[Haskell-cafe] A small (?) problem with type families

David Menendez dave at zednenem.com
Fri Nov 13 15:36:59 EST 2009

On Fri, Nov 13, 2009 at 3:26 PM, Andy Gimblett <haskell at gimbo.org.uk> wrote:
> First a type family where the type Y is functionally dependent on
> the type X, and we have a function from Y to ().
>> class X a where
>>   type Y a
>>   enact :: Y a -> ()

This is ambiguous. Type families are not injective (that is, Y a ~ Y b
does not imply a ~ b), so there's no way for the compiler to figure
out which instance of X is being used when it encounters enact.

Given these instances,

instance X Int where
    type Y Int = Bool
    enact _ = ()

instance X Char where
    type Y Char = Bool
    enact _ = undefined

What is "enact False"?

I recall seeing a discussion of this in the GHC documentation, but I
can't seem to locate it.

Dave Menendez <dave at zednenem.com>

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