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// http://higherkindedtripe.wordpress.com/2011/08/10/impredicative-polymorphism-nah-macros/
type Program = Program // dummy
type ProgramState = {
Exec : Program list
Integer : int list
Float : float list
Code : Program list
Boolean : bool list
}
let dup =
function
| [] -> []
| x::xs -> x::x::xs
let swap =
function
| a::b::rest -> b::a::rest
| x -> x
let rot =
function
| a::b::c::rest -> c::a::b::rest
| x -> x
type ListFunction =
abstract Invoke: 'a list -> 'a list
let DUP = { new ListFunction with member this.Invoke a = dup a }
let SWAP = { new ListFunction with member this.Invoke a = swap a }
let ROT = { new ListFunction with member this.Invoke a = rot a }
// this doesn't compile:
//let make_instructions (func : 'a list -> 'a list) =
// [
// fun ps -> { ps with Integer = func ps.Integer };
// fun ps -> { ps with Boolean = func ps.Boolean };
// fun ps -> { ps with Exec = func ps.Exec };
// fun ps -> { ps with Float = func ps.Float };
// fun ps -> { ps with Code = func ps.Code };
// ]
// but this does:
let make_instructions (func : ListFunction) =
[
fun ps -> { ps with Integer = func.Invoke ps.Integer };
fun ps -> { ps with Boolean = func.Invoke ps.Boolean };
fun ps -> { ps with Exec = func.Invoke ps.Exec };
fun ps -> { ps with Float = func.Invoke ps.Float };
fun ps -> { ps with Code = func.Invoke ps.Code };
]
let eval f s = List.fold (|>) s f
[<EntryPoint>]
let main argv =
let duplication = make_instructions DUP
let state =
{ Exec = []
Integer = [1]
Float = []
Code = []
Boolean = [true; true; false] }
let newState = eval duplication state
let expected =
{ Exec = []
Integer = [1; 1]
Float = []
Code = []
Boolean = [true; true; true; false] }
if newState <> expected
then failwith "nooooooooooooo"
else printfn "success!"
0
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