Skip to content

GitLab

GHC
GHC
Open
Opened by Jakob Brünker@JakobBruenker

:kind! should adhere to -freduction-depth

Summary

:kind! consumes gigabytes of memory for a while and then fails with *** Exception: stack overflow (GHCi itself does not crash, as long as enough memory is available) if given a non-terminating type-level expression, while using such a type elsewhere will immediately result in ghc announcing that it has reached the limit of it's reduction stack (with size 200 by default, specified by -freduction-depth). I can't be sure if this is intended, since -freduction-depth doesn't appear to be documented in the user guide, but it's not something I would have expected.

Steps to reproduce

Type the following in GHCi:

:set -XTypeFamilies -XUndecidableInstances
type family F where F = F
:kind! F

Typing something like undefined :: F works as expected, with an error message about having reached a reduction stack size of 201.

Expected behavior

I would expect to get the current error message from undefined :: F for both undefined :: F and :kind! F.

Environment

  • GHC version used:

8.6.5, 8.8.1, HEAD (8.9.0.20191001)

  • Operating System:

Windows, nixos

  • System Architecture:

x86_64

Edited by Jakob Brünker