diff --git a/src/debouncedState.ts b/src/debouncedState.ts
index d33fee9..4fe6287 100644
--- a/src/debouncedState.ts
+++ b/src/debouncedState.ts
@@ -13,7 +13,7 @@ type OriginalInfo<V> = Pick<IState<V>, 'activated' | 'touched' | 'error' | 'ownE
  * The state for debounce purpose.
  * Changes from the original state (`$`) will be debounced.
  */
-export class DebouncedState<S extends IState<V>, V = ValueOf<S>> extends ValidatableState<V> implements IState<V> {
+export class DebouncedState<S extends IState<V>, V = ValueOf<S>, SV extends V = V> extends ValidatableState<V, SV> implements IState<V, SV> {
 
   /**
    * The original state.
@@ -136,7 +136,7 @@ export class DebouncedState<S extends IState<V>, V = ValueOf<S>> extends Validat
  * The field state with debounce.
  * Value changes from `onChange` will be debounced.
  */
-export class DebouncedFieldState<V> extends DebouncedState<FieldState<V>, V> {
+export class DebouncedFieldState<V, SV extends V = V> extends DebouncedState<FieldState<V, SV>, V, SV> {
   constructor(initialValue: V, delay = defaultDelay) {
     super(new FieldState(initialValue), delay)
   }
diff --git a/src/fieldState.ts b/src/fieldState.ts
index 5e46a00..9683a0c 100644
--- a/src/fieldState.ts
+++ b/src/fieldState.ts
@@ -5,7 +5,7 @@ import { ValidatableState } from './state'
 /**
  * The state for a field.
  */
-export class FieldState<V> extends ValidatableState<V> implements IState<V> {
+export class FieldState<V, SV extends V = V> extends ValidatableState<V, SV> implements IState<V, SV> {
 
   @observable.ref value!: V
 
diff --git a/src/formState.ts b/src/formState.ts
index 08e8ecf..a72398c 100644
--- a/src/formState.ts
+++ b/src/formState.ts
@@ -1,8 +1,8 @@
 import { observable, computed, isObservable, action, reaction, makeObservable, override } from 'mobx'
-import { IState, ValidateStatus, ValidateResult, ValueOfStatesObject } from './types'
+import { IState, ValidateStatus, ValidateResult, ValueOfStatesObject, SafeValueOfStatesObject } from './types'
 import { ValidatableState } from './state'
 
-abstract class AbstractFormState<T, V> extends ValidatableState<V> implements IState<V> {
+abstract class AbstractFormState<T, V, SV extends V = V> extends ValidatableState<V, SV> implements IState<V, SV> {
 
   /** Reference of child states. */
   abstract readonly $: T
@@ -66,7 +66,7 @@ abstract class AbstractFormState<T, V> extends ValidatableState<V> implements IS
     this.resetChildStates()
   }
 
-  override async validate(): Promise<ValidateResult<V>> {
+  override async validate(): Promise<ValidateResult<SV>> {
     if (this.disabled) {
       return this.validateResult
     }
@@ -113,7 +113,7 @@ export type StatesObject = { [key: string]: IState }
 export class FormState<
   TStates extends StatesObject
 > extends AbstractFormState<
-  TStates, ValueOfStatesObject<TStates>
+  TStates, ValueOfStatesObject<TStates>, SafeValueOfStatesObject<TStates>
 > {
 
   @observable.ref readonly $: Readonly<TStates>
@@ -171,9 +171,9 @@ export class FormState<
  * The state for a array form (list of child states).
  */
 export class ArrayFormState<
-  V, T extends IState<V> = IState<V>
+  V, SV extends V, T extends IState<V, SV> = IState<V, SV>
 > extends AbstractFormState<
-  readonly T[], V[]
+  readonly T[], V[], SV[]
 > {
 
   @observable.ref protected childStates: T[]
diff --git a/src/index.spec.ts b/src/index.spec.ts
index d638f3a..fed0175 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -126,3 +126,41 @@ describe('Composition', () => {
     expect(hostState.error).toBe('empty hostname')
   })
 })
+
+interface HostInput {
+  hostname: string | null
+  port: number | null
+}
+
+function parseHost(input: string): HostInput {
+  const [hostname, portStr] = input.split(':')
+  const port = parseInt(portStr, 10)
+  return { hostname, port }
+}
+
+function stringifyHost(host: HostInput) {
+  const suffix = (host.port == null || Number.isNaN(host.port)) ? '' : `:${host.port}`
+  return host.hostname + suffix
+}
+
+function createDebouncedHostState(hostStr: string) {
+  const host = parseHost(hostStr)
+  const hostnameState = new FieldState<string | null>(host.hostname).withValidator<string>(
+    v => !v && 'empty hostname'
+  )
+  const portState = new FieldState<number | null, number>(host.port)
+  const rawState = new FormState({
+    hostname: hostnameState,
+    port: portState
+  })
+  const state = new DebouncedState(
+    new TransformedState(rawState, stringifyHost, parseHost),
+    defaultDelay
+  ).withValidator(
+    v => !v && 'empty'
+  )
+
+  state.safeValue
+
+  return state
+}
\ No newline at end of file
diff --git a/src/state.ts b/src/state.ts
index c104caf..54fba6d 100644
--- a/src/state.ts
+++ b/src/state.ts
@@ -1,5 +1,5 @@
 import { action, autorun, computed, makeObservable, observable, when } from 'mobx'
-import { ValidationError, IState, Validation, ValidateResult, ValidateStatus, Validator } from './types'
+import { ValidationError, IState, Validation, ValidateResult, ValidateStatus, Validator, WithSafeValue } from './types'
 import Disposable from './disposable'
 import { applyValidators, isValid, isPromiseLike } from './utils'
 
@@ -37,13 +37,18 @@ export abstract class BaseState extends Disposable implements Pick<
 }
 
 /** Extraction for State's validating logic */
-export abstract class ValidatableState<V> extends BaseState implements IState<V> {
+export abstract class ValidatableState<V, SV extends V> extends BaseState implements IState<V, SV> {
 
   abstract value: V
   abstract touched: boolean
   abstract onChange(value: V): void
   abstract set(value: V): void
 
+  @computed get safeValue(): SV {
+    if (!this.validated || this.hasError) throw new Error('TODO')
+    return this.value as SV
+  }
+
   /** The original validate status (regardless of `validationDisabled`) */
   @observable protected _validateStatus: ValidateStatus = ValidateStatus.NotValidated
 
@@ -76,9 +81,9 @@ export abstract class ValidatableState<V> extends BaseState implements IState<V>
   /** List of validator functions. */
   @observable.shallow private validatorList: Validator<V>[] = []
 
-  @action withValidator(...validators: Validator<V>[]) {
+  @action withValidator<NSV = SV>(...validators: Validator<V>[]): WithSafeValue<this, NSV> {
     this.validatorList.push(...validators)
-    return this
+    return this as WithSafeValue<this, NSV>
   }
 
   /** Current validation info. */
@@ -108,15 +113,15 @@ export abstract class ValidatableState<V> extends BaseState implements IState<V>
     })()
   }
 
-  @computed protected get validateResult(): ValidateResult<V> {
+  @computed protected get validateResult(): ValidateResult<SV> {
     return (
       this.error
-      ? { hasError: true, error: this.error } as const
-      : { hasError: false, value: this.value } as const
+      ? { hasError: true, error: this.error }
+      : { hasError: false, value: this.value as SV }
     )
   }
 
-  async validate(): Promise<ValidateResult<V>> {
+  async validate(): Promise<ValidateResult<SV>> {
     if (this.disabled) {
       return this.validateResult
     }
diff --git a/src/transformedState.ts b/src/transformedState.ts
index 4528687..60f870e 100644
--- a/src/transformedState.ts
+++ b/src/transformedState.ts
@@ -1,8 +1,8 @@
 import { computed } from 'mobx'
 import { BaseState } from './state'
-import { IState, Validator, ValueOf } from './types'
+import { IState, Validator, ValueOf, WithSafeValue } from './types'
 
-export class TransformedState<S extends IState<$V>, V, $V = ValueOf<S>> extends BaseState implements IState<V> {
+export class TransformedState<S extends IState<$V>, V, $V = ValueOf<S>, SV extends V = V> extends BaseState implements IState<V, SV> {
 
   /** The original state, whose value will be transformed. */
   public $: S
@@ -23,6 +23,11 @@ export class TransformedState<S extends IState<$V>, V, $V = ValueOf<S>> extends
     return this.parseOriginalValue(this.$.value)
   }
 
+  @computed get safeValue() {
+    if (!this.validated || this.hasError) throw new Error('TODO')
+    return this.value as SV
+  }
+
   @computed get ownError() {
     return this.$.ownError
   }
@@ -46,7 +51,7 @@ export class TransformedState<S extends IState<$V>, V, $V = ValueOf<S>> extends
   async validate() {
     const result = await this.$.validate()
     if (result.hasError) return result
-    return { ...result, value: this.value }
+    return { ...result, value: this.value as SV }
   }
 
   set(value: V) {
@@ -61,12 +66,12 @@ export class TransformedState<S extends IState<$V>, V, $V = ValueOf<S>> extends
     this.$.reset()
   }
 
-  withValidator(...validators: Array<Validator<V>>) {
+  withValidator<NSV = SV>(...validators: Array<Validator<V>>): WithSafeValue<this, NSV> {
     const rawValidators = validators.map(validator => (
       (rawValue: $V) => validator(this.parseOriginalValue(rawValue))
     ))
     this.$.withValidator(...rawValidators)
-    return this
+    return this as WithSafeValue<this, NSV>
   }
 
   disableWhen(predict: () => boolean) {
diff --git a/src/types.ts b/src/types.ts
index e8530ce..2883124 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -25,9 +25,10 @@ export type ValidateResultWithValue<T> = { hasError: false, value: T }
 export type ValidateResult<T> = ValidateResultWithError | ValidateResultWithValue<T>
 
 /** interface for State */
-export interface IState<V = unknown> {
+export interface IState<Value = unknown, SafeValue extends Value = Value> {
   /** Value in the state. */
-  value: V
+  value: Value
+  safeValue: SafeValue
   /** If value has been touched. */
   touched: boolean
   /** The error info of validation. */
@@ -50,15 +51,15 @@ export interface IState<V = unknown> {
    */
   validated: boolean
   /** Fire a validation behavior. */
-  validate(): Promise<ValidateResult<V>>
+  validate(): Promise<ValidateResult<this['safeValue']>>
   /** Set `value` on change event. */
-  onChange(value: V): void
+  onChange(value: Value): void
   /** Set `value` imperatively. */
-  set(value: V): void
+  set(value: Value): void
   /** Reset to initial status. */
   reset(): void
   /** Append validator(s). */
-  withValidator(...validators: Array<Validator<V>>): this
+  withValidator<NSV = SafeValue>(...validators: Array<Validator<Value>>): WithSafeValue<this, NSV>
   /**
    * Configure when state should be disabled, which means:
    * - corresponding UI is invisible or disabled
@@ -71,6 +72,13 @@ export interface IState<V = unknown> {
   dispose(): void
 }
 
+/** Safe Value of `IState` */
+export type SafeValueOf<S> = S extends IState ? S['safeValue'] : never
+
+export type WithSafeValue<S extends IState, SV> = S & {
+  safeValue: SV
+}
+
 /** Function to do dispose. */
 export interface Disposer {
   (): void
@@ -81,6 +89,11 @@ export type ValueOfStatesObject<StatesObject> = {
   [K in keyof StatesObject]: ValueOf<StatesObject[K]>
 }
 
+/** Safe Value of states object. */
+export type SafeValueOfStatesObject<StatesObject> = {
+  [K in keyof StatesObject]: SafeValueOf<StatesObject[K]>
+}
+
 /** Value of `IState` */
 export type ValueOf<S> = S extends IState<infer V> ? V : never