gala/json"
    "strings"
)

struct Email(address string)

func (e Email) Validate() bool = strings.Contains(e.address, "@")

func main() {
    val codec = json.Codec[Email](json.AsIs())
    Println(codec.Decode("{\"address\":\"not an address\"}"))
}

Compiler message

error[GALA-E0057]: cannot generate a codec for Email: Email has private fields, and its Validate method is `func (e Email) Validate() bool`, not `func (e Email) Validate() Try[Email]`
  --> main.gala:13:35
   |
13 |     val codec = json.Codec[Email](json.AsIs())
   |                                   ^^^^ Validate needs a value receiver and a Try[Email] result
   |
   = hint: Validate needs a value receiver and a Try[Email] result; declare it as `func (e Email) Validate() Try[Email]`: decoding builds the raw value and returns what Validate returns, so a Failure rejects the input with its error

How to fix it

Return the value itself on success and the reason it is invalid on failure:

package main

import (
    "errors"
    "martianoff/gala/json"
    "strings"
)

struct Email(address string)

func (e Email) Validate() Try[Email] =
    if (strings.Contains(e.address, "@")) Success(e) else Failure(errors.New(s"not an email address: ${e.address}"))

func main() {
    val codec = json.Codec[Email](json.AsIs())
    Println(codec.Decode("{\"address\":\"not an address\"}"))
}

Decode now returns Failure(not an email address: not an address) — the error your Validate returned, unchanged.

Why the rule exists

A struct with an unexported field is an encapsulated value: its package builds it only through a constructor that checks it. Decoding builds the struct from whatever the input holds, so the codec builds the raw value and returns what Validate returns, and a value the constructor would refuse is refused by the codec too. That needs one fixed signature: a bool or error result would leave the codec to invent the failure, and a pointer receiver or a parameter cannot be called on the freshly built value. A method with the right name and the wrong shape is reported rather than ignored, since ignoring it would leave the struct undecodable for a reason the author cannot see.