Add support for user specified functions for structs

[mathieu-lemay]

Description

Add the ability to register custom functions and use them to dynamically set fields in a struct. This is done by setting the fake tag on a struct field to fn=some_function and then calling faker.RegisterFunction("some_function", func() interface{} {...}). When fake.Struct().Fill(&o) is called, it will call the registered function and use that value to fill the struct. If the return value of the function can't be assigned to the field, the normal logic will be used.

Are you trying to fix an existing issue?

No

Go Version

$ go version
go version go1.24.5 darwin/arm64

Go Tests

$ go test
2025/07/17 09:59:07 Error while requesting https://loremflickr.com/300/200 : request failed
2025/07/17 09:59:07 Error while creating a temp file: temp file creation failed
2025/07/17 09:59:08 Error while requesting https://randomuser.me : request failed
2025/07/17 09:59:08 Error while creating a temp file: temp file creation failed
PASS
ok      github.com/jaswdr/faker/v2      3.394s

[jaswdr]

Sorry for the delay, I'll take a look on this between today and tomorrow.

[jaswdr]

LGTM, I reviewed but forgot to merge 😂 my bad. I'll do a new release soon. Thank you for the contribution, nice work there.

[mathieu-lemay]

No worries! I actually had this sitting on my computer for months before I remembered to open the PR, so what's a few more days at that point? 😂