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