> Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can > write: > > testBla1 = bla fromInteger 1 :: (Int, Float) > testBla2 = bla read "1" :: (Int, Float) > > My question is how to define 'bla'. > > I can write: > > > bla :: (forall b. a -> b) -> a -> (c, d) > > bla f x = (f x, f x) > > But then 'testBla1' gives the following expected error: > > Could not deduce (Num b) from the context () > arising from a use of `fromInteger' > > [...] > > So, somehow I need to quantify over the type class. > > bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> > (c, d) > > But this isn't legal. > > Is there another way of defining 'bla'? >
The simple, from-the-gut answer involves the enumeration of all the classes that you need in the context. > bla :: (Read c, Read d, Num c, Num d) => (forall b. (Read b, Num b) => a -> b) -> a -> (c, d) > bla f x = (f x, f x) But perhaps you're looking for potentially unknown classes? Sean
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