Bits is not derived from Num?

Question

Trying to compile data-memocombinators 0.4.3, got the following error:

Data/MemoCombinators.hs:119:10:
    Could not deduce (Num a) arising from a use of `IntTrie.apply'
    from the context (Ord a, Bits a)

I think Bits used to be derived from Num. Perhaps it was bad and the dependency was removed, but now the package is broken. Is there any known fix for this? Perhaps I'm using a bad version of something?

I have added Num a to the function signature in my copy of the package for the time being, but this can't be right in the long term.

I'm using GHC 7.6.1, base is 4.6.0.0.

Answer 1

It's mentioned in the changelog for GHC 7.6.1 (base 4.6.0.0).

The Bits class does not have a Num superclass anymore.

You can make code that works with both Haskell98/Haskell2010 and GHC by:

• Whenever you make a Bits instance of a type, also make Num instance, and
• Whenever you give a function, instance or class a Bits t constraint, also give it a Num t constraint.

See GHC #5593 and the mailing list discussion for more information about this decision.