How to fix DatatypeContexts?

Daniel Wagner wagnerdm at
Thu Jul 18 19:47:21 CEST 2013

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? 
> 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.


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