This code example shows a use of Top.Fold to count the fields of a type level record.

con rr :: {Type} = [One = int, Two = string, Three = bool, Four = list float]

val q : int = @@fold [fn (rx :: {Type}) => (int)]

               (fn [nm :: Name] [t :: Type] [r :: {Type}] [[nm] ~ r] acc => 1 + acc)
               (0) [rr] (_ : folder rr)

fun main () = return <xml>

 <head>
   <title>Count fields of a record type</title>
 </head>
 <body>

Count fields of a record type

We can use fold to count the fields in a type level record.

Here is the value of q: {[q]}

 </body>

</xml>

Using Top.Fold to count fields of a record