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There is currently a warning for hand-rolled instances of Typeable. However, this leads to some pretty surprising error messages: because the instance is ignored any code that relies on that instance in the same module just fails, which appears pretty weird to the user.

import Data.Typeable

data K = K

instance Typeable K where
    typeRep _ = undefined

ex :: TypeRep
ex = typeRep (undefined :: Proxy K)

Gives as an error:

    No instance for (Typeable * K) arising from a use of ‛typeRep’
    In the expression: typeRep (undefined :: Proxy K)
    In an equation for ‛ex’: ex = typeRep (undefined :: Proxy K)

Which is of course surprising because the user just defined the instance.

After commenting out the code that uses the instance, one gets:

TypEx.hs:1:1: Warning:
    Typeable instances can only be derived; ignoring the following instance:
      instance Typeable * K -- Defined at TypEx.hs:5:10

This information should clearly be somehow presented before or with the previous error if the user is to have any way to figure out what is going on.

I suppose this could be fixed by:

1. Turning the Typeable instance warning into an error
2. Checking the missing instance in the existing error for Typeable and suggest that the user should look for hand written instances as a source of the problem.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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