How to fix DatatypeContexts?

[Guy]

Daniel Wagner wrote:
> On 2013-07-18 10:46, harry wrote:
>> Why not let all types carry the dictionary automatically, or at least every
>> time that it's used, if that would incur a memory/performance penalty? GHC
>> tells me which context to add when it's missing, so it clearly knows.
>
> I'm not sure the claim in your second sentence is true. For example,
>
> foo :: a -> a -> Bool
> foo = (==)
>
> bar :: a -> a -> Bool
> bar = foo
>
> GHC (rightly) complains about foo, but makes no complaint about bar. That is, if a thing only works in the presence of a
> constraint, you need to have that constraint visible in the type, every time, or else downstream dependencies can
> reasonably make wrong assumptions.

It does complain about bar after adding the context to foo, so the proposal still seems to hold. In any case, would 
there actually be a performance penalty in adding it everywhere?