go
import (
    "github.com/supertokens/supertokens-golang/recipe/passwordless"
    "github.com/supertokens/supertokens-golang/recipe/passwordless/plessmodels"
    "github.com/supertokens/supertokens-golang/supertokens"
)

func main() {
    passwordless.Init(plessmodels.TypeInput{
        Override: &plessmodels.OverrideStruct{
            APIs: func(originalImplementation plessmodels.APIInterface) plessmodels.APIInterface {
                ogCreateCodePOST := *originalImplementation.CreateCodePOST
                (*originalImplementation.CreateCodePOST) = func(email, phoneNumber *string, options plessmodels.APIOptions, userContext supertokens.UserContext) (plessmodels.CreateCodePOSTResponse, error) {
                    user, err := passwordless.GetUserByEmail(*email)
                    if err != nil {
                        return plessmodels.CreateCodePOSTResponse{}, err
                    }
                    if user != nil {
                        // this means that the user with this email already exist, so it's a sign in
                        // so we allow it
                        return ogCreateCodePOST(email, phoneNumber, options, userContext)
                    }
                    // this means that the user does not exist and is trying to sign up, which we
                    // disallow
                    return plessmodels.CreateCodePOSTResponse{
                        GeneralError: &supertokens.GeneralErrorResponse{
                            Message: "Sign up is disabled",
                        },
                    }, nil
                }
                return originalImplementation
            },
        },
    })
}