Conditional When

Learn how to use conditional.When in NLG Form to run validation rules only when custom runtime predicates match.

conditional.When is the low-level primitive for conditional validation.

It allows one or more validation rules to run only when a custom predicate returns true.

Use When when predefined conditional helpers are not expressive enough.

TL;DR

Helper	Description
`conditional.When(cond, rules...)`	Runs nested rules only when `cond` returns `true`

Basic Usage

conditional.When(
	func(r *Request) bool {
		return r.AccountType == "business"
	},
	rules.Required(Form.CompanyName),
)

If the condition is false, validation is skipped.

If the condition is true, all nested rules are executed.

Common Use Cases

business account validation
password change flows
billing sections
role-based validation
feature-specific fields
multi-step forms
PATCH APIs
advanced business rules

Complete Example

schema.go

package main

import (
	"github.com/netlifeguru/form"
	"github.com/netlifeguru/form/conditional"
	"github.com/netlifeguru/form/rules"
)

const (
	CodeCompanyRequired = form.Code("company_required")
	CodePasswordMinLen  = form.Code("password_min_len")
)

type ConditionalWhenRequest struct {
	AccountType     string `json:"account_type"`
	CompanyName    string `json:"company_name"`
	ChangePassword bool   `json:"change_password"`
	NewPassword     string `json:"new_password"`
}

func ConditionalWhenSchema() form.Schema[ConditionalWhenRequest] {
	ConditionalWhenForm := struct {
		AccountType     form.StringField[ConditionalWhenRequest]
		CompanyName    form.StringField[ConditionalWhenRequest]
		ChangePassword form.BoolField[ConditionalWhenRequest]
		NewPassword    form.StringField[ConditionalWhenRequest]
	}{
		AccountType: form.Str[ConditionalWhenRequest]("account_type", func(r *ConditionalWhenRequest) string {
			return r.AccountType
		}),
		CompanyName: form.Str[ConditionalWhenRequest]("company_name", func(r *ConditionalWhenRequest) string {
			return r.CompanyName
		}),
		ChangePassword: form.Bool[ConditionalWhenRequest]("change_password", func(r *ConditionalWhenRequest) bool {
			return r.ChangePassword
		}),
		NewPassword: form.Str[ConditionalWhenRequest]("new_password", func(r *ConditionalWhenRequest) string {
			return r.NewPassword
		}),
	}

	return form.Schema[ConditionalWhenRequest]{
		conditional.When(
			func(r *ConditionalWhenRequest) bool {
				return r.AccountType == "business"
			},
			rules.RequiredWithCode(ConditionalWhenForm.CompanyName, CodeCompanyRequired),
		),

		conditional.When(
			func(r *ConditionalWhenRequest) bool {
				return r.ChangePassword
			},
			rules.Required(ConditionalWhenForm.NewPassword),
			rules.MinLenWithCode(ConditionalWhenForm.NewPassword, 8, CodePasswordMinLen),
		),
	}
}

main.go

package main

import (
	"encoding/json"
	"fmt"
	"log/slog"
	"net/http"
	"os"

	"github.com/netlifeguru/form"
	"github.com/netlifeguru/form/httpform"
	"github.com/netlifeguru/router"
)

func main() {
	r := router.New()

	r.HandleFunc("/conditional-when", "POST", func(w http.ResponseWriter, req *http.Request, ctx *router.Context) {
		var in ConditionalWhenRequest

		if !httpform.BindAndValidate(w, req, &in, ConditionalWhenSchema(), 1<<20) {
			fmt.Println("conditional when validation failed")
			return
		}

		fmt.Println("conditional when validation passed:", in)

		w.Header().Set("Content-Type", "application/json")

		_ = json.NewEncoder(w).Encode(map[string]any{
			"message": "conditional when validation passed",
			"data":    in,
		})
	})

	validPayload := map[string]any{
		"account_type":    "business",
		"company_name":    "Acme s.r.o.",
		"change_password": true,
		"new_password":    "secret123",
	}

	invalidPayload := map[string]any{
		"account_type":    "business",
		"company_name":    "",
		"change_password": true,
		"new_password":    "short",
	}

	skippedPayload := map[string]any{
		"account_type":    "personal",
		"company_name":    "",
		"change_password": false,
		"new_password":    "",
	}

	fmt.Println("\n--- Valid request ---")
	form.SendTestPost(":8080/conditional-when", validPayload)

	fmt.Println("\n--- Invalid request ---")
	form.SendTestPost(":8080/conditional-when", invalidPayload)

	fmt.Println("\n--- Skipped conditional validation request ---")
	form.SendTestPost(":8080/conditional-when", skippedPayload)

	if err := r.ListenAndServe(8080); err != nil {
		slog.Error("failed to start server", "error", err)
		os.Exit(1)
	}
}

Validation Flow

condition is false
    ↓
nested validation skipped

condition is true
    ↓
nested rules executed

Notes

When is the most flexible conditional validation primitive.
Conditions receive the full request structure.
A single When block can contain multiple nested rules.
Use When for custom business logic.
Use convenience helpers such as RequiredIfStr for common conditional cases.
Conditional validation remains explicit and transport-independent.

conditional.When is the low-level primitive for conditional validation.

It allows one or more validation rules to run only when a custom predicate returns true.

Use When when predefined conditional helpers are not expressive enough.

TL;DR

Helper	Description
`conditional.When(cond, rules...)`	Runs nested rules only when `cond` returns `true`

Basic Usage

conditional.When(
	func(r *Request) bool {
		return r.AccountType == "business"
	},
	rules.Required(Form.CompanyName),
)

If the condition is false, validation is skipped.

If the condition is true, all nested rules are executed.

Common Use Cases

business account validation
password change flows
billing sections
role-based validation
feature-specific fields
multi-step forms
PATCH APIs
advanced business rules

Complete Example

schema.go

package main

import (
	"github.com/netlifeguru/form"
	"github.com/netlifeguru/form/conditional"
	"github.com/netlifeguru/form/rules"
)

const (
	CodeCompanyRequired = form.Code("company_required")
	CodePasswordMinLen  = form.Code("password_min_len")
)

type ConditionalWhenRequest struct {
	AccountType     string `json:"account_type"`
	CompanyName    string `json:"company_name"`
	ChangePassword bool   `json:"change_password"`
	NewPassword     string `json:"new_password"`
}

func ConditionalWhenSchema() form.Schema[ConditionalWhenRequest] {
	ConditionalWhenForm := struct {
		AccountType     form.StringField[ConditionalWhenRequest]
		CompanyName    form.StringField[ConditionalWhenRequest]
		ChangePassword form.BoolField[ConditionalWhenRequest]
		NewPassword    form.StringField[ConditionalWhenRequest]
	}{
		AccountType: form.Str[ConditionalWhenRequest]("account_type", func(r *ConditionalWhenRequest) string {
			return r.AccountType
		}),
		CompanyName: form.Str[ConditionalWhenRequest]("company_name", func(r *ConditionalWhenRequest) string {
			return r.CompanyName
		}),
		ChangePassword: form.Bool[ConditionalWhenRequest]("change_password", func(r *ConditionalWhenRequest) bool {
			return r.ChangePassword
		}),
		NewPassword: form.Str[ConditionalWhenRequest]("new_password", func(r *ConditionalWhenRequest) string {
			return r.NewPassword
		}),
	}

	return form.Schema[ConditionalWhenRequest]{
		conditional.When(
			func(r *ConditionalWhenRequest) bool {
				return r.AccountType == "business"
			},
			rules.RequiredWithCode(ConditionalWhenForm.CompanyName, CodeCompanyRequired),
		),

		conditional.When(
			func(r *ConditionalWhenRequest) bool {
				return r.ChangePassword
			},
			rules.Required(ConditionalWhenForm.NewPassword),
			rules.MinLenWithCode(ConditionalWhenForm.NewPassword, 8, CodePasswordMinLen),
		),
	}
}

main.go

package main

import (
	"encoding/json"
	"fmt"
	"log/slog"
	"net/http"
	"os"

	"github.com/netlifeguru/form"
	"github.com/netlifeguru/form/httpform"
	"github.com/netlifeguru/router"
)

func main() {
	r := router.New()

	r.HandleFunc("/conditional-when", "POST", func(w http.ResponseWriter, req *http.Request, ctx *router.Context) {
		var in ConditionalWhenRequest

		if !httpform.BindAndValidate(w, req, &in, ConditionalWhenSchema(), 1<<20) {
			fmt.Println("conditional when validation failed")
			return
		}

		fmt.Println("conditional when validation passed:", in)

		w.Header().Set("Content-Type", "application/json")

		_ = json.NewEncoder(w).Encode(map[string]any{
			"message": "conditional when validation passed",
			"data":    in,
		})
	})

	validPayload := map[string]any{
		"account_type":    "business",
		"company_name":    "Acme s.r.o.",
		"change_password": true,
		"new_password":    "secret123",
	}

	invalidPayload := map[string]any{
		"account_type":    "business",
		"company_name":    "",
		"change_password": true,
		"new_password":    "short",
	}

	skippedPayload := map[string]any{
		"account_type":    "personal",
		"company_name":    "",
		"change_password": false,
		"new_password":    "",
	}

	fmt.Println("\n--- Valid request ---")
	form.SendTestPost(":8080/conditional-when", validPayload)

	fmt.Println("\n--- Invalid request ---")
	form.SendTestPost(":8080/conditional-when", invalidPayload)

	fmt.Println("\n--- Skipped conditional validation request ---")
	form.SendTestPost(":8080/conditional-when", skippedPayload)

	if err := r.ListenAndServe(8080); err != nil {
		slog.Error("failed to start server", "error", err)
		os.Exit(1)
	}
}

Validation Flow

condition is false
    ↓
nested validation skipped

condition is true
    ↓
nested rules executed

Notes

When is the most flexible conditional validation primitive.
Conditions receive the full request structure.
A single When block can contain multiple nested rules.
Use When for custom business logic.
Use convenience helpers such as RequiredIfStr for common conditional cases.
Conditional validation remains explicit and transport-independent.

Conditional When

TL;DR

Basic Usage

Common Use Cases

Complete Example

schema.go

main.go

Validation Flow

Notes

On this page

Conditional When

TL;DR

Basic Usage

Common Use Cases

Complete Example

schema.go

main.go

Validation Flow

Notes

On this page