Original bug ID: 3113
Reporter: administrator
Status: closed
Resolution: not a bug
Priority: normal
Severity: minor
Category: ~DO NOT USE (was: OCaml general)

Bug description

Hi!

I posted to the mailing list about my attempts to learn how to use phantom
types. The only response I have received (from brogoff) seemed to confirm my
fears I had found a bug in the OCaml type checker. I was using 3.08.0 but
just upgraded to 3.08.1 and the problem still appears. This may well be my
misunderstanding, however. The following:

module PhantomInt : sig
type 'p t

val add : 'p t -> 'p t -> 'p t

val add_even_even : [< Even ] t -> [< Even ] t -> [< Even ] t val add_even_odd : [< Even ] t -> [< Odd ] t -> [< Odd ] t
val add_odd_even : [ Odd ] t -> [ Even ] t -> [ Odd ] t val add_odd_odd : [ Odd ] t -> [ Odd ] t -> [ Even ] t
val neg : 'a t -> 'a t
val make_even : int -> [ Even ] t val make_odd : int -> [ Odd ] t
end = struct
type 'p t = int

let add = (+)
let add_even_even = add
let add_even_odd = add
let add_odd_even = add
let add_odd_odd = add
let neg n = -n
let make_even n = if n mod 2 = 0 then n else failwith "not even"
let make_odd n = if n mod 2 <> 0 then n else failwith "not odd"
end;;

let i = PhantomInt.make_even 2;;
let j = PhantomInt.make_odd 3;;
let k = PhantomInt.add_even_odd i j;;
PhantomInt.add_even_even i k;;
PhantomInt.neg i;;

<<<<<

works as expected, but changing the type to be non-abstract (i.e. putting
"type 'p t = int" in the signature) changes the behaviour and the phantom
type checking ceases to work. Perhaps this is the intended behaviour though,
I'm not sure.

Cheers,
Jon.