Checking user inputs and providing clear feedback is essential for the usability of web applications. WebDSL allows declarative specification of such input validation rules using the validate feature.

Validation rules in WebDSL are of the form validate(e,s) and consist of a Boolean expression e to be validated, and a String expression s to be displayed as error message. Any globally visible functions or data can be accessed as well as any of the properties and functions in scope of the validation rule context.

Value well-formedness checks (e.g. whether the user enters a valid integer in an Int input) are added automatically to each input field.

Validation can be specified on entities in property annotations:

entity User { 
  username :: String (id, validate(isUniqueUser(this), "Username is taken")) 
  password :: Secret (validate(password.length() >= 8, "Password needs to be at least 8 characters") 
  , validate(/[a-z]/.find(password), "Password must contain a lower-case character") 
  , validate(/[A-Z]/.find(password), "Password must contain an upper-case character") 
  , validate(/[0-9]/.find(password), "Password must contain a digit")
  email :: Email)) 
extend entity User { 
  username(validate(isUniqueUser(this),"Username is taken")) 
  password(validate(password.length() >= 8, "Password needs to be at least 8 characters") 
  ,validate(/[a-z]/.find(password), "Password must contain a lower-case character") 
  ,validate(/[A-Z]/.find(password), "Password must contain an upper-case character") 
  ,validate(/[0-9]/.find(password), "Password must contain a digit")) 

Validation can be specified directly in pages:

define page editUser(u:User) { 
  var p: Secret; 
  form { 
    group("User") { 
      label("Username") { input(u.username) } 
      label("Email") { input( } 
      label("New Password") { 
      label("Re-enter Password") { 
        input(p) { 
          validate(u.password == p, "Password does not match") 
      action("Save", action{ } ) 

Validation can be specified in actions:

define page createGroup() { 
  var ug := UserGroup {} 
  form { 
    group("User Group") { 
    label("Name") { input( } 
    label("Owner") { input(ug.owner) } 
    action("Save", save()) } } 
    action save() { 
      validate(email(newGroupNotify(ug)),"Owner could not be notified by email"); 
      return userGroup(ug); 

Customizing Validation Output

Validation output can be customized by overriding the templates used to display validation messages. Currently, there are 4 global validation templates:

define ignore-access-control errorTemplateInput(messages : List<String>)

Displays validation message related to an input.

define ignore-access-control errorTemplateForm(messages : List<String>)

Displays validation message for validation in a form.

define ignore-access-control errorTemplateAction(messages : List<String>)

Displays validation message for validation in an action.

define ignore-access-control templateSuccess(messages : List<String>)

Displays validation message for success messages.

When overriding these validation templates, use a validateInput() templatecall to refer to the element being validated.


define ignore-access-control errorTemplateInput(messages : List<String>){
  for(ve: String in messages){

Ajax Validation

WebDSL provides input components that validate the inputs using ajax.

built-in value types:


reference types:


provide selection options:


Selection options can also be provided using the allowed annotation on an entity property. Example:

entity Person{
  parent -> Person (allowed=from Person as p where p != this)