Simple example in ghci passes when it shouldn't

[MarcCoquand]

If we run

Prelude Test.QuickCheck> quickCheck (\xs -> reverse xs == xs)
+++ OK, passed 100 tests.

It passes and it's ambiguous why it does that (and it shouldn't)

[phadej]

because GHCi defaults to ()

λ Test.QuickCheck> quickCheck (\xs -> reverse xs == xs)
+++ OK, passed 100 tests.
λ Test.QuickCheck> quickCheck (\xs -> reverse xs == (xs :: [()]))
+++ OK, passed 100 tests.
λ Test.QuickCheck> quickCheck (\xs -> reverse xs == (xs :: [Int]))
*** Failed! Falsifiable (after 5 tests and 2 shrinks):    
[0,1]
λ Test.QuickCheck> :set -XNoExtendedDefaultRules 
λ Test.QuickCheck> quickCheck (\xs -> reverse xs == xs)

<interactive>:9:1: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘quickCheck’

Read more in the GHC manual: https://downloads.haskell.org/~ghc/8.4.4/docs/html/users_guide/ghci.html#type-defaulting-in-ghci

[nick8325]

Yes, this is due to GHCi defaulting all type variables to (). Annoying, but I don't think there's anything we can do about it.

Would it be worth warning about this in the documentation?