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Strage error message probably caused by universal variable escape (with polymorphic variants) #6987

vicuna opened this issue Sep 11, 2015 · 2 comments


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@vicuna vicuna commented Sep 11, 2015

Original bug ID: 6987
Reporter: mandrykin
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-03-14T07:18:14Z)
Resolution: fixed
Priority: low
Severity: minor
Platform: x86_64
OS: Linux (3.2.0)
OS Version: Ubuntu 12.04
Version: 4.02.3
Target version: undecided
Fixed in version: 4.05.0 +dev/beta1/beta2/beta3/rc1
Category: typing
Monitored by: @gasche

Bug description

The following code:

type 'a t = V1 of 'a

type ('c,'t) pvariant = [ `V of ('c * 't t) ]

class ['c] clss =
method mthod : 't . 'c -> 't t -> ('c, 't) pvariant = fun c x ->
`V (c, x)

let f2 = fun o c x -> match x with | V1 _ -> x

let rec f1 o c x =
match (o :> _ clss)#mthod c x with
| `V c -> f2 o c x

Is rejected with a strange inclusion error (of a value within itself):

Error: Values do not match:
val f1 :
< mthod : 't. 'a -> 't t -> [< ('a, 't) pvariant ]; .. > ->
'a -> 'b t -> 'b t
is not included in
val f1 :
< mthod : 't. 'a -> 't t -> [< ('a, 't) pvariant ]; .. > ->
'a -> 'b t -> 'b t

However, after substituting the type ('c, 't) pvariant' directly into the type of mthod' the function `f1' is assigned essentially the same type:

f1 : < mthod : 't. 'a -> 't t -> [< `V of 'a * 't t ]; .. > -> 'a -> 'b t -> 'b t

An attempt to call the function `f1' in the following way:

let v' = f1 (object method mthod = fun c (V1 x) -> `V (c, V1 x) end) 0 (V1 0)

casues an error message about a universal variable escaping its scope:

Error: This expression has type
< mthod : 'a -> 'b t -> [> V of int * 'c t ] > but an expression was expected of type < mthod : 't. 'a -> 't t -> [< V of int * 't0 t ]; .. >
The universal variable 't would escape its scope

The function can be successfully called with an object inherited form int clss'. I'm not sure if the initial function f1' should be accepted or what the appropriate error message for it should be, though (in practice the coercion :> _ clss' can be removed or replaced with :> _ #clss', which solves the problem).

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@vicuna vicuna commented Nov 30, 2015

Comment author: @garrigue

Actually, V1 and f2 are irrelevant. This a problem with universal variables appearing under a non-fixed polymorphic variant. I.e. the following is ok

let f1 o = (o : < mthod : 't. 'a -> 't t -> ('a,'t) pvariant; .. > :> 'a clss);;

but this one fails;

let f2 o = (o : < mthod : 't. 'a -> 't t -> [< ('a, 't) pvariant ]; .. > :> 'a clss);;

Note that in the theory developed in my papers, it is correct to refuse f2.
There is a hack in ocaml to make it possible to express ('t. 'a -> 't t -> [< ('a, 't) pvariant ]), but it doesn't play well with unification.

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@vicuna vicuna commented Mar 14, 2017

Comment author: @garrigue

Resolved somewhere on the road to 4.05.
Test added in commits da9535e and d588cf6.

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