This function is used to check if the given variable in the form has correct validity and if yes, then returns this variable with her value. Otherwise returns an Err errorMessage.

import Validation exposing (preValidatedField, Field)


type A = A Int
type B = B Float


type alias Model = { a : A, b : B}
type alias Form = { a : Field String Int, b : Field String Float}


form = Form (preValidatedField 1) (preValidatedField 0.3)


condPosInt : Int -> Result String A
condPosInt val =
    if val > 0 then
        Ok (A val)
    else
        Err "Value must be positive number!!!"


condFloatRange : Float -> Result String B
condFloatRange val =
    if val < 10 && val > 0 then
        Ok (B val)
    else
        Err "Value isn't in range!!!"


model : Result String Model
model =
    toModel
        Model
        ( withField condPosInt .a
        >> withField condFloatRange .b
        )
        form


-- Ok { a = 1, b = 0.3 }

This function is similar to function above and is used for non-field type of variables.

import Validation exposing (preValidatedField, Field)


type Planet = Venus | Earth | Mars
type alias Model = { planet : Planet, a: Int, b: Float}
type alias Form = { planet : Planet, a : Field String Int, b : Field String Float}


form = Form Earth (preValidatedField 1) (preValidatedField 0.3)
--form = Form Mars (preValidatedField 1) (Validation.field "40.5")


model : Result String Model
model =
    let
        fieldOk = withField Ok
        valueOk = withoutField Ok
    in
        toModel
            Model
            ( valueOk .planet
            >> fieldOk .a
            >> fieldOk .b
            )
            form


-- Ok { planet = Earth, a = 1, b = 0.3 }
-- for form with Mars we get:
--      Err "Form is invalid!!!",
-- because validity of initial value b is NotValidated.

This function transforms form to model and returns an Err errorMessage if any variable in the form is NotValidated either Invalid. Otherwise returns Ok model. See examples above.