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|
(* TEST
* expect
*)
(* Injectivity *)
type (_, _) eq = Refl : ('a, 'a) eq
let magic : 'a 'b. 'a -> 'b =
fun (type a b) (x : a) ->
let module M =
(functor (T : sig type 'a t end) ->
struct
let f (Refl : (a T.t, b T.t) eq) = (x :> b)
end)
(struct type 'a t = unit end)
in M.f Refl
;;
[%%expect{|
type (_, _) eq = Refl : ('a, 'a) eq
Line 8, characters 44-52:
8 | let f (Refl : (a T.t, b T.t) eq) = (x :> b)
^^^^^^^^
Error: Type a is not a subtype of b
|}];;
(* Variance and subtyping *)
type (_, +_) eq = Refl : ('a, 'a) eq
let magic : 'a 'b. 'a -> 'b =
fun (type a) (type b) (x : a) ->
let bad_proof (type a) =
(Refl : (< m : a>, <m : a>) eq :> (<m : a>, < >) eq) in
let downcast : type a. (a, < >) eq -> < > -> a =
fun (type a) (Refl : (a, < >) eq) (s : < >) -> (s :> a) in
(downcast bad_proof ((object method m = x end) :> < >)) # m
;;
[%%expect{|
Line 1, characters 0-36:
1 | type (_, +_) eq = Refl : ('a, 'a) eq
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: In this GADT definition, the variance of some parameter
cannot be checked
|}];;
(* Record patterns *)
type _ t =
| IntLit : int t
| BoolLit : bool t
let check : type s . s t * s -> bool = function
| BoolLit, false -> false
| IntLit , 6 -> false
;;
[%%expect{|
type _ t = IntLit : int t | BoolLit : bool t
Lines 5-7, characters 39-23:
5 | .......................................function
6 | | BoolLit, false -> false
7 | | IntLit , 6 -> false
Warning 8 [partial-match]: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
(BoolLit, true)
val check : 's t * 's -> bool = <fun>
|}];;
type ('a, 'b) pair = { fst : 'a; snd : 'b }
let check : type s . (s t, s) pair -> bool = function
| {fst = BoolLit; snd = false} -> false
| {fst = IntLit ; snd = 6} -> false
;;
[%%expect{|
type ('a, 'b) pair = { fst : 'a; snd : 'b; }
Lines 3-5, characters 45-38:
3 | .............................................function
4 | | {fst = BoolLit; snd = false} -> false
5 | | {fst = IntLit ; snd = 6} -> false
Warning 8 [partial-match]: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
{fst=BoolLit; snd=true}
val check : ('s t, 's) pair -> bool = <fun>
|}];;
|
