Derived Functor instance for void types

[David Feuer]

I don't know what you mean. fmap for an uninhabited type is vacuously
strict: its result is always _|_.

On Sun, Jan 15, 2017 at 11:00 PM, Kevin Cotrone <kevincotrone at gmail.com> wrote:
> That seems to have a surprising strictness.
>
> I'm not sure if it would be the best idea to try and evaluate a type with no
> inhabitants.
>
> On Sun, Jan 15, 2017 at 2:37 PM, David Feuer <david.feuer at gmail.com> wrote:
>>
>> Currently, if you write
>>
>> data V a deriving Functor
>>
>> GHC generates
>>
>> fmap _ _ = error "Void fmap"
>>
>> This seems quite unfortunate, because it loses potentially useful error
>> information:
>>
>> fmap (+ 3) (error "Too many snozzcumbers!")
>>
>> throws "Void fmap", rather than the much more precise "Too many
>> snozzcumbers!" I've opened Trac #13117 to fix this, but I figured I should
>> double check that no one is opposed.
>>
>> David Feuer
>>
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>