Reduction stack overflow

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It compiles with GHC 9.8.2? Hmm:

  • # ghc -V
The Glorious Glasgow Haskell Compilation System, version 9.8.2

  • # ghc -c Foo.hs

Foo.hs:10:7: error: [GHC-40404]
    • Reduction stack overflow; size = 201
      When simplifying the following type: Monoid [Foo]
    • In the expression: pure 1
      In an equation for ‘foo’: foo = pure 1
    Suggested fix:
      Use -freduction-depth=0 to disable this check
      (any upper bound you could choose might fail unpredictably with
       minor updates to GHC, so disabling the check is recommended if
       you're sure that type checking should terminate)
   |
10 | foo = pure 1
   |       ^^^^

  • # ghc -c -XUndecidableInstances Foo.hs

Foo.hs:10:7: error: [GHC-40404]
    • Reduction stack overflow; size = 201
      When simplifying the following type: Monoid [Foo]
    • In the expression: pure 1
      In an equation for ‘foo’: foo = pure 1
    Suggested fix:
      Use -freduction-depth=0 to disable this check
      (any upper bound you could choose might fail unpredictably with
       minor updates to GHC, so disabling the check is recommended if
       you're sure that type checking should terminate)
   |
10 | foo = pure 1
   |       ^^^^

  • # ghc -c -freduction-depth=0 Foo.hs

^C
#

  • # ghc -c -XUndecidableInstances -freduction-depth=0 Foo.hs

^C
#

Regression or not, it’s certainly something that needs careful monitoring; principle of least surprise and all…