Re: Derived Functor instance for void types

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 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 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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