Ian Lynagh
On Mon, Sep 17, 2007 at 03:23:46PM +0200, Josef Svenningsson wrote:
It is becoming increasingly popular to import Control.Arrow just to get access to the functions &&& and *** specialized to the function arrow. I propose to add the specialized version of these two functions
I think that having the same function with different types is likely to cause user confusion, and also ambiguous function errors.
(I also think that using a different name for the specialised versions would be a bad idea.)
This seems to be a recurrent problem, and I keep wondering whether there might not be a general solution along the lines of declaring that an instance of a class at a particular type subsumes the functions declared with those names at the specialised type. Taking the current example, in Data.Tuple there'd be (***):: (b->c)->(b'->c')->(b,b')->(c,c') (&&&):: (b->c)->(b->c')->b->(c,c') f *** g = ... f &&& g = ... and in Control.Arrow there'd be class Arrow a where ... (***):: a b c -> a b' c' -> a (b, b') (c, c') (&&&):: a b c -> a b c' -> a b (c, c') and instance Arrow (->) where ... (***) subsumes (Data.Tuple.***) (&&&) subsumes (Data.Tuple.&&&) the effect (and type checks) within the Arrow module would be the same as import qualified Data.Tuple ((***),(&&&)) instance Arrow (->) where ... (***) = (Data.Tuple.***) (&&&) = (Data.Tuple.&&&) and modules that imported only Arrow (not Tuple) would see no difference from the present state of affairs. Where things would be different would be if a module imported both Tuple and Arrow, when, instead of a name clash, *** and &&& would get their Arrow meanings (albeit with a specialised instance for ->). So if someone wrote a module that used only Tuple but later needed to import Arrow, nothing untoward would happen -- no renaming or complicated import hidings would be needed. I suppose one could describe this as shadowing of names with the restriction that the shadowed name has to fit the shadow exactly. [I have no attachment to the syntax suggested above] -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk