elm-validate

elm-validate provides convenience functions for validating data.

It is based around the idea of a Validator, which runs checks on a subject and returns a list of errors representing anything invalid about that subject. If the list is empty, the subject is valid.

For example:

import Validate exposing (ifBlank, ifNotInt, validate)


type alias Model =
    { name : String, email : String, age : String, selections : List String }


modelValidator : Validator Model String
modelValidator =
    Validate.all
        [ ifBlank .name "Please enter a name."
        , ifBlank .email "Please enter an email address."
        , ifNotInt .age "Age must be a whole number."
        , ifEmptyList .selections "Please select at least one."
        ]


validate modelValidator
    { name = "Sam", email = "", age = "abc", selections = [ "cats" ] }
    == [ "Please enter an email address.", "Age must be a whole number." ]
    --> True

You can represent your errors however you like. One nice approach is to use tuple of the error message and the field responsible for the error:

type Field =
    Name | Email | Age | Selections


modelValidator : Validator ( Field, String ) Model
modelValidator =
    Validate.all
        [ ifBlank .name ( Name, "Please enter a name." )
        , ifBlank .email ( Email, "Please enter an email address." )
        , ifNotInt .age ( Age, "Age must be a whole number." )
        , ifEmptyList .selections ( Selections, "Please select at least one." )
        ]


type alias Model =
    { name : String, email : String, age : String }


validate modelValidator
    { name = "Sam", email = "", age = "abc", selections = [ "cats" ] }
    == [ ( Email, "Please enter an email address." )
       , ( Age, "Age must be a whole number." )
       ]
    --> True