[clean-list] Default values for record fields

[Peter Achten]

Dear Bas,

At 12:13 PM 12/3/2004, you wrote:
>Hi,
>
>Is it possible to define default values for record fields?
>
>If it's possible: how?
>If it isn't: why wouldn't this be a good feature?
>
>Bas.

If you don't mind using an experimental feature of Clean, viz. generics, 
you can define the following generic function (create new project and set 
environment to Everything):

generic gNew a :: a
gNew{|Int|}                                     = 0
gNew{|Bool|}                            = False
gNew{|Real|}                            = 0.0
gNew{|String|}                          = ""
gNew{|Char|}                            = '\0'
gNew{|UNIT|}                            = UNIT
gNew{|PAIR|}   newA newB        = PAIR   newA newB
gNew{|EITHER|} newA newB        = RIGHT  newB
gNew{|CONS|}   newA                     = CONS   newA
gNew{|OBJECT|} newA                     = OBJECT newA
gNew{|FIELD|}  newA                     = FIELD  newA

It defines default values for the basic types Int, Bool, Real, String and 
Char. It also defines default values for any type that you can come up with 
that is a record, algebraic type or arbitrary composition of these types. 
It does not work for function types.

E.g., define:

:: MyR = { name::String,age::Int,married::Bool }
derive gNew MyR

Start :: MyR
Start = gNew{|*|}

This lets Start create a default value for MyR: (MyR "" 0 False).

If you define:
:: MyT :== [(Int,MyR)]
derive gNew MyR, (,), []

and change Start's type to:
Start :: MyT

then Start yields: [].

Hope this helps,

Peter