Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

nonrec misbehaves with GADTs #6934

Closed
vicuna opened this issue Jul 21, 2015 · 5 comments

Comments

Projects
None yet
2 participants
@vicuna
Copy link

commented Jul 21, 2015

Original bug ID: 6934
Reporter: @mmottl
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-03-15T23:38:28Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.2
Fixed in version: 4.06.0 +dev/beta1/beta2/rc1
Category: typing
Monitored by: runhang @gasche @yallop @hcarty @mmottl

Bug description

Consider the following example:

type nonrec t = A : t

The above will fail with an unbound type constructor for "t". But it isn't really a recursive type, it's a GADT. I think "nonrec" shouldn't apply to the return type of a GADT, at least not to the outermost type constructor defining the GADT, though possibly to types in its parameters.

@vicuna

This comment has been minimized.

Copy link
Author

commented Jul 22, 2015

Comment author: @diml

I'd be in favor of just updating the error message to say that you can't use [nonrec] when defining a GADT. I don't like the idea of allowing [type nonrec t = A : t -> t] where the last two [t]s means two different things.

@vicuna

This comment has been minimized.

Copy link
Author

commented Jul 22, 2015

Comment author: @mmottl

@dim I agree that the syntax of GADTs doesn't compose nicely with "nonrec" here, and we don't want to break existing syntax. Maybe it would have been better to do something like:

type t as u = t * u

where the "t" on the RHS refers to the previous definition of "t", and "u" is the recursive alias for the current definition of "t".

@vicuna

This comment has been minimized.

Copy link
Author

commented Mar 14, 2017

Comment author: @garrigue

My understanding is that nonrec is essentially intended for type abbreviations.
So is its not working with GADTs really a problem?
I tend to concur with @dim.

@vicuna

This comment has been minimized.

Copy link
Author

commented Mar 15, 2017

Comment author: @mmottl

It's surely not a big problem. It's conceivable that a developer has opened a module and just wants to define type "t" in terms of a type "t" in the opened module. Though dropping "nonrec" and using an explicit module path would easily solve the issue, the error message will likely confuse the developer.

@vicuna

This comment has been minimized.

Copy link
Author

commented Mar 15, 2017

Comment author: @garrigue

Fixed in commit f900888 by introducing a new error message:

type nonrec t = A : t;;
Line _, characters 16-21:
Error: GADT case syntax cannot be used in a 'nonrec' block.

@vicuna vicuna closed this Mar 15, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.