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Misleading error message with GADTs and polymorphic variants #7181

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vicuna opened this issue Mar 14, 2016 · 4 comments

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commented Mar 14, 2016

Original bug ID: 7181
Reporter: @chambart
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-09-24T15:32:09Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.03.0+dev / +beta1
Target version: 4.03.1+dev
Fixed in version: 4.03.0+dev / +beta1
Category: typing
Monitored by: @hcarty

Bug description

The example is

type 'a tree =
| Root : [>Root] tree | Leaf : [Root | Leaf] tree -> [>Leaf] tree

let rec root (type x) (tree : x tree) : [`Root] tree =
match tree with
| Root -> tree
| Leaf parent -> root parent

which fails with

| Root -> tree
^^^^
Error: This expression has type x tree but an expression was expected of type [ Root ] tree Type x = [> Root ] is not compatible with type [ `Root ]
The first variant type does not allow tag(s)

Additional information

By the way, I'm not completely certain that this type should not be accepted

@vicuna

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commented Mar 15, 2016

Comment author: @gasche

There is something fishy in trying to take an (exists 'r. [ Root | 'r ]) and turn it into a [ Root ]. In this case, you have no parameter to the Root constructor, so it seems harmless, but you could equally write the following which type-checks:

let rec root (type x) (tree : x tree) : [`Root] tree =
match tree with
| Root -> Root
| Leaf parent -> root parent

if you added a parameter in the Root case, if this parameter does not depend on 'a then you can reuse the trick above, and if it does then your version seems unsound:

type 'a tree =
| Root : ([> Root] as 'a) -> 'a tree | Leaf : [Root | Leaf] tree -> [>Leaf] tree

let rec root (type x) (tree : x tree) : [`Root] tree =
match tree with
| Root _ -> tree
| Leaf parent -> root parent

if I could type-check that I could use it to coerce (Root Leaf) into a ([Root] tree).

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commented Mar 15, 2016

Comment author: @garrigue

Indeed, the error message is confusing.
I suppose you should get something like
Type x = [> Root ] is not compatible with type [ Root ]
The second variant type does not allow tag(s) `AnyOtherTag

Here type x has an existential row, which means that it could contain
anything, so you cannot coerce it to just [`Root].

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commented Mar 15, 2016

Comment author: @chambart

By the way this also cannot be coerced to [>`Root] for a different reason. The row variable will escape:

| Root -> tree
          ^^^^

Error: This expression has type x tree but an expression was expected of type
([> Root ] as 'a) tree Type x = [> Root ] is not compatible with type [> `Root ] as 'a
The type constructor $0 would escape its scope

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commented Mar 30, 2016

Comment author: @garrigue

Fixed in 4.03 by commit 56b337.
Just removed the incorrect last line in that case.

@vicuna vicuna closed this Sep 24, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added this to the 4.03.1 milestone Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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