On 14/09/2011, at 1:44 PM, Chris Smith wrote:
> On Wed, 2011-09-14 at 13:35 +1200, Richard O'Keefe wrote:
>> I would like to have
>>
>> tuple (f1,f2) x = (f1 x, f2 x)
>> tuple (f1,f2,f3) x = (f1 x, f2 x, f3 x)
> There is no polymorphism across tuple structures,
I know that. I know how tuples get to be instances of Ix,
one instance declaration for each of (,) (,,) (,,,) ....
> so if you absolutely
> *must* have n-tuples instead of nested 2-tuples, then you just need to
> implement the new functions as needed. You can't implement that only
> once.
I don't *expect* to implement anything just once. I am perfectly
happy writing as many instance declarations as I have tuple sizes
that I care about. It's just that I can't see how to get the types
right, because in
class Thingy t ... where
tuple :: t -> a -> b
b depends on t and possibly a, so
instance Thingy (,,) where
tuple (f,g,h) x = (f x, g x, h x)
it's not an arbitrary b. Can this be done with functional dependencies?
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