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This program doesn't typecheck due to the monomorphism restriction:

f = (==)

In GHC 8.0.2, the error message was quite helpful in informing you of this fact:

GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:1:1: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘==’
      prevents the constraint ‘(Eq a0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance Eq Ordering -- Defined in ‘GHC.Classes’
        instance Eq Integer
          -- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’
        instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
        ...plus 22 others
        ...plus 7 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • When instantiating ‘f’, initially inferred to have
      this overly-general type:
        forall a. Eq a => a -> a -> Bool
      NB: This instantiation can be caused by the monomorphism restriction.

Notice the NB: This instantiation can be caused by the monomorphism restriction part. But in GHC 8.2.1, this advice has mysteriously vanished!

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:1:5: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘==’
      prevents the constraint ‘(Eq a0)’ from being solved.
      Relevant bindings include
        f :: a0 -> a0 -> Bool (bound at Bug.hs:1:1)
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance Eq Ordering -- Defined in ‘GHC.Classes’
        instance Eq Integer
          -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
        instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
        ...plus 22 others
        ...plus 9 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: (==)
      In an equation for ‘f’: f = (==)
  |
1 | f = (==)
  |     ^^^^

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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