What is the easiest way to name a combination of type classes, i.e., to
abbreviate the fact that a certain type is an instance of several
classes simultaneously? I have a vague sense that this is do-able, but
that I am messing up by trying to use an empty class body as below.
So in the code below, I try to use FooBar to abbreviate the conjunction
of Foo and Bar. But while f (which uses a FooBar constraint) has a
valid definition, it can't be used. On the other hand, g (which uses
the long-winded constraint), is both a valid defined and useable.
(In a real example, imagine that FooBar names a conjunction of a half
dozen things, so that the g-like form really is onerous, whereas the
f-like form would be sweet and tidy :) .)
-- Fritz
-------------------
class Foo a where
foo :: a -> Int
class Bar a where
bar :: a -> a
class (Foo a, Bar a) => FooBar a
f :: FooBar a => a -> Int
f a = foo (bar a)
g :: (Foo a, Bar a) => a -> Int
g a = foo (bar a)
instance Foo Char where
foo = ord
instance Bar Char where
bar = id
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