• state: Typically a record containing fields and possibly other forms.
• accessors: Typically a record containing getters/setters and anything else that needs access to the state of the form.
• error: The errors produced by an invalid form.
• output: The data produced by a valid form.

Combines a way to get a value from a property on state and a way to modify that property's value.

type alias State =
    { count : Int
    , name : String
    }


countAccessor : Accessor State Int
countAccessor =
    { get = .count
    , modify = \f state -> { state | count = f state.count }
    }


nameAccessor : Accessor State String
nameAccessor =
    { get = .name
    , modify = \f state -> { state | name = f state.name }
    }

Get the value of a property on the given form.

Set the value of a property on the given form.

Modify the value of a property on the given form.

Update the value of a property on the given form.

True if the form does not have any errors.

True if the form has errors. isInvalid form is equivalent to not (isValid form).