Conditional Rules

Learn how to use NLG Form conditional rule helpers for required-if, prohibited-if, required-with, and required-without validation flows.

Conditional rule helpers provide shortcuts for common conditional validation patterns.

They are built on top of conditional.When, but make frequent validation cases easier to read and reuse.

Use conditional rule helpers when validation depends on other fields in the same request.

Common Use Cases

Conditional rules are commonly used for:

company-only fields
country-specific billing fields
admin-only notes
contact method validation
address-dependent fields
backup contact information
onboarding flows
profile completion
dynamic form sections

TL;DR

Helper	Description
`conditional.RequiredIfStr(field, cond)`	Requires a string field when `cond` returns `true`
`conditional.RequiredIfStrWithCode(field, cond, code)`	Same as `RequiredIfStr` with a custom error code
`conditional.ProhibitedIfStr(field, cond)`	Forbids a non-blank string value when `cond` returns `true`
`conditional.ProhibitedIfStrWithCode(field, cond, code)`	Same as `ProhibitedIfStr` with a custom error code
`conditional.RequiredWithAnyStr(field, others...)`	Requires a string field when any other field is non-blank
`conditional.RequiredWithAnyStrWithCode(field, code, others...)`	Same as `RequiredWithAnyStr` with a custom error code
`conditional.RequiredWithoutAnyStr(field, others...)`	Requires a string field when any other field is blank
`conditional.RequiredWithoutAnyStrWithCode(field, code, others...)`	Same as `RequiredWithoutAnyStr` with a custom error code

Defining Conditional Fields

Conditional validation starts with regular typed string fields.

ConditionalRulesForm := struct {
	AccountType form.StringField[ConditionalRulesRequest]
	CompanyName form.StringField[ConditionalRulesRequest]
	Country     form.StringField[ConditionalRulesRequest]
	VatNumber   form.StringField[ConditionalRulesRequest]
	Role        form.StringField[ConditionalRulesRequest]
	AdminNote   form.StringField[ConditionalRulesRequest]
}{
	AccountType: form.Str[ConditionalRulesRequest]("account_type", func(r *ConditionalRulesRequest) string {
		return r.AccountType
	}),
	CompanyName: form.Str[ConditionalRulesRequest]("company_name", func(r *ConditionalRulesRequest) string {
		return r.CompanyName
	}),
	Country: form.Str[ConditionalRulesRequest]("country", func(r *ConditionalRulesRequest) string {
		return r.Country
	}),
	VatNumber: form.Str[ConditionalRulesRequest]("vat_number", func(r *ConditionalRulesRequest) string {
		return r.VatNumber
	}),
	Role: form.Str[ConditionalRulesRequest]("role", func(r *ConditionalRulesRequest) string {
		return r.Role
	}),
	AdminNote: form.Str[ConditionalRulesRequest]("admin_note", func(r *ConditionalRulesRequest) string {
		return r.AdminNote
	}),
}

Applying Conditional Rules

Conditional helpers attach validation behavior to fields based on runtime values.

return form.Schema[ConditionalRulesRequest]{
	conditional.RequiredIfStr(
		ConditionalRulesForm.CompanyName,
		func(r *ConditionalRulesRequest) bool {
			return r.AccountType == "company"
		},
	),

	conditional.RequiredIfStrWithCode(
		ConditionalRulesForm.VatNumber,
		func(r *ConditionalRulesRequest) bool {
			return r.Country == "SK"
		},
		CodeVatNumberRequired,
	),

	conditional.ProhibitedIfStr(
		ConditionalRulesForm.AdminNote,
		func(r *ConditionalRulesRequest) bool {
			return r.Role != "admin"
		},
	),
}

Rule Examples

RequiredIfStr

Requires a string field when the condition returns true.

conditional.RequiredIfStr(
	ConditionalRulesForm.CompanyName,
	func(r *ConditionalRulesRequest) bool {
		return r.AccountType == "company"
	},
)

This is useful when one field becomes mandatory based on another field.

RequiredIfStrWithCode

Requires a string field when the condition returns true and returns a custom validation code.

const CodeVatNumberRequired = form.Code("vat_number_required")

conditional.RequiredIfStrWithCode(
	ConditionalRulesForm.VatNumber,
	func(r *ConditionalRulesRequest) bool {
		return r.Country == "SK"
	},
	CodeVatNumberRequired,
)

This is useful for country-specific, account-specific, or workflow-specific validation.

ProhibitedIfStr

Forbids a non-blank string value when the condition returns true.

conditional.ProhibitedIfStr(
	ConditionalRulesForm.AdminNote,
	func(r *ConditionalRulesRequest) bool {
		return r.Role != "admin"
	},
)

Validation behavior:

Condition	Field value	Result
`false`	any value	skipped
`true`	`""`	valid
`true`	`"secret note"`	invalid

ProhibitedIfStrWithCode

Forbids a non-blank string value when the condition returns true and returns a custom validation code.

const CodeInternalNoteBlocked = form.Code("internal_note_blocked")

conditional.ProhibitedIfStrWithCode(
	ConditionalRulesForm.InternalNote,
	func(r *ConditionalRulesRequest) bool {
		return r.UserType == "external"
	},
	CodeInternalNoteBlocked,
)

This is useful for protecting internal-only fields from public or external input.

RequiredWithAnyStr

Requires a field when any of the referenced fields is non-blank.

conditional.RequiredWithAnyStr(
	ConditionalRulesForm.Email,
	ConditionalRulesForm.Phone,
)

This means:

if phone is filled, email is required

Typical use cases:

contact method pairs
dependent address fields
partially completed form sections

RequiredWithAnyStrWithCode

Requires a field when any referenced field is non-blank and returns a custom validation code.

const CodeContactRequired = form.Code("contact_required")

conditional.RequiredWithAnyStrWithCode(
	ConditionalRulesForm.ContactName,
	CodeContactRequired,
	ConditionalRulesForm.Address,
	ConditionalRulesForm.City,
)

This means:

if address or city is filled, contact_name is required

RequiredWithoutAnyStr

Requires a field when any referenced field is blank.

conditional.RequiredWithoutAnyStr(
	ConditionalRulesForm.Phone,
	ConditionalRulesForm.Email,
)

This means:

if email is blank, phone is required

Typical use cases:

fallback contact fields
at-least-one contact method
alternative input paths

RequiredWithoutAnyStrWithCode

Requires a field when any referenced field is blank and returns a custom validation code.

const CodeBackupRequired = form.Code("backup_required")

conditional.RequiredWithoutAnyStrWithCode(
	ConditionalRulesForm.BackupEmail,
	CodeBackupRequired,
	ConditionalRulesForm.BackupPhone,
)

This means:

if backup_phone is blank, backup_email is required

Complete Example

schema.go

package main

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

const (
	CodeVatNumberRequired  = form.Code("vat_number_required")
	CodeInternalNoteBlocked = form.Code("internal_note_blocked")
	CodeContactRequired    = form.Code("contact_required")
	CodeBackupRequired     = form.Code("backup_required")
)

type ConditionalRulesRequest struct {
	AccountType string `json:"account_type"`
	CompanyName string `json:"company_name"`

	Country   string `json:"country"`
	VatNumber string `json:"vat_number"`

	Role         string `json:"role"`
	AdminNote    string `json:"admin_note"`
	UserType      string `json:"user_type"`
	InternalNote string `json:"internal_note"`

	Phone       string `json:"phone"`
	Email       string `json:"email"`
	Address     string `json:"address"`
	City        string `json:"city"`
	ContactName string `json:"contact_name"`

	BackupEmail string `json:"backup_email"`
	BackupPhone string `json:"backup_phone"`
}

func ConditionalRulesSchema() form.Schema[ConditionalRulesRequest] {
	ConditionalRulesForm := struct {
		AccountType form.StringField[ConditionalRulesRequest]
		CompanyName form.StringField[ConditionalRulesRequest]
		Country     form.StringField[ConditionalRulesRequest]
		VatNumber   form.StringField[ConditionalRulesRequest]

		Role         form.StringField[ConditionalRulesRequest]
		AdminNote    form.StringField[ConditionalRulesRequest]
		UserType      form.StringField[ConditionalRulesRequest]
		InternalNote form.StringField[ConditionalRulesRequest]

		Phone       form.StringField[ConditionalRulesRequest]
		Email       form.StringField[ConditionalRulesRequest]
		Address     form.StringField[ConditionalRulesRequest]
		City        form.StringField[ConditionalRulesRequest]
		ContactName form.StringField[ConditionalRulesRequest]

		BackupEmail form.StringField[ConditionalRulesRequest]
		BackupPhone form.StringField[ConditionalRulesRequest]
	}{
		AccountType: form.Str[ConditionalRulesRequest]("account_type", func(r *ConditionalRulesRequest) string {
			return r.AccountType
		}),
		CompanyName: form.Str[ConditionalRulesRequest]("company_name", func(r *ConditionalRulesRequest) string {
			return r.CompanyName
		}),
		Country: form.Str[ConditionalRulesRequest]("country", func(r *ConditionalRulesRequest) string {
			return r.Country
		}),
		VatNumber: form.Str[ConditionalRulesRequest]("vat_number", func(r *ConditionalRulesRequest) string {
			return r.VatNumber
		}),

		Role: form.Str[ConditionalRulesRequest]("role", func(r *ConditionalRulesRequest) string {
			return r.Role
		}),
		AdminNote: form.Str[ConditionalRulesRequest]("admin_note", func(r *ConditionalRulesRequest) string {
			return r.AdminNote
		}),
		UserType: form.Str[ConditionalRulesRequest]("user_type", func(r *ConditionalRulesRequest) string {
			return r.UserType
		}),
		InternalNote: form.Str[ConditionalRulesRequest]("internal_note", func(r *ConditionalRulesRequest) string {
			return r.InternalNote
		}),

		Phone: form.Str[ConditionalRulesRequest]("phone", func(r *ConditionalRulesRequest) string {
			return r.Phone
		}),
		Email: form.Str[ConditionalRulesRequest]("email", func(r *ConditionalRulesRequest) string {
			return r.Email
		}),
		Address: form.Str[ConditionalRulesRequest]("address", func(r *ConditionalRulesRequest) string {
			return r.Address
		}),
		City: form.Str[ConditionalRulesRequest]("city", func(r *ConditionalRulesRequest) string {
			return r.City
		}),
		ContactName: form.Str[ConditionalRulesRequest]("contact_name", func(r *ConditionalRulesRequest) string {
			return r.ContactName
		}),

		BackupEmail: form.Str[ConditionalRulesRequest]("backup_email", func(r *ConditionalRulesRequest) string {
			return r.BackupEmail
		}),
		BackupPhone: form.Str[ConditionalRulesRequest]("backup_phone", func(r *ConditionalRulesRequest) string {
			return r.BackupPhone
		}),
	}

	return form.Schema[ConditionalRulesRequest]{
		conditional.RequiredIfStr(
			ConditionalRulesForm.CompanyName,
			func(r *ConditionalRulesRequest) bool {
				return r.AccountType == "company"
			},
		),

		conditional.RequiredIfStrWithCode(
			ConditionalRulesForm.VatNumber,
			func(r *ConditionalRulesRequest) bool {
				return r.Country == "SK"
			},
			CodeVatNumberRequired,
		),

		conditional.ProhibitedIfStr(
			ConditionalRulesForm.AdminNote,
			func(r *ConditionalRulesRequest) bool {
				return r.Role != "admin"
			},
		),

		conditional.ProhibitedIfStrWithCode(
			ConditionalRulesForm.InternalNote,
			func(r *ConditionalRulesRequest) bool {
				return r.UserType == "external"
			},
			CodeInternalNoteBlocked,
		),

		conditional.RequiredWithAnyStr(
			ConditionalRulesForm.Email,
			ConditionalRulesForm.Phone,
		),

		conditional.RequiredWithAnyStrWithCode(
			ConditionalRulesForm.ContactName,
			CodeContactRequired,
			ConditionalRulesForm.Address,
			ConditionalRulesForm.City,
		),

		conditional.RequiredWithoutAnyStr(
			ConditionalRulesForm.Phone,
			ConditionalRulesForm.Email,
		),

		conditional.RequiredWithoutAnyStrWithCode(
			ConditionalRulesForm.BackupEmail,
			CodeBackupRequired,
			ConditionalRulesForm.BackupPhone,
		),
	}
}

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-rules", "POST", func(w http.ResponseWriter, req *http.Request, ctx *router.Context) {
		var in ConditionalRulesRequest

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

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

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

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

	validPayload := map[string]any{
		"account_type": "company",
		"company_name": "Acme s.r.o.",
		"country":      "SK",
		"vat_number":   "SK1234567890",

		"role":          "user",
		"admin_note":    "",
		"user_type":     "external",
		"internal_note": "",

		"phone":        "+421900123456",
		"email":        "john@example.com",
		"address":      "Main Street 1",
		"city":         "",
		"contact_name": "John Doe",

		"backup_email": "backup@example.com",
		"backup_phone": "",
	}

	invalidPayload := map[string]any{
		"account_type": "company",
		"company_name": "",
		"country":      "SK",
		"vat_number":   "",

		"role":          "user",
		"admin_note":    "internal only",
		"user_type":     "external",
		"internal_note": "secret note",

		"phone":        "+421900123456",
		"email":        "",
		"address":      "Main Street 1",
		"city":         "",
		"contact_name": "",

		"backup_email": "",
		"backup_phone": "",
	}

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

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

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

Notes

Conditional rule helpers are shortcuts for common When patterns.
Conditions receive the full request structure.
String conditional helpers use blank-string behavior based on trimmed string values.
Use RequiredIfStr when a field becomes required in a specific state.
Use ProhibitedIfStr when a field must stay empty in a specific state.
Use RequiredWithAnyStr when one field depends on another field being filled.
Use RequiredWithoutAnyStr when one field is required as a fallback.
Use When directly for advanced rule composition or multiple nested rules.

Conditional rule helpers provide shortcuts for common conditional validation patterns.

They are built on top of conditional.When, but make frequent validation cases easier to read and reuse.

Use conditional rule helpers when validation depends on other fields in the same request.

Common Use Cases

Conditional rules are commonly used for:

company-only fields
country-specific billing fields
admin-only notes
contact method validation
address-dependent fields
backup contact information
onboarding flows
profile completion
dynamic form sections

TL;DR

Helper	Description
`conditional.RequiredIfStr(field, cond)`	Requires a string field when `cond` returns `true`
`conditional.RequiredIfStrWithCode(field, cond, code)`	Same as `RequiredIfStr` with a custom error code
`conditional.ProhibitedIfStr(field, cond)`	Forbids a non-blank string value when `cond` returns `true`
`conditional.ProhibitedIfStrWithCode(field, cond, code)`	Same as `ProhibitedIfStr` with a custom error code
`conditional.RequiredWithAnyStr(field, others...)`	Requires a string field when any other field is non-blank
`conditional.RequiredWithAnyStrWithCode(field, code, others...)`	Same as `RequiredWithAnyStr` with a custom error code
`conditional.RequiredWithoutAnyStr(field, others...)`	Requires a string field when any other field is blank
`conditional.RequiredWithoutAnyStrWithCode(field, code, others...)`	Same as `RequiredWithoutAnyStr` with a custom error code

Defining Conditional Fields

Conditional validation starts with regular typed string fields.

ConditionalRulesForm := struct {
	AccountType form.StringField[ConditionalRulesRequest]
	CompanyName form.StringField[ConditionalRulesRequest]
	Country     form.StringField[ConditionalRulesRequest]
	VatNumber   form.StringField[ConditionalRulesRequest]
	Role        form.StringField[ConditionalRulesRequest]
	AdminNote   form.StringField[ConditionalRulesRequest]
}{
	AccountType: form.Str[ConditionalRulesRequest]("account_type", func(r *ConditionalRulesRequest) string {
		return r.AccountType
	}),
	CompanyName: form.Str[ConditionalRulesRequest]("company_name", func(r *ConditionalRulesRequest) string {
		return r.CompanyName
	}),
	Country: form.Str[ConditionalRulesRequest]("country", func(r *ConditionalRulesRequest) string {
		return r.Country
	}),
	VatNumber: form.Str[ConditionalRulesRequest]("vat_number", func(r *ConditionalRulesRequest) string {
		return r.VatNumber
	}),
	Role: form.Str[ConditionalRulesRequest]("role", func(r *ConditionalRulesRequest) string {
		return r.Role
	}),
	AdminNote: form.Str[ConditionalRulesRequest]("admin_note", func(r *ConditionalRulesRequest) string {
		return r.AdminNote
	}),
}

Applying Conditional Rules

Conditional helpers attach validation behavior to fields based on runtime values.

return form.Schema[ConditionalRulesRequest]{
	conditional.RequiredIfStr(
		ConditionalRulesForm.CompanyName,
		func(r *ConditionalRulesRequest) bool {
			return r.AccountType == "company"
		},
	),

	conditional.RequiredIfStrWithCode(
		ConditionalRulesForm.VatNumber,
		func(r *ConditionalRulesRequest) bool {
			return r.Country == "SK"
		},
		CodeVatNumberRequired,
	),

	conditional.ProhibitedIfStr(
		ConditionalRulesForm.AdminNote,
		func(r *ConditionalRulesRequest) bool {
			return r.Role != "admin"
		},
	),
}

Rule Examples

RequiredIfStr

Requires a string field when the condition returns true.

conditional.RequiredIfStr(
	ConditionalRulesForm.CompanyName,
	func(r *ConditionalRulesRequest) bool {
		return r.AccountType == "company"
	},
)

This is useful when one field becomes mandatory based on another field.

RequiredIfStrWithCode

Requires a string field when the condition returns true and returns a custom validation code.

const CodeVatNumberRequired = form.Code("vat_number_required")

conditional.RequiredIfStrWithCode(
	ConditionalRulesForm.VatNumber,
	func(r *ConditionalRulesRequest) bool {
		return r.Country == "SK"
	},
	CodeVatNumberRequired,
)

This is useful for country-specific, account-specific, or workflow-specific validation.

ProhibitedIfStr

Forbids a non-blank string value when the condition returns true.

conditional.ProhibitedIfStr(
	ConditionalRulesForm.AdminNote,
	func(r *ConditionalRulesRequest) bool {
		return r.Role != "admin"
	},
)

Validation behavior:

Condition	Field value	Result
`false`	any value	skipped
`true`	`""`	valid
`true`	`"secret note"`	invalid

ProhibitedIfStrWithCode

Forbids a non-blank string value when the condition returns true and returns a custom validation code.

const CodeInternalNoteBlocked = form.Code("internal_note_blocked")

conditional.ProhibitedIfStrWithCode(
	ConditionalRulesForm.InternalNote,
	func(r *ConditionalRulesRequest) bool {
		return r.UserType == "external"
	},
	CodeInternalNoteBlocked,
)

This is useful for protecting internal-only fields from public or external input.

RequiredWithAnyStr

Requires a field when any of the referenced fields is non-blank.

conditional.RequiredWithAnyStr(
	ConditionalRulesForm.Email,
	ConditionalRulesForm.Phone,
)

This means:

if phone is filled, email is required

Typical use cases:

contact method pairs
dependent address fields
partially completed form sections

RequiredWithAnyStrWithCode

Requires a field when any referenced field is non-blank and returns a custom validation code.

const CodeContactRequired = form.Code("contact_required")

conditional.RequiredWithAnyStrWithCode(
	ConditionalRulesForm.ContactName,
	CodeContactRequired,
	ConditionalRulesForm.Address,
	ConditionalRulesForm.City,
)

This means:

if address or city is filled, contact_name is required

RequiredWithoutAnyStr

Requires a field when any referenced field is blank.

conditional.RequiredWithoutAnyStr(
	ConditionalRulesForm.Phone,
	ConditionalRulesForm.Email,
)

This means:

if email is blank, phone is required

Typical use cases:

fallback contact fields
at-least-one contact method
alternative input paths

RequiredWithoutAnyStrWithCode

Requires a field when any referenced field is blank and returns a custom validation code.

const CodeBackupRequired = form.Code("backup_required")

conditional.RequiredWithoutAnyStrWithCode(
	ConditionalRulesForm.BackupEmail,
	CodeBackupRequired,
	ConditionalRulesForm.BackupPhone,
)

This means:

if backup_phone is blank, backup_email is required

Complete Example

schema.go

package main

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

const (
	CodeVatNumberRequired  = form.Code("vat_number_required")
	CodeInternalNoteBlocked = form.Code("internal_note_blocked")
	CodeContactRequired    = form.Code("contact_required")
	CodeBackupRequired     = form.Code("backup_required")
)

type ConditionalRulesRequest struct {
	AccountType string `json:"account_type"`
	CompanyName string `json:"company_name"`

	Country   string `json:"country"`
	VatNumber string `json:"vat_number"`

	Role         string `json:"role"`
	AdminNote    string `json:"admin_note"`
	UserType      string `json:"user_type"`
	InternalNote string `json:"internal_note"`

	Phone       string `json:"phone"`
	Email       string `json:"email"`
	Address     string `json:"address"`
	City        string `json:"city"`
	ContactName string `json:"contact_name"`

	BackupEmail string `json:"backup_email"`
	BackupPhone string `json:"backup_phone"`
}

func ConditionalRulesSchema() form.Schema[ConditionalRulesRequest] {
	ConditionalRulesForm := struct {
		AccountType form.StringField[ConditionalRulesRequest]
		CompanyName form.StringField[ConditionalRulesRequest]
		Country     form.StringField[ConditionalRulesRequest]
		VatNumber   form.StringField[ConditionalRulesRequest]

		Role         form.StringField[ConditionalRulesRequest]
		AdminNote    form.StringField[ConditionalRulesRequest]
		UserType      form.StringField[ConditionalRulesRequest]
		InternalNote form.StringField[ConditionalRulesRequest]

		Phone       form.StringField[ConditionalRulesRequest]
		Email       form.StringField[ConditionalRulesRequest]
		Address     form.StringField[ConditionalRulesRequest]
		City        form.StringField[ConditionalRulesRequest]
		ContactName form.StringField[ConditionalRulesRequest]

		BackupEmail form.StringField[ConditionalRulesRequest]
		BackupPhone form.StringField[ConditionalRulesRequest]
	}{
		AccountType: form.Str[ConditionalRulesRequest]("account_type", func(r *ConditionalRulesRequest) string {
			return r.AccountType
		}),
		CompanyName: form.Str[ConditionalRulesRequest]("company_name", func(r *ConditionalRulesRequest) string {
			return r.CompanyName
		}),
		Country: form.Str[ConditionalRulesRequest]("country", func(r *ConditionalRulesRequest) string {
			return r.Country
		}),
		VatNumber: form.Str[ConditionalRulesRequest]("vat_number", func(r *ConditionalRulesRequest) string {
			return r.VatNumber
		}),

		Role: form.Str[ConditionalRulesRequest]("role", func(r *ConditionalRulesRequest) string {
			return r.Role
		}),
		AdminNote: form.Str[ConditionalRulesRequest]("admin_note", func(r *ConditionalRulesRequest) string {
			return r.AdminNote
		}),
		UserType: form.Str[ConditionalRulesRequest]("user_type", func(r *ConditionalRulesRequest) string {
			return r.UserType
		}),
		InternalNote: form.Str[ConditionalRulesRequest]("internal_note", func(r *ConditionalRulesRequest) string {
			return r.InternalNote
		}),

		Phone: form.Str[ConditionalRulesRequest]("phone", func(r *ConditionalRulesRequest) string {
			return r.Phone
		}),
		Email: form.Str[ConditionalRulesRequest]("email", func(r *ConditionalRulesRequest) string {
			return r.Email
		}),
		Address: form.Str[ConditionalRulesRequest]("address", func(r *ConditionalRulesRequest) string {
			return r.Address
		}),
		City: form.Str[ConditionalRulesRequest]("city", func(r *ConditionalRulesRequest) string {
			return r.City
		}),
		ContactName: form.Str[ConditionalRulesRequest]("contact_name", func(r *ConditionalRulesRequest) string {
			return r.ContactName
		}),

		BackupEmail: form.Str[ConditionalRulesRequest]("backup_email", func(r *ConditionalRulesRequest) string {
			return r.BackupEmail
		}),
		BackupPhone: form.Str[ConditionalRulesRequest]("backup_phone", func(r *ConditionalRulesRequest) string {
			return r.BackupPhone
		}),
	}

	return form.Schema[ConditionalRulesRequest]{
		conditional.RequiredIfStr(
			ConditionalRulesForm.CompanyName,
			func(r *ConditionalRulesRequest) bool {
				return r.AccountType == "company"
			},
		),

		conditional.RequiredIfStrWithCode(
			ConditionalRulesForm.VatNumber,
			func(r *ConditionalRulesRequest) bool {
				return r.Country == "SK"
			},
			CodeVatNumberRequired,
		),

		conditional.ProhibitedIfStr(
			ConditionalRulesForm.AdminNote,
			func(r *ConditionalRulesRequest) bool {
				return r.Role != "admin"
			},
		),

		conditional.ProhibitedIfStrWithCode(
			ConditionalRulesForm.InternalNote,
			func(r *ConditionalRulesRequest) bool {
				return r.UserType == "external"
			},
			CodeInternalNoteBlocked,
		),

		conditional.RequiredWithAnyStr(
			ConditionalRulesForm.Email,
			ConditionalRulesForm.Phone,
		),

		conditional.RequiredWithAnyStrWithCode(
			ConditionalRulesForm.ContactName,
			CodeContactRequired,
			ConditionalRulesForm.Address,
			ConditionalRulesForm.City,
		),

		conditional.RequiredWithoutAnyStr(
			ConditionalRulesForm.Phone,
			ConditionalRulesForm.Email,
		),

		conditional.RequiredWithoutAnyStrWithCode(
			ConditionalRulesForm.BackupEmail,
			CodeBackupRequired,
			ConditionalRulesForm.BackupPhone,
		),
	}
}

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-rules", "POST", func(w http.ResponseWriter, req *http.Request, ctx *router.Context) {
		var in ConditionalRulesRequest

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

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

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

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

	validPayload := map[string]any{
		"account_type": "company",
		"company_name": "Acme s.r.o.",
		"country":      "SK",
		"vat_number":   "SK1234567890",

		"role":          "user",
		"admin_note":    "",
		"user_type":     "external",
		"internal_note": "",

		"phone":        "+421900123456",
		"email":        "john@example.com",
		"address":      "Main Street 1",
		"city":         "",
		"contact_name": "John Doe",

		"backup_email": "backup@example.com",
		"backup_phone": "",
	}

	invalidPayload := map[string]any{
		"account_type": "company",
		"company_name": "",
		"country":      "SK",
		"vat_number":   "",

		"role":          "user",
		"admin_note":    "internal only",
		"user_type":     "external",
		"internal_note": "secret note",

		"phone":        "+421900123456",
		"email":        "",
		"address":      "Main Street 1",
		"city":         "",
		"contact_name": "",

		"backup_email": "",
		"backup_phone": "",
	}

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

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

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

Notes

Conditional rule helpers are shortcuts for common When patterns.
Conditions receive the full request structure.
String conditional helpers use blank-string behavior based on trimmed string values.
Use RequiredIfStr when a field becomes required in a specific state.
Use ProhibitedIfStr when a field must stay empty in a specific state.
Use RequiredWithAnyStr when one field depends on another field being filled.
Use RequiredWithoutAnyStr when one field is required as a fallback.
Use When directly for advanced rule composition or multiple nested rules.

Conditional Rules

Common Use Cases

TL;DR

Defining Conditional Fields

Applying Conditional Rules

Rule Examples

RequiredIfStr

RequiredIfStrWithCode

ProhibitedIfStr

ProhibitedIfStrWithCode

RequiredWithAnyStr

RequiredWithAnyStrWithCode

RequiredWithoutAnyStr

RequiredWithoutAnyStrWithCode

Complete Example

schema.go

main.go

Notes

On this page

Conditional Rules

Common Use Cases

TL;DR

Defining Conditional Fields

Applying Conditional Rules

Rule Examples

RequiredIfStr

RequiredIfStrWithCode

ProhibitedIfStr

ProhibitedIfStrWithCode

RequiredWithAnyStr

RequiredWithAnyStrWithCode

RequiredWithoutAnyStr

RequiredWithoutAnyStrWithCode

Complete Example

schema.go

main.go

Notes

On this page