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type 'a x = [> ] as 'a and 'a t = { z : 'a x } doesn't compile #6320

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vicuna opened this issue Feb 5, 2014 · 2 comments
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type 'a x = [> ] as 'a and 'a t = { z : 'a x } doesn't compile #6320

vicuna opened this issue Feb 5, 2014 · 2 comments
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@garrigue
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@vicuna
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vicuna commented Feb 5, 2014

Original bug ID: 6320
Reporter: pw374
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2015-12-11T18:25:40Z)
Resolution: not a bug
Priority: normal
Severity: minor
Version: 4.01.0
Category: typing

Bug description

type 'a x = [> ] as 'a
type 'a t = { z : 'a x }

compiles just fine but

type 'a x = [> ] as 'a
and 'a t = { z : 'a x }

doesn't.

I'm not sure that it's a bug but if it's not, I really don't see why and I'd love some explanations.
Cheers.

Steps to reproduce

  1. Compile a file that only has
    type 'a x = [> ] as 'a
    type 'a t = { z : 'a x }

  2. Compile a file that only has
    type 'a x = [> ] as 'a
    and 'a t = { z : 'a x }

Additional information

The error message is:

Error: Constraints are not satisfied in this type.
Type 'a x should be an instance of [> ] x
@vicuna
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vicuna commented Feb 6, 2014

Comment author: @garrigue

When a type abbreviation has a constraint, it is not propagated automatically to mutually recursive types.
This is to allow type abbreviations to be used polymorphically inside the recursion.

As a result you must propagate the constraint by hand:

type 'a x = [> ] as 'a
and 'a t = { z : ([> ] as 'a) x }

@vicuna
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vicuna commented Feb 6, 2014

Comment author: pw374

Ok, thanks.

However it's quite inconvenient if instead of just [> ] we have a rather big sum [> Int of int | String of string | ...etc... ], since we'd have to repeat that big sum as many times at it's used by different types...

@vicuna vicuna closed this as completed Dec 11, 2015
@vicuna vicuna added the typing label Mar 14, 2019
@vicuna vicuna added the bug label Mar 20, 2019
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